Nature Guide_ Rocks and Minerals

MINERALS | SILICATES: PHYLLOSILICATES 199 Vermiculite layers pseudohexagonal This specimen of vermiculite, outline mined in Pennsylvania, shows the mineral’s foliated habit. foliated habit PROFILE r (Mg,Fe,Al)3(Al,Si)4O10(OH)∙4H2O Monoclinic VERMICULITE 4 1–2 The name vermiculite is applied to a group of mica m 2.6 n Perfect minerals in which various chemical substitutions occur in o Uneven p White the molecular structure. Vermiculite may be completely q Oily to earthy interlayered with other micas and claylike minerals. Specimens are green, golden yellow, or brown in color. Vermiculite usually forms tabular, pseudohexagonal crystals or platy aggregates. Vermiculite occurs as large pseudomorphs replacing biotite (p.197), as small particles in soils and ancient sediments, and at the interface between feldspar-rich and iron- and magnesium-rich igneous rocks. It also forms by hydrothermal alteration of iron-bearing micas. When heated to nearly 572°F (300°C), Potting soil vermiculite can expand quickly Expanded vermiculite is a and strongly to 20 times its good growing medium for original thickness. new plants; it retains water and offers good aeration.

200 MINERALS | SILICATES: PHYLLOSILICATES PROFILE dull, earthy surface Monoclinic 4 1–2 m 2.1 n Perfect o Uneven p White to buff q Earthy massive habit Massive bentonite r (Na,Ca0.5)0.33(Al,Mg)2Si4O10(OH)2∙nH2O Bentonite is always found in massive form, as in the specimen BENTONITE shown here. Under magnification, tiny, hexagonal plates are visible. This group of minerals are all kinds of clay that expand VARIANTS as they absorb water and shrink as they dry. In regions Desert cracking Loss of underlain by bentonites, this property causes immense water from bentonite clays causes shrinkage and cracking problems with building foundations. There are three Bentonite sediments Layers bentonite minerals, each named after the respective of bentonite-rich clay dominant element: potassium bentonite, sodium bentonite, and calcium bentonite. The minerals are generally yellow, white, or gray in color. They occur as microscopic crystals and are earthy and frequently stained. Although the term bentonite has been used for clay beds of uncertain origin, this mineral group generally forms from volcanic ash that has weathered in the presence of water. Potting clay Important industrial minerals, The potting clay used to bentonites are used as sealants make this bowl contains and in oil drilling. bentonite, which is also used in bricks and ceramics.

PROFILE MINERALS | SILICATES: PHYLLOSILICATES 201 Triclinic Powdery kaolinite In this specimen, powdery 4 2–2 1⁄2 kaolinite coats a piece m 2.6 of granite. n Perfect o Unobservable powdery kaolinite p White q Earthy earthy luster VARIANTS r Al2Si2O5(OH)4 Blocky kaolinite Blocky, KAOLINITE typically white kaolinite Clay minerals are far removed in their outward Iron staining A specimen of appearance from more attractive and glamorous kaolinite mixed with iron oxides, minerals, such as gold and diamond. Yet, by providing which give it an orange color the raw material for brick, pottery, and tiles, they have played a vital part in the progress of human civilization. Important among these minerals is kaolinite. Kaolinite forms white, microscopic, pseudohexagonal plates in compact or granular masses and in micalike piles. Three other minerals—dickite, nacrite, and halloysite— are chemically identical to kaolinite but crystallize in the monoclinic system. All four have been found together and are often visually indistinguishable. Kaolinite is a natural product of the alteration of mica, plagioclase, and sodium–potassium feldspars under the influence of water, dissolved carbon dioxide, and organic acids. It is used in agriculture; as a filler in food, such as chocolate; mixed with pectin as an antidiarrheal; as a paint extender; as a strengthener in rubber; and as a dusting agent in foundry operations.

202 MINERALS | SILICATES: PHYLLOSILICATES PROFILE pale, earthy mass of illite Monoclinic 4 1–2 m 2.6–2.9 n Perfect o None p White q Dull Illite mass One of the major clay minerals, illite is usually found as pale, earthy masses. VARIANT r K0.65Al2Al0.65Si3.35O10(OH)2 Solid illite Solid masses of ILLITE illite are occasionally found Once regarded as a clay mineral, illite is now classified as a group of mica minerals that bear many structural similarities to the white mica muscovite (p.195). Illite takes its name from its type location in Illinois. It is white, but impurities may tint it gray and other pale colors. It occurs as fine-grained aggregates. Individual hexagonal crystals can only be seen using an electron microscope. Because of its minute crystals, illite can only be positively identified by x-ray diffraction. The degree of crystallization of illite has been used as an indicator of metamorphic grade in clay- bearing metamorphic rocks. Illite is found in sedimentary rocks and soils. It is the most abundant clay mineral in shales (pp.313–14) and clays. It Mud bricks appears to be derived from the Ancient buildings, such weathering of muscovite and as the Funerary Temple in feldspar (pp.173–81). Egypt, were often made from clays bearing illite.

MINERALS | SILICATES: PHYLLOSILICATES 203 Chrysocolla with azurite rock matrix In this specimen, chrysocolla can be seen with the carbonate mineral azurite in a rock matrix. chrysocolla PROFILE Orthorhombic azurite 4 2–4 m 2.0–2.4 n None o Uneven to conchoidal p Pale blue, tan, gray q Vitreous to earthy VARIANTS r Cu2H2(Si2O5)(OH)4∙nH2O Rough chrysocolla CHRYSOCOLLA A specimen intergrown with turquoise and malachite The term chrysocolla was first used by the Greek chrysocolla philosopher Theophrastus in 315 BCE to refer to various Cabochon Green chrysocolla materials used in soldering gold. The name is derived within reddish iron oxide from two Greek words: chrysos, which means “gold,” and kolla, which means “glue.” A copper aluminum silicate, chrysocolla is generally blue-green in color. Specimens are commonly fine grained and massive. Crystals are very rare but when found occur as botryoidal, radiating aggregates. An occasional ore of copper, chrysocolla is a decomposition product of copper minerals, especially in arid regions. It is frequently intergrown with other minerals, such as quartz (p.168), chalcedony (p.169), and opal Chrysocolla bracelet (p.172), to yield a gemstone Rich blue-green chrysocolla, variety. Gemstones can weigh such as the cabochon in this more than 5lb (2.3kg). antique bracelet, is highly prized as a gemstone.

204 MINERALS | SILICATES: PHYLLOSILICATES Green apophyllite PROFILE In this specimen, green apophyllite occurs in a Tetragonal basalt matrix with a white zeolite mineral. 4 4 1⁄2–5 m 2.3–2.4 green apophyllite n Perfect o Uneven, brittle p Colorless q Vitreous white zeolite basalt VARIANTS r KCa4Si8O20(F,OH)∙8H2O (fluorapophyllite) Pink crystal APOPHYLLITE An unusual single crystal The name apophyllite comes from the Greek words of pink apo and phyllazein, which mean “to get” and “leaf” apophyllite respectively—a reference to the way in which the mineral separates into flakes or layers when it is heated. Once Blocky crystals Green considered to be a single mineral, apophyllite is now crystals on stilbite divided into two distinct species—fluorapophyllite and hydroxyapophyllite. These species form a solid-solution apophyllite series in which fluorine can predominate over oxygen stilbite and hydrogen, and vice versa. Apophyllite specimens are green, pink, colorless, or white. Crystals are transparent or Reddish pink crystals translucent and up to 8in (20cm) in length. They occur as Apophyllite with white stilbite square-sided, striated prisms with flat ends and may appear cubic. Apophyllite crystals may also show steep pyramidal terminations. The mineral frequently occurs with zeolite minerals in basalt (p.273) and less commonly in cavities in granites (pp.258–59). It is also found in metamorphic rocks and in hydrothermal deposits. Colorless and green specimens from India are faceted as collectors’ gems.

PROFILE MINERALS | SILICATES: PHYLLOSILICATES 205 Orthorhombic Botryoidal prehnite A group of radiating crystal 4 6–6 1⁄2 masses of prehnite resting m 2.9 on a rock matrix gives a botryoidal n Distinct basal form to this specimen. o Uneven, brittle p White radiating prehnite q Vitreous crystal rock matrix vitreous luster VARIANT r Ca2Al2Si3O10(OH)2 calcite PREHNITE prehnite A calcium aluminum silicate, prehnite was named Green prehnite Spherical masses of prehnite occuring in 1789 after its discoverer Hendrik von Prehn, a Dutch with white calcite on a rock matrix military officer. Specimens can be pale to mid-green, tan, pale yellow, gray, light blue, or white. Prehnite commonly occurs as globular, spherical, or stalactitic aggregates of fine to coarse crystals. Rare individual crystals are short prismatic to tabular with square cross sections. Many of these have curved faces. Prehnite is often found lining cavities in volcanic rocks, associated with calcite (p.114) and zeolites (pp.185–90), and in mineral veins in granite (pp.258–59). Crystals up to several inches long come from Canada. Transparent White cabochon specimens from Australia Prehnite gems, such as this and Scotland are faceted for creamy white cabochon gem collectors. with dark inclusions, are almost too soft to wear.

206 MINERALS | SILICATES: PHYLLOSILICATES PROFILE Massive sepiolite This specimen of massive sepiolite shows a characteristic dull, earthy luster. Orthorhombic massive habit 4 2–2 1⁄2 dull luster m 2.1–2.3 n Good but rarely seen o Uneven p White q Dull to earthy VARIANT r Mg4Si6O15(OH)2.6H2O massive SEPIOLITE habit A compact, claylike, often porous mineral, sepiolite is best known by its popular name meerschaum, which is the German word for “sea-foam.” The name sepiolite comes from the mineral’s resemblance to the light and porous bone of cuttlefish from the genus Sepia. Sepiolite is usually white or gray and may be tinted yellow, brown, or green. It is Meerschaum A specimen of usually found in nodular masses massive white sepiolite of interlocking fibers, which give it a toughness contrary to its mineralogical softness. Sepiolite also occurs in porous aggregates. Sepiolite is an alteration product of minerals such as magnesite (p.118) and rocks, such as serpentinite (p.298). It is found Meerschaum cigar holder as irregular nodules in Turkey Sepiolite is used in carved and elsewhere, and in large tobacco pipes and cigar sedimentary deposits. holders, which develop a brown patina when smoked.

Purple-brown pigeonite MINERALS | SILICATES: INOSILICATES 207 This specimen of pigeonite comes from the Kovdor Pit, perfect cleavage Kola Peninsula, Russia. PROFILE cleavable mass Monoclinic r (Mg,Fe,Ca)2(Si2O6) 46 PIGEONITE m 3.2–3.5 n Good A member of the pyroxene group of minerals, o Uneven to conchoidal, pigeonite is named after Pigeon Point, Minnesota, USA— the locality where it was first identified. Specimens are brittle brown, purplish brown, or greenish brown to black in color. Pigeonite is generally found as disseminated p White to pale brown grains. Well-formed crystals are relatively rare. An iron-rich q Vitreous variety of pigeonite is sometimes called ferropigeonite. Pigeonite is an important mineral in lunar rocks and also occurs in meteorites (pp.335–37). It is found in lavas and smaller intrusive rock bodies as the dominant pyroxene and as an important component of dolerites (p.268) and andesites (p.275). The temperature limit of pigeonite formation indicates the crystallization temperature of the magma from which it has originated. Mare—the large, dark, flat areas of the Moon once believed to be seas—are in fact basalts (p.273) containing pigeonite. Notable localities on Earth include Skaergaard, Greenland; Mull, Scotland; Labrador, Canada; Mount Wellington, Tasmania; and Goose Creek, Virginia, and New Jersey, USA.

208 MINERALS | SILICATES: INOSILICATES PROFILE Prismatic crystals This mass of small, prismatic enstatite crystals rests on a rock matrix. Orthorhombic 4 5–6 m 3.1–3.9 n Good to perfect o Uneven p Gray to white q Vitreous small, prismatic crystals rock matrix VARIANTS r Mg2Si2O6 Single crystal A large crystal ENSTATITE from Telemark, Norway The pyroxene mineral enstatite takes its name from fibrous mass the Greek word enstates, which means “opponent”— Fibrous a reference to the use of this mineral as a refractory enstatite A fibrous “opponent” of heat in the linings of ovens and kilns. mass of parallel enstatite crystals Specimens are colorless, pale yellow, or pale green. They become darker and turn greenish brown to black with increasing iron content. Enstatite generally occurs as massive aggregates or disseminated grains. Well-formed crystals, when found, tend to be short prisms, often with complex terminations. Enstatite is also found as fibrous masses of parallel, needlelike crystals. A widespread mineral, enstatite forms a solid-solution series with ferrosilite. The mineral usually Mixed-cut enstatite occurs in magnesium- and iron- Recovered from Myanmar rich igneous rocks and in and Sri Lanka, facet-grade meteorites (pp.335–37). enstatite, such as this gem, is mainly cut for collectors.

PROFILE MINERALS | SILICATES: INOSILICATES 209 Monoclinic Terminated crystals This specimen is composed of 46 a group of prismatic aegirine m 3.5–3.6 crystals with feldspar. The crystals n Good to perfect have full terminations. o Uneven p Yellow-green to prismatic crystal pale green q Vitreous feldspar VARIANTS r NaFe(Si2O6) Parallel AEGIRINE crystals A mass of prismatic The sodium iron silicate aegirine was discovered aegirine crystals in Norway and named in 1835 after Aegir, the Scandinavian aligned in parallel god of the sea. Aegirine is also known as acmite after the Greek word acme, which means “point” or “edge”— Prismatic aegirine Prismatic a reference to the mineral’s typically pointed crystals. crystals of aegirine in a Specimens are dark green, reddish brown, or black in rock matrix color. Aegirine occurs as needlelike or fibrous crystals that form attractive, radiating sprays. The crystals have steep or blunt terminations and are often striated along the length. Prism faces are often lustrous and striated, while the faces of terminations are often etched and dull. A pyroxene, aegirine forms a solid-solution series with hedenbergite and diopside (p.210). It is found in magnesium- and iron-rich igneous rocks, especially syenitic pegmatites (p.260) and syenites (p.262). It is also found in schists (pp.291–92), metamorphosed iron-rich sediments, and metamorphic rocks altered by circulating fluids. Notable localities include Kongsberg, Norway, and Mont St.-Hilaire, Canada.

210 MINERALS | SILICATES: INOSILICATES PROFILE quartz Prismatic diopside This specimen of diopside Monoclinic in a rock matrix comes from St. Marcel, Valle d’Aosta, Italy. 46 m 3.3 prismatic n Distinct in two directions diopside crystal at almost right angles o Uneven p White to pale green q Vitreous rock matrix VARIANT r CaMg(Si2O6) Violane A blue, crystalline DIOPSIDE variant of diopside A member of the pyroxene family, diopside takes its name from the Greek for “double” and “appearance,” a reference to the variable appearance of the mineral. Specimens can be colorless but are more often bottle green, brownish green, or light green in color. Diopside occurs in the form of equant to prismatic crystals that are usually nearly square in section. Crystals are less commonly tabular. This mineral can also form columnar, sheetlike, granular, or massive aggregates. Most diopside is metamorphic and found in metamorphosed silica-rich limestones (p.319) and dolomites (p.320) and in iron-rich contact metamorphic rocks. It also Chrome diopside occurs in peridotites (p.266), Emerald-green diopside, kimberlites (p.269), and other such as the gem shown here, igneous rocks. is chromium-rich and is also known as chrome diopside.

MINERALS | SILICATES: INOSILICATES 211 PROFILE dark, nearly Single crystal opaque crystal In this specimen, a single, large, Monoclinic dark-colored augite crystal rests on a matrix of volcanic tuff. 4 5 1⁄2–6 m 3.3 good cleavage n Distinct in two directions in two directions nearly at right at almost right angles angles o Uneven to subconchoidal p Pale brown to greenish gray q Vitreous to dull volcanic tuff VARIANTS r (Ca,Na)(Mg,Fe,Ti,Al)(Al,Si)2O6 Greenish black augite AUGITE A mass of greenish black, prismatic augite crystals The most common pyroxene, augite is named after the Greek word augites, which means “brightness”—a Prismatic crystal A short reference to its occasional shiny appearance. Most augite prismatic augite crystal from has a dull, dark green, brown, or black finish. Augite occurs the Czech Republic chiefly as short, thick, prismatic crystals with a square or octagonal cross section and sometimes as large, cleavable masses. It occurs in a solid-solution series in which diopside (p.210) and hedenbergite are the end-members. Augite is common in silica-poor rocks and various other dark-colored igneous rocks, as well as igneous rocks of intermediate silica content. It also occurs in some metamorphic rocks formed at high temperatures (1,065°F/575°C or above). Augite is a common constituent of lunar basalts and some meteorites (pp.335–37). Notable crystal localities are in Germany, the Czech Republic, Italy, Russia, Japan, Mexico, Canada, and USA. Because it is difficult to distinguish between augite, diopside, and hedenbergite in hand specimens, all pyroxenes are often identified as augite.

212 MINERALS | SILICATES: INOSILICATES PROFILE vertical striation Monoclinic 4 6 1⁄2–7 m 3.0–3.2 n Perfect o Subconchoidal to splintery p White q Vitreous prismatic habit Common spodumene Most spodumene that is mined as an industrial source of lithium occurs as opaque crystals, such as the one shown here. VARIANTS r LiAl(Si2O6) subconchoidal SPODUMENE fracture A member of the pyroxene group, spodumene is Kunzite The lavender gemstone named after the Greek word spodumenos, which means form of spodumene “reduced to ashes”—a reference to the mineral’s elongate crystal common ash-gray color. It can also be pink, lilac, or Hiddenite Green, gem-variety specimen green. Crystals are prismatic, flattened, and typically of spodumene striated along their length. Gem varieties of the mineral usually exhibit strong pleochroism. Spodumene is an important ore of lithium. It occurs in lithium-bearing granite pegmatite dykes, often with other lithium- bearing minerals, such as eucryptite and lepidolite (p.198). One of the largest single crystals of any mineral ever found was a spodumene Strengthened glass specimen from South Dakota, Spodumene is a key source USA, 47 ft (14.3m) long and of lithium, which forms the 90 tons in weight. lithium fluoride that is used to add strength to glass.

MINERALS | SILICATES: INOSILICATES 213 PROFILE polished Weathered jadeite surface This massive jadeite specimen is Monoclinic colored lavender by impurities. It has been partly polished to show 4 6–7 its characteristic brown “rind.” m 3.2–3.4 n Good greasy luster o Splintery p White brown q Vitreous to rind greasy “orange peel” surface VARIANTS r Na(Al,Fe)Si2O6 Lilac jadeite A rare and JADEITE valuable variety of jadeite A pyroxene mineral, jadeite is one of the two minerals polished surface that are referred to as jade. The other is nephrite (p.217), Imperial jade A rich green which is a variety of either tremolite (p.219) or actinolite specimen of imperial jade (p.220). Pure jadeite is white in color. Specimens can be rough, violet colored green by iron, lilac by manganese and iron, or mass pink, purple, brown, red, blue, black, orange, or yellow Violet jadeite A specimen of by inclusions of other minerals. Jadeite rare, valuable, violet jadeite is made up of interlocking, blocky, granular crystals and commonly has a sugary or granular texture. Crystals are short prisms. They are rare but when found are usually in hollows within massive material. Jadeite occurs in metamorphic rocks formed at high pressure. Although usually recovered as Jade mask alluvial pebbles and boulders, it is Jadeite, such as that used also found in the rocks in which it in this 18th-century mask, originally formed. had cultural value for Central and South American Indians.

214 MINERALS | SILICATES: INOSILICATES PROFILE Crystalline wollastonite The mass of parallel crystals in this Triclinic specimen are shaped like coarse blades. They show silky luster and 4 4 1⁄2–5 a splintery fracture. m 2.9 n Perfect fibrous mass o Uneven to splintery of crystals p White q Vitreous to silky splintery fracture VARIANTS r CaSiO3 Massive wollastonite WOLLASTONITE A piece of massive wollastonite A valuable industrial mineral, wollastonite is white, gray, Coarse crytals A mass of coarsely crystalline wollastonite or pale green in color. It occurs as rare, tabular crystals or from New York massive, coarse-bladed, foliated, or fibrous masses. Its crystals are usually triclinic, although its structure has seven variants, one of which is monoclinic. These variations are however, indistinguishable in hand specimens. Wollastonite forms as a result of the contact metamorphism of limestones (p.319) and in igneous rocks that are contaminated by carbon-rich inclusions. It can be accompanied by other calcium- containing silicates, such as diopside (p.210), tremolite (p.219), epidote (p.230), and grossular garnet (p.245). Wollastonite also appears in regionally Ceramic tile metamorphosed rocks in schists Wollastonite is widely used (pp.291–92), slates (p.293), and in ceramics, such as the tile phyllites (p.294). shown here. It is also an ideal base for fluxes and glazes.

MINERALS | SILICATES: INOSILICATES 215 PROFILE vitreous Massive rhodonite luster This specimen of rough rhodonite Triclinic shows the intense coloring and fine texture of the best 46 gem-quality material. m 3.5–3.7 n Perfect o Conchoidal to uneven p White q Vitreous massive habit uneven fracture VARIANT r (Mn,Ca)5(Si5O15) manganese- tabular RHODONITE oxide matriz crystal The semiprecious gemstone rhodonite takes its name from the Greek word rhodon, which means “rose”— a reference to the mineral’s typical pink color. Crystals are uncommon but are rounded when found. Rhodonite usually occurs as masses or grains and is often coated Tabular crystals Aggregates or veined with manganese oxides. of tabular crystals Rhodonite is found in various manganese ores, often with rhodochrosite (p.121) or as a product of rhodochrosite that has undergone metamorphism. It has been used as a manganese ore in India but is more often mined as a gem and an ornamental stone. Rhodonite is primarily cut en cabochon as beads. Massive rhodonite is relatively tough and is good as a carving medium. Transparent rhodonite Rhodonite box is rare. Although extremely Black-veined rhodonite, fragile, it can be faceted such as that used here, for collectors. is relatively tough and is preferred by many carvers.

216 MINERALS | SILICATES: INOSILICATES Radiating crystals vitreous luster This specimen is a mass of fibrous, radiating crystals of anthophyllite with vitreous luster. mass of fibrous, radiating crystals PROFILE r (Mg,Fe)7Si8O22(OH)2 Orthorhombic ANTHOPHYLLITE 4 5 1⁄2–6 The name anthophyllite comes from the Latin m 2.8–3.6 word anthophyllum, which means “clove”—a reference n Perfect, imperfect to the mineral’s clove-brown to dark brown color. o Uneven Specimens can also be pale green, gray, or white. p Colorless to gray Anthophyllite is usually found in columnar to fibrous q Vitreous masses. Single crystals are uncommon; when found, they are prismatic and usually unterminated. The iron and magnesium content in anthophyllite is variable. The mineral is called ferroanthophyllite when it is iron-rich, sodium-anthophyllite when sodium is present, and magnesioanthophyllite when magnesium is dominant. Titanium and manganese may also be present in the anthophyllite structure. Anthophyllite forms by the regional metamorphism of iron- and magnesium-rich rocks, especially silica-poor igneous rocks. It is an important component of some gneisses (p.288) and crystalline schists (pp.291–92) and is found worldwide. Anthophyllite is one of several minerals referred to as asbestos.

MINERALS | SILICATES: INOSILICATES 217 Polished nephrite PROFILE This small boulder of nephrite has been sliced and polished to reveal its quality. splintery fracture Monoclinic 4 6 1⁄2 m 2.9–3.4 n Perfect o Splintery, brittle p White q Dull to waxy polished surface VARIANT r Ca2(Mg,Fe)5(Si8O22)(OH)2 Light green nephrite A mass NEPHRITE of light green nephrite Not a true mineral name, the term nephrite applies to the tough, compact form of either tremolite (p.219) or actinolite (p.220). Both are calcium magnesium silicate hydroxides and structurally identical, except that in actinolite some of the magnesium is replaced by iron. Nephrite is dark green when iron-rich and creamy white when magnesium-rich. Specimens are composed of a mat of tightly interlocked fibers, creating a stone that is tougher than steel. Nephrite forms in metamorphic environments, especially in metamorphosed iron- and magnesium-rich rocks, where it is associated with serpentine (p.191) and talc (p.193). It is also found in regionally metamorphosed Nephrite tiki areas where dolomites (p.320) are Hei tikis, such as this intruded by iron- and magnesium- one made of nephrite, rich igneous rocks. are worn by the Maori of New Zealand.

218 MINERALS | SILICATES: INOSILICATES PROFILE prismatic hornblende Monoclinic crystal 4 5–6 vitreous luster m 3.1–3.3 n Perfect o Uneven, brittle p White to gray q Vitreous vertical striation Hornblende crystals r eg: Ca2(Fe2,Mg)4(Al,Fe3)(Si7Al)O22(OH,F)2 This specimen consists of a group of prismatic hornblende crystals HORNBLENDE embedded in a rock matrix. The name hornblende is applied to a group of minerals VARIANTS that can be distinguished from each other only by detailed chemical analysis. The two end-member white rock hornblendes—iron-rich ferrohornblende and magnesium- matrix rich magnesiohornblende—are both calcium-rich and monoclinic in crystal structure. Other elements, such Prismatic as chromium, titanium, and nickel, can also appear in hornblende the crystal structures of the group. The concentrations Prismatic of these elements are an indicator of the metamorphic crystals in a grade of the mineral. rock matrix Specimens are green, dark green, or brownish green to Massive black in color. Hornblende crystals are usually bladed and hornblende unterminated, and they often show a pseudohexagonal A piece of cross section. Well-formed crystals are short to long massive prisms. Hornblende also occurs as cleavable masses hornblende and radiating groups. The mineral forms in metamorphic rocks, especially gneisses (p.288), hornblende schists, six-sided amphibolites (p.296), and magnesium- and iron-rich crystal igneous rocks. Single crystal A single, short prismatic crystal of hornblende

Tremolite crystals MINERALS | SILICATES: INOSILICATES 219 This specimen has plumelike aggregates of white, bladed vitreous to tremolite crystals. silky luster PROFILE Monoclinic featherlike aggregate 4 5–6 of crystals m 2.9–3.4 n Perfect r Ca2(Mg,Fe2)5Si8O22(OH)2 o Splintery, brittle p White TREMOLITE q Vitreous to silky A calcium magnesium silicate, tremolite forms a VARIANTS solid-solution series with ferroactinolite (p.220), where iron substitutes in increasing amounts for magnesium. silvery, The color of tremolite varies with increasing iron content radiating from colorless to white in pure tremolite to gray, crystals gray-green, green, dark green and nearly black in other specimens. Traces of manganese may tint tremolite pink Radiating tremolite Radiating or violet. When well-formed, crystals are short to long crystals of silvery tremolite prisms. More commonly, tremolite forms unterminated bladed crystals, parallel aggregates of bladed crystals, or Asbestos fibers Tremolite radiating groups. Tremolite and actinolite both form thin, in fibrous form parallel, flexible fibers up to 10 in (25 cm) in length, which are used commercially as asbestos. Tremolite is known as nephrite jade when it is massive and fine-grained. The mineral is abundant and widespread. It is the product of both thermal and regional metamorphism and is an indicator of metamorphic grade because it converts to diopside (p.210) at high temperatures (1,065°F/575°C or above).

220 MINERALS | SILICATES: INOSILICATES PROFILE talc schist Monoclinic thin, prismatic crystal 4 5–6 m 2.9–3.4 n Good, crossing at 56° and 124° o Splintery, brittle p White q Vitreous or silky green, bladed r Ca2(Mg,Fe2+)5Si8O22(OH)2 crystal ACTINOLITE Actinolite crystals This specimen contains a group Actinolite is an abundant mineral. It is in the middle of thin, prismatic crystals of of a solid-solution series of calcium, iron, and magnesium actinolite in a talc matrix. silicates that also includes ferroactinolite and tremolite (p.219). There is complete substitution in the series VARIANT between iron and magnesium, but all have the same structure. Actinolite was named in 1794 after the Greek Gray-green actinolite word aktis, which means “ray”—an allusion to its radiating, Crystals of actinolite, some of prismatic habit. Specimens range from green to dark green which have been powdered to black. Well-formed crystals are short to long prisms. Actinolite usually occurs as unterminated bladed crystals, parallel aggregates of bladed crystals, or radiating groups. It is sometimes found as needlelike or fibrous crystals up to 10 in (25 cm) long. When in this form, it is one of the minerals that are called asbestos. Massive, fine-grained actinolite and tremolite are both called nephrite jade. Actinolite is an amphibole mineral and forms as a product of low- to medium-grade thermal and regional metamorphism. Good crystals come from Edwards, New York, USA, and Kantiwa, Afghanistan.

MINERALS | SILICATES: INOSILICATES 221 PROFILE dark blue-green glaucophane Monoclinic fuchsite 46 m 3.2 n Distinct o Uneven to conchoidal p Gray-blue q Vitreous to pearly pyrite VARIANT Italian glaucophane This specimen from Polloni in prismatic crystal Piedmont, Italy, shows glaucophane Glaucophane crystals with fuchsite and pyrite. Crystals of glaucophane in a rock matrix r Na2(Mg3Al2)Si8O22(OH)2 GLAUCOPHANE The mineral is named after two Greek words: glaukos, which means “bluish green”; and phainesthai, which means “to appear.” Specimens can be gray, lavender blue, or bluish black. Crystals are slender, often lathlike prisms, with lengthwise striations. Twinning is common. Glaucophane can also be massive, fibrous, or granular. When iron replaces the magnesium in its structure, it is known as ferroglaucophane. Glaucophane occurs in schists (pp.291–92) formed by high-pressure metamorphism of sodium-rich sediments at low temperatures (up to 400°F/200°C) or by the introduction of sodium into the process. Glaucophane is often accompanied by jadeite (p.213), epidote (p.230), almandine (p.243), and chlorite. It is one of the minerals that are referred to as asbestos. Glaucophane and its associated minerals are known as the glaucophane metamorphic facies. The presence of these minerals indicates the range of temperatures and pressures under which metamorphism occurs.

222 MINERALS | SILICATES: INOSILICATES PROFILE group of prismatic Riebeckite crystals crystals The long, striated crystals Monoclinic characteristic of riebeckite are clearly visible in this specimen. 46 m 3.3–3.4 n Perfect o Uneven p Blue-gray q Vitreous, silky deep grayish blue color vertical, parallel striations VARIANT r Na2(Fe2+3Fe3+2)Si8O22(OH)2 blue RIEBECKITE asbestos A sodium iron silicate, riebeckite is one of the several Crocidolite Fibers of blue riebeckite, which constitute minerals called asbestos. It was named after Emil Riebeck, blue asbestos a 19th-century German explorer. Although riebeckite specimens are generally grayish blue to dark blue, their color can vary depending on the concentration of iron in their structure. Riebeckite can occur as prismatic, striated crystals or sometimes as massive or fibrous aggregates. This mineral occurs in feldspar- and quartz-rich igneous rocks. These include granites (pp.258–59), syenites (p.262) and, feldspar- and quartz- rich volcanic rocks, especially sodium-rich rhyolites (p.278). Riebeckite granite is found on the island of Ailsa Craig in western Scotland and is locally Tiger’s eye ring known as ailsite. Ailsite is used to Crocidolite, a variant manufacture stones used in the of riebeckite, forms the sport of curling. gemstone tiger’s eye when it is silica-saturated.

PROFILE MINERALS | SILICATES: CYCLOSILICATES 223 Orthorhombic Cordierite crystals This group of short prismatic, 4 7–7 1⁄2 dark gray cordierite crystals m 2.6 occurs in a rock matrix. n Moderate to poor o Conchoidal to uneven p White q Vitreous to greasy rock matrix cordierite crystal VARIANTS r (Mg,Fe)2Al4Si5O18 Single crystal A large crystal CORDIERITE of cordierite in matrix The mineral is named after the French geologist Pierre Iolite A polished gemstone of cordierite called iolite L.A. Cordier, who first described it in 1813. Specimens can be blue, violet-blue, gray, or blue-green. Gem-quality blue cordierite or iolite is also known as water sapphire because of its color. Cordierite is pleochroic, exhibiting three different colors when viewed from different angles. Its crystals are prismatic, and the best blue color is seen along their length. Cordierite occurs in high-grade, thermally metamorphosed, alumina-rich rocks. It is also found in gneisses (p.288) and schists (pp.291–92) and more rarely in granites (pp.258–59), pegmatites (p.260), and veins of quartz (p.168). Cordierite Cordierite jewelry is important in the production A variety of cordierite, of ceramics used in catalytic iolite is used in ornaments converters in cars. because of its color and brilliance.

224 MINERALS | SILICATES: CYCLOSILICATES PROFILE green or red Watermelon tourmaline crystal rim Color can vary either along or Hexagonal or trigonal across a tourmaline crsytal. This zoning takes its most dramatic 4 7–7 1⁄2 form in “watermelon” tourmaline. m 3.0–3.2 n Indistinct crystal sliced o Uneven to across its width conchoidal p Colorless q Vitreous red or pink center VARIANTS r FeWO4 Schorl Probably the most TOURMALINE common tourmaline mineral Tourmaline is the name given to a family of minerals Elbaite A gemstone-quality variant of tourmaline of complex and variable composition, but all members Indicolite A blue-colored have the same basic crystal structure. The 11 minerals in variant of tourmaline the group include elbaite, schorl, dravite, and liddicoacite. Gemstone varieties based on their color are also recognized, including indicolite (blue), rubellite (pink or red), verdelite (green), and achroite (colorless). These variety names can be applied to more than one mineral. Most tourmaline is dark, opaque, and not particularly attractive, but many of its transparent varieties are valued as gems. Tourmaline is abundant, and its best formed crystals are usually found in pegmatites (p.260) and metamorphosed limestones (p.319) in contact with granitic magmas. It accumulates in Cut rubellite gravel deposits and occurs as This specimen shows the an accessory mineral in some rich red coloration and sedimentary rocks. transparency found in some specimens of rubellite.

PROFILE MINERALS | SILICATES: CYCLOSILICATES 225 Hexagonal or trigonal Aquamarine This mass of prismatic aquamarine 4 7 1⁄2–8 crystals is from the Karakoram m 2.6–2.8 Range in Pakistan. The name n Indistinct aquamarine means “seawater.” o Uneven to vitreous transparent conchoidal luster sky blue p White q Vitreous iron-stained coating r MnWO4 VARIANTS BERYL Heliodor Crystalline heliodor Few people have ever heard of the mineral beryl with hexagonal prisms but almost everyone has heard of its principal gemstone varieties—emerald and aquamarine. Before 1925, beryl’s Emerald solitary use was as a gemstone but since then many An unusually important uses have been found for beryllium. As a result, long prismatic common beryl, which is usually pale green or white, has crystal of emerald become widely sought after as the ore of this rare element. Most beryl is found in granites (pp.258–59), granite Morganite A variant with pegmatites (p.260), and rhyolites (p.278), but it can also crystals in shades of pink occur in metamorphic rocks, such as schists (pp.291–92). Emerald owes its grass-green color to the presence of traces of chromium and sometimes vanadium. Flawless emeralds are rare, but since 1937 the manufacture of synthetic crystals has become possible. Aquamarine is the most common gemstone variety of beryl. Nearly always found in cavities in pegmatites or in alluvial deposits, it forms larger and clearer crystals than emerald. Other gemstone varieties of beryl include heliodor, morganite, and goshenite.

226 MINERALS | SILICATES: CYCLOSILICATES PROFILE short, prismatic dioptase crystal Hexagonal vitreous luster 45 m 3.3 n Perfect o Uneven to conchoidal p Pale greenish blue q Vitreous to greasy chrysocolla matrix Prismatic dioptase r CuSiO2(OH2) This spectacular encrustation of dioptase crystals on quartz shows DIOPTASE why it is a favorite with collectors. The bright green crystals of dioptase can superficially VARIANTS resemble emerald. Dioptase crystals mined from a rich deposit in Kazakhstan were wrongly identified as emerald Clustered prisms A green when they were sent to Czar Paul of Russia in 1797. Were dioptase specimen it not for its softness and good cleavage, dioptase would make a superb gemstone to rival emerald (p.225) in Lustrous dioptase color. Its prismatic crystals, often with rhombohedral A specimen found in terminations, can be highly transparent. This explains Central Africa why the name dioptase is derived from two Greek words: dia, which means “through,” and optazein, which means “visible” or “to see.” Transparent specimens of dioptase appear in different colors depending on the direction from which they are seen, and intensely colored specimens can be translucent. The mineral can also occur in granular or massive habits. Dioptase forms in areas where copper veins have been altered by oxidation. Its vibrant color and its typical occurence as well-formed crystals make it popular with mineral collectors.

MINERALS | SILICATES: SOROSILICATES 227 PROFILE Orthorhombic 4 4 1⁄2–5 m 3.4–3.5 n Perfect, good, poor o Uneven, brittle p White q Vitreous botryoidal aggregate of tiny crystals Botryoidal hemimorphite This superb specimen of sky-blue hemimorphite has a botryoidal habit. VARIANTS r Zn4Si2O7(OH)2·H2O crystal HEMIMORPHITE cluster One of two minerals formerly called calamine in the White hemimorphite Tabular USA, hemimorphite takes its name from the Greek words crystals on a rock matrix hemi, which means “half,” and morphe, which means “form”—a reference to its crystalline form. Hemimorphite rounded crystals are double-terminated prisms with a differently mass shaped termination at each end—pointed at one and flat at the other. Crystals are often grouped in fan-shaped Crystalline clusters. Hemimorphite can also be botryoidal, chalky, aggregates massive, granular, fibrous, or form encrustations. Usually A green colorless or white, specimens can also be pale yellow, specimen pale green, or sky blue. Some specimens show strong, on a rock green fluorescence in shortwave ultraviolet light and weak, matrix light pink fluorescence in longwave ultraviolet light. Hemimorphite is a secondary mineral formed in the alteration zones of zinc deposits, especially as an alteration product of sphalerite (p.53). It can be half zinc by weight and is an important ore of that metal. Well-crystallized specimens come from Algeria, Namibia, Germany, Mexico, Spain, USA, and China.

228 MINERALS | SILICATES: SOROSILICATES PROFILE Axinite crystals This mass of well-formed, Triclinic transparent, wedge-shaped, tabular axinite crystals rests 4 6 1⁄2–7 on a rock matrix. m 3.2–3.3 n Good, poor vitreous o Uneven to luster conchoidal, brittle p Colorless to light brown q Vitreous characteristic clove-brown color VARIANTS r Ca2FeAl2(BSi4O15)(OH) wedge-shaped AXINITE axinite crystal This group of minerals takes its name from the Gem quality Wedge-shaped crystals of brown axinite axehead shape of its crystals. Axinite minerals also occur distinctive axe shape as rosettes and in massive and granular forms. The most Unusual growth A small familiar color of axinite is clove brown. Varieties can also crystal growing on a larger one be gray to bluish gray; honey-, gray-, or golden-brown; violet-blue, pink, yellow, orange, or red. There are four minerals in the group: ferroaxinite, the most common; magnesioaxinite, in which magnesium replaces the iron in ferroaxinite; manganaxinite, in which manganese replaces the iron in ferroaxinite; and tinzenite, which is intermediate in composition between ferroaxinite and manganaxinite. Axinite is commonly found in contact and low-temperature metamorphic rocks (those formed Axinite gemstone at up to 400°F/200°C) and Brilliant-cut axinite crystals, in magnesium- and iron-rich such as this specimen in an igneous rocks. unusual shade of violet, are popular with collectors.

MINERALS | SILICATES: SOROSILICATES 229 PROFILE Tetragonal Monoclinic 4 6 1⁄2 m 3.4 n Poor o Subconchoidal to uneven, brittle p White to pale greenish brown q Vitreous to resinous tetragonal crystal vertical striation Striated vesuvianite r Ca10(Mg,Fe)2Al4(Si04)5(Si207)2(OH,F)4 This superb specimen consists of prismatic, vertically striated VESUVIANITE vesuvianite crystals. Formerly called idocrase, vesuvianite is named after its VARIANTS place of discovery—Mount Vesuvius in Italy. Usually green Cyprine A specimen of blue vesuvianite, or cyprine or yellow-green, it can also be yellow to brown, red, black, Tetragonal crystal blue, or purple. A greenish blue copper-bearing vesuvianite A single, well-formed crystal of vesuvianite is called cyprine. An unusual bismuth-bearing vesuvianite from Langben, Sweden, is bright red. The mineral forms pyramidal or prismatic and glassy crystals. Crystals 23⁄4 in (7 cm) or more long have been found. Elements such as tin, lead, manganese, chromium, zinc, and sulfur may substitute in the vesuvianite structure. The mineral is formed by the metamorphism of impure limestones (p.319). It is also found in granulites (p.297) and marbles (p.301) accompanied Vesuvianite gem by calcite (p.114), diopside (p.210), Occasionally, vesuvianite wollastonite (p.214), and is found in translucent to grossular (p.245). transparent crystals suitable for cutting into gems.

230 MINERALS | SILICATES: SOROSILICATES PROFILE vitreous luster Epidote crystals This superb group of striated Monoclinic epidote crystals, some reaching 1 in (2.5 cm) in length, shows 4 6–7 typical prismatic development. m 3.4 n Good prismatic crystal o Uneven to splintery p Colorless or grayish q Vitreous striations perfect cleavage VARIANTS r Ca2Al2(Fe,Al)(SiO4)(Si2O7)O(OH) Pistachio epidote Long, EPIDOTE striated crystals from Peru An abundant rock-forming mineral, epidote derives Acicular epidote Yellowish brown, needlelike crystals its name from the Greek word epidosis, which means “increase”—a reference to the fact that one side of the prism is always wider than the others. Epidote is most easily recognized by its characteristic color—light to dark pistachio green. Gray or yellow specimens are also found. Epidote is pleochroic, exhibiting different colors when viewed from different directions. The mineral frequently forms well-developed crystals. These may be columnar prisms or thick, tabular crystals with faces that are finely striated parallel to the crystal’s length. Twinning is common. Specimens can also be needlelike, massive, or granular. Epidote is found in low-grade, regionally metamorphosed rocks. Epidote gemstone It also occurs as a product of Clear, yellowish green to dark the alteration of plagioclase brown epidote gems are feldspars (pp.173–81). rare. Transparent crystals are cut for collectors.

MINERALS | SILICATES: SOROSILICATES 231 PROFILE Orthorhombic 4 6–7 m 3.2–3.4 n Perfect o Conchoidal to uneven, brittle p White q Vitreous perfect cleavage deep vertical Zoisite crystals striation This specimen of ordinary zoisite shows a typical prismatic shape VARIANTS and vertical striations. The crystals are in a pegmatite matrix. Thulite A pink, manganese- rich variety of zoisite r Ca2Al3(SiO4)3(OH) Tanzanite A rich purple ZOISITE gem-quality zoisite variety A calcium aluminum silicate hydroxide, most zoisite is gray, white, light brown, yellowish green, or pale greenish gray. A massive, pinkish red variety is called thulite. A lilac- blue to sapphire-blue variety of zoisite is called tanzanite and is sometimes mistaken for sapphire (p.95). Zoisite with inclusions of ruby is called ruby-in-zoisite. Zoisite crystals exhibit gray, purple, or blue colors depending on the angle from which they are viewed. Zoisite is found as deeply striated, prismatic crystals and also as disseminated grains and columnar or massive aggregates. The mineral is characteristic of regional metamorphism and hydrothermal alteration of igneous rocks. It results from metamorphism of calcium- rich rocks and typically occurs in medium-grade schists (pp.291– 92), gneisses (p.288), and Ruby-in-zoisite amphibolites (p.296). It is also Considered a good carving found in veins of quartz (p.168) medium, ruby-in-zoisite has and pegmatites (p.260). been used to make this 19th-century desk seal.

232 MINERALS | SILICATES: NESOSILICATES PROFILE rounded, transparent Orthorhombic olivine crystal 4 6 1⁄2–7 m 3.3–4.3 n Imperfect o Conchoidal p White q Vitreous secondary clay minerals Peridot crystal This gem-quality specimen of olivine, or peridot, is from Pakistan. Other important sources include China and Myanmar. VARIANTS r (Mg,Fe)2SiO4 Forsterite Magnesium-rich OLIVINE olivine is called forsterite tabular The name olivine may be unfamiliar but most people crystal know of its gemstone variety, peridot, which has been Fayalite Iron-rich olivine is mined for over 3,500 years. The name olivine is applied called fayalite to any mineral belonging to a solid-solution series in which iron and magnesium substitute freely in the structure. Fayalite is the iron end-member of the solid-solution series, and forsterite is the magnesium end-member. Olivine specimens are usually yellowish green, but they can also be yellow, brown, gray, or colorless. Crystals are tabular, often with wedge-shaped terminations, although well- formed crystals of olivine are rare. Olivine may also occur in Peridot gemstone massive or granular habits. It is Green peridot, such as a major component of Earth‘s the one in this brooch, was upper mantle. used by Egyptians since the second millennium BCE.

PROFILE MINERALS | SILICATES: NESOSILICATES 233 twinned zircon crystal Tetragonal 4 7 1⁄2 m 4.6–4.7 n Imperfect o Uneven to conchoidal p White q Adamantine to oily feldspar-and-biotite matrix biotite Afghan zircon r ZrSiO4 This specimen of zircon crystals in a feldspar-and-biotite matrix ZIRCON is from Afghanistan. The crystals are up to 11⁄4 in (3 cm) long. A superb gem and one of the few stones to approach VARIANTS diamond (p.47) in fire and brilliance, zircon has a high Purple zircon Crystals of refractive index and color dispersion. Known since zircon in a rock matrix antiquity, zircon takes its name from the Arabic word Crystalline cluster Zircon crystals that are embedded zargun, derived in turn from the Persian words zar, which in pegmatite means ”gold,“ and gun, which means ”color.“ Specimens can also be colorless, yellow, gray, green, brown, blue, and red. Brown zircon is frequently heat-treated to turn it blue. The mineral forms prismatic to dipyramidal crystals. Single crystals can reach a considerable size: specimens weighing up to 41⁄2 lb (2kg) and 83⁄4 lb (4kg) have been found in Australia and Russia, respectively. Zircon is found in metamorphic rocks and silica-rich igneous rocks. It resists weathering and, because of its relatively high Zircon bracelet specific gravity, concentrates Gem zircons, such as the in stream and river gravels and colorless, faceted zircons beach deposits. in this bracelet, have been mined for over 2,000 years.

234 MINERALS | SILICATES: NESOSILICATES PROFILE prismatic crystal Orthorhombic 48 m 3.4–3.6 n Perfect basal o Subconchoidal to uneven p Colorless q Vitreous albite termination face Topaz crystal This prismatic, pinkish brown topaz from Afghanistan is over 31⁄4 in (8cm) tall and weighs more than 1 lb (0.5 kg). VARIANTS r Al2SiO4(F,OH)2 vitreous TOPAZ luster The name topaz is thought to have been derived from Brown topaz A fine, natural the Sanskrit tapaz, which means “fire.” Topaz occurs in crystal of brown topaz a wide range of colors, with sherry-yellow and pink line of cleavage stones being particularly valuable. Colorless topaz has Light blue such a high refractive index that brilliant-cut specimens topaz A specimen of are mistaken for diamond (p.47). Well-formed prismatic blue topaz crystals have a characteristic lozenge-shaped cross Imperial topaz A golden imperial section and striations parallel to their length. The mineral topaz from a deposit in Brazil is also found in massive, granular, and columnar habits. Topaz is formed by fluorine-bearing vapors released in the late stages of crystallization of igneous rocks. It occurs in granites (pp.258–59), rhyolites (p.278), pegmatite dykes, and hydrothermal veins. Rounded pebbles are also found in river Pink topaz deposits. The world’s largest A clear, octagonal step faceted topaz weighs over cut, pink topaz is set here 36,000 carats. in a gold ring. Natural pink topaz is rare.

Titanite crystals MINERALS | SILICATES: NESOSILICATES 235 These interpenetrating, wedge-shaped titanite wedge-shaped crystals, 1 1⁄2 in (3.5 cm) crystal long, are from Russia. rock groundmass vitreous luster PROFILE Monoclinic r CaTiSiO5 4 5–5 1⁄2 TITANITE m 3.5–3.6 n Imperfect Formerly called sphene, titanite is a calcium titanium o Conchoidal p White silicate. The name sphene originates from the Greek word q Vitreous to greasy sphen, which means ”wedge“—a reference to the typical VARIANT wedge-shaped crystals of the mineral. Crystals can also Crystal group Wedge-shaped titanite crystals be prismatic. Gem-quality crystals occur in yellow, green, or brown colors. Specimens can also be black, pink, red, blue, or colorless. Titanite is strongly pleochroic, exhibiting different colors when seen from different directions. The mineral can also be massive, lamellar, or compact. Faceted titanite is one of the few stones with a color dispersion higher than that of diamond (p.47). Titanite is widely distributed as a minor component of silica-rich igneous rocks and associated pegmatites (p.260). It is also found Titanite ring in the metamorphic rocks gneiss Faceted titanites, such as (p.288), schist (pp.291–92), and the brilliant cut set in this marble (p.301). gold ring, have superb fire and intense colors.

236 MINERALS | SILICATES: NESOSILICATES PROFILE Andalusite crystals This group of prismatic andalusite Orthorhombic crystals from the Austrian Tyrol is in a matrix of quartz. 4 6 1⁄2–7 1⁄2 m 3.2 quartz matrix n Good to perfect, poor o Conchoidal prismatic andalusite p White crystal q Vitreous VARIANTS r Al2OSiO5 Brown andalusite Prismatic ANDALUSITE crystals on a rock matrix Named after the locality in Andalusia, Spain, where it Chiastolite A yellowish brown andalusite crystal with cross- was first described, andalusite is aluminum silicate. It is shaped inclusions of carbon pink to reddish brown, white, gray, violet, yellow, green, or blue. Gem-quality andalusite exhibits yellow, green, and red colors when viewed from different directions. Andalusite crystals are commonly prismatic with a square cross section. They can also be elongated or tapered. Andalusite can also occur in massive form. A yellowish gray variety called chiastolite occurs as long prisms enclosing symmetrical wedges of carbon-rich material. Andalusite is found in regional and low-grade metamorphic rocks, where it is associated with corundum (p.95), cordierite (p.223), sillimanite (p.237), and Rectangular step cut kyanite (p.238). It is rarely found in Relatively uncommon, granites (pp.258–59) and granitic transparent andalusite is pegmatites (p.260) . too brittle to be worn. It is faceted for gem collectors.

PROFILE MINERALS | SILICATES: NESOSILICATES 237 Orthorhombic vitreous luster 47 m 3.2–3.3 n Perfect o Uneven p White q Silky elongated, prismatic sillimanite crystal rock matrix Prismatic sillimanite In this specimen, elongated, prismatic crystals of sillimanite can be seen in a rock matrix. VARIANTS r Al2OSiO5 Crystals in rock Sillimanite in SILLIMANITE a rock matrix Named after the American chemist Benjamin Silliman, silky luster sillimanite is one of three polymorphs of aluminum Fibrous sillimanite A mass of parallel, fibrous crystals silicate. Commonly colorless to white, sillimanite can also be pale yellow to brown, pale blue, green, or violet. A single specimen may appear yellowish green, dark green, or blue when seen from different angles. The mineral occurs in long, slender, glassy crystals or in blocky, poorly terminated prisms. Sillimanite is characteristic of clay-rich metamorphic rocks formed at high temperatures (1,065°F/575°C or above). The mineral is often found with corundum (p.95), cordierite (p.223), and kyanite (p.238). Specimens also occur in Collectors’ gem gneisses (p.288), sillimanite Facet-grade sillimanite, such schists, hornfels (p.303), and as this specimen, occurs in detrital sediments. the gem gravels of Sri Lanka and Myanmar, and in Brazil.

238 MINERALS | SILICATES: NESOSILICATES PROFILE Blady kyanite Triclinic This specimen of kyanite with quartz from northern Brazil shows 4 4 1⁄2; 6 the characteristic elongated, m 3.6 bladed habit of kyanite crystals. n Perfect o Splintery vitreous luster p Colorless q Vitreous long, bladed crystal rock matrix quartz VARIANT r Al2SiO5 rock matrix KYANITE Named after the Greek word kyanos, which means “dark blue,” kyanite is blue and blue-gray, the colors generally zoned within a single crystal. Kyanite can also be green, orange, or colorless. Specimens have variable hardness: about 41⁄2 when scratched parallel to the long axis but 6 when scratched perpendicular to the long axis. Kyanite occurs mainly as elongated, flattened blades that kyanite are often bent and sometimes as staurolite radiating, columnar aggregates. Kyanite in rock Kyanite Kyanite is formed during the crystals with staurolite in a rock matrix regional metamorphism of clay-rich sediments. It occurs in mica schists, gneisses (p.288), and associated hydrothermal quartz veins and pegmatites (p.260). It is used to Spark plugs estimate the temperature, depth, Kyanite is mined for and pressure at which a rock the aluminum silicate has metamorphosed. mullite, which is used in spark plugs.

MINERALS | SILICATES: NESOSILICATES 239 PROFILE prismatic twinned staurolite crystal staurolite Monoclinic crystals 4 7–7 1⁄2 m 3.7 n Distinct o Conchoidal p Colorless to gray q Vitreous to resinous vitreous luster Staurolite crystals mica schist This is a specimen of staurolite matrix in a mica schist matrix. Single and twinned crystals can be seen here. VARIANTS r (Fe,Mg)4Al17(Si,Al)8O45(OH)3 kyanite STAUROLITE A widespread mineral, staurolite takes its name from two Greek words: stauros, which means “cross,” and lithos, which means “stone”—a reference to its typical crosslike twinned form. Cross-shaped penetration twins of the mineral are common and are in great demand as staurolite charms. Staurolite specimens are yellowish brown, Staurolite in schist Kyanite reddish brown, or nearly black in color. The mineral and staurolite in schist normally occurs as prismatic crystals, which are either hexagonal or diamond-shaped in section and often have rough surfaces. Staurolite occurs in mica schists, gneisses (p.288), and other metamorphosed, aluminum- rich rocks. It forms only under a specific range of pressure Fairy cross A twinned staurolite, and temperature, which helps Trapeze-cut staurolite or “fairy cross,” crystal determine the various conditions Transparent staurolite, as in under which the metamorphic this stone, is a rare faceting rock formed. material because of its dark color and lack of brilliance.

240 MINERALS | SILICATES: NESOSILICATES PROFILE prismatic euclase crystal Monoclinic 4 7 1⁄2 m 3.0 n Perfect o Conchoidal, brittle p White q Vitreous rock matrix striated crystal Blue euclase r BeAlSiO4(OH) This mass of well-developed, prismatic crystals of blue euclase EUCLASE is on a rocky matrix. Euclase takes its name from two Greek words: eu, VARIANT which means “good,” and klasis, which means “fracture”— Transparent euclase A near-transparent, striated a reference to its perfect cleavage. Generally white or euclase crystal colorless, euclase can also be pale green or pale to deep blue—a color for which it is particularly noted. It forms striated prisms, often with complex terminations. Massive and fibrous specimens are also found. Euclase occurs in hydrothermal veins formed at low temperatures (up to 400°F/200°C), granitic pegmatites (p.260), and some metamorphic schists (pp.291–92) and phyllites (p.294). It is also found in stream gravels. Exquisite, colorless, and deep blue color- zoned crystals come from Karoi in Euclase gemstone Zimbabwe. Cut euclase resembles This square-cut euclase certain types of beryl (p.225) gemstone shows small, and topaz (p.234). dark inclusions of another mineral.

MINERALS | SILICATES: NESOSILICATES 241 PROFILE yellowish brown Humite crust humite crystal In this specimen, a crust of Orthorhombic yellowish brown humite crystals covers a rock matrix that 4 6–6 1⁄2 also contains accessory mica. m 3.2–3.3 n Poor o Subconchoidal to uneven p Yellow to orange q Vitreous rock matrix VARIANTS r (Mg,Fe)7(SiO4)3(F,OH)2 Yellow humite A specimen of HUMITE massive, yellow humite Named in 1813 after the English mineral and art Orange humite Massive, collector Sir Abraham Hume (1749–1838), humite is a orange humite with a brown silicate of iron and magnesium. Manganese substitutes weathering crust for iron in the structure to form a complete solid-solution series with manganhumite. Specimens are yellow to dark orange or reddish orange in color, tending toward brown with increasing manganese content. Humite is generally found in granular masses. Well- formed crystals are rare and grow in parallel with one another. Crystals rarely exceed 3⁄8 in (1cm) in length and are occasionally twinned. Humite is found with pyrite (p.62), cassiterite (p.79), hematite (p.91), quartz (p.168), tourmaline (p.224), and mica in contact and regionally metamorphosed limestones (p.319) and dolomites (p.320). Although this mineral occurs worldwide, noteworthy locations include Persberg and elsewhere in Sweden; Isle of Skye, Scotland; Mount Vesuvius, Italy; Valais, Switzerland; and Brewster, New York, USA.

242 MINERALS | SILICATES: NESOSILICATES pyrope crystal PROFILE conchoidal fracture Cubic 4 7–7 1⁄2 m 3.6 n None o Conchoidal, brittle p White q Vitreous rock matrix Pyrope in matrix This specimen from Mexico includes several pyrope garnets in a matrix. Most pyrope is found as pebbles in placer deposits with other gems. VARIANT r Mg3Al2(SiO4)3 Gemstone rough Water- PYROPE rounded pyrope recovered from a placer deposit The magnesium aluminum garnet pyrope was named in 1803 after the Greek words pyr and õps, which mean “fire” and “eye” respectively—a reference to the typical fiery color of specimens. Manganese, chromium, iron, and titanium substitute in the mineral’s structure and act as coloring agents to some degree. Specimens can be rich red, dark red, violet-red, rose red, or reddish orange depending on the composition. Crystals are dodecahedral and trapezohedral, with hexoctahedra sometimes present. The mineral is most often found as rounded grains or pebbles. Pyrope occurs as a high- pressure mineral in metamorphic rocks. It is also found in high- Pyrope gemstones pressure, silica-poor igneous Beautiful garnet jewelry rocks and in detrital deposits comes from Bohemia, Czech derived from them. Republic, where pyropes as big as hens’ eggs are found.

MINERALS | SILICATES: NESOSILICATES 243 PROFILE modified Cubic dodecahedron 4 7–7 1⁄2 m 4.3 n None o Subconchoidal, brittle p White q Vitreous schist well-formed crystal Almandine crystal This almandine in schist from an area near Wrangell, Alaska, USA, shows a modified dodecahedral form. VARIANTS r Fe3Al2(SiO4)3 schist ALMANDINE almandine The most common garnet, almandine is named after crystal Crystals in schist Numerous Alabanda (now Araphisar) in Turkey, where it has been almandine dodecahedrons rest on schist cut since antiquity. Almandine is always red, often with almandine crystal a pink or violet tinge. Specimens can sometimes be nearly Almandine in granulite black. This mineral tends to be a pinker red than other Crystals of almandine in a granulite matrix garnets. Crystals often have well-developed faces and are dodecahedral or trapezohedral or have other more complex forms. Massive aggregates and rounded grains are also found. Rutile (p.78) needles can show as a four-rayed star when almandine is cut en cabochon. Almandine occurs worldwide. It is found in gneisses (p.288) and mica schists, igneous rocks (pp.256–57), and occasionally as inclusions in diamond (p.47). When Faceted almandine it occurs in metamorphic rocks, Three faceted almandine its presence indicates the grade gems and a seed pearl of metamorphism. create a central flower motif in this antique gold brooch.

244 MINERALS | SILICATES: NESOSILICATES Spessartine crystals PROFILE In this specimen from Norway, well-formed dodecahedral crystals Cubic encrust a rock matrix. 4 7–7 1⁄2 rock matrix m 4.2 n None uneven fracture o Conchoidal, brittle p White q Vitreous dodecahedral spessartine crystal VARIANT r Mn3Al2(SiO4)3 Translucent spessartine SPESSARTINE An attractive crystal of translucent spessartine The manganese aluminum silicate spessartine is named after Spessart—the locality in Germany where it was first described. The mineral is pale yellow when nearly pure and orange to deep red, brown, or black in other specimens. A color change known as the alexandrite effect is occasionally found in spessartine grossular garnet. Crystals are dodecahedral or trapezohedral. Spessartine may also occur as either granular or massive aggregates. Spessartine almost always contains some amount of iron (p.39). Pure spessartine is relatively rare and is found in manganese-rich metamorphic rocks, granites (pp.258–59), and pegmatite veins (p.260). Octagonal step cut The heaviest spessartine Because of spessartine’s rich ever discovered weighs color, the liquid inclusions 6,720 carats. under the edge facets in this gem are not very noticeable.

PROFILE MINERALS | SILICATES: NESOSILICATES 245 Cubic diopside 4 6 1⁄2–7 m 3.6 n None o Conchoidal p White q Vitreous grossular crystal Grossular on diopside r Ca3Al2(SiO4)3 These grossular crystals from Piedmont, Italy, are set on GROSSULAR a matrix of diopside. The calcium aluminum silicate grossular, a type of VARIANTS garnet, is named after the Latin word grossularia, which Hessonite Reddish brown dodecahedral grossular crystals translates into “gooseberry”—a reference to the mineral’s impure marble gooseberry-green color. Specimens can also be pale to Pink grossular Rounded, pink emerald green, white, colorless, cream, orange, red, grossular crystals in marble honey, brown, or black. Reddish brown or pink grossular is called hessonite or cinnamon stone. Grossular is usually translucent to opaque but can be transparent. It occurs as rounded dodecahedral or trapezohedral crystals that are up to 5 in (13 cm) wide. Specimens can also be granular or massive. Grossular forms in impure calcareous rocks that have undergone regional or contact metamorphism, in some schists Grossular beads (pp.291–92) and serpentinites This strung group is (p.298), and occasionally in made up of size-graded, meteorites (pp.335–37). round, luminescent green grossular beads.

246 MINERALS | ORGANICS ORGANICS Generated by organic (biological) processes, the organics may or may not be crystalline. In some cases, they contain the same mineral matter—such as calcite or aragonite—as that generated through inorganic processes. Organics are sometimes used as gems. COMPOSITION as carbonate sediment, either to be The organics have a highly varied reincorporated into other organics or to composition. Coral, pearl, and shell be lithified. Other organics incorporate contain mineral matter generated by carbon in their essential composition. biological processes. Amber is fossilized resin, mainly from extinct coniferous trees, USES although amberlike substances from even Coal and bitumen are older trees are also known. Copal the organics that find the is a modern equivalent of widest use. Organically fossilized amber. Coal is derived formed carbonate rocks from buried organic material, are also extensively used as such as peat. Bitumen is a building stone and ballast very heavy oil. and in the manufacture of cement. Other organics are Shell used to make ornaments The shells of marine invertebrates and jewelry. capture large amounts of carbon in the form of aragonite. Their remains form Coral necklace red coral extensive beds of carbonate rocks. This branchlike Native American OCCURRENCE necklace is The organics are relatively widespread. The made from shells of freshwater and marine organisms small, polished are part of a carbonate cycle that extracts branches and carbon from the environment and returns it tiny beads of red coral. Coral reef A coral reef forms in shallow ocean areas. Corals are the most important part of the reef and form its main structural framework. Coral skeletons are made of aragonite.

PROFILE MINERALS | ORGANICS 247 Trigonal Orthorhombic wood-grain pattern Amorphous coral branch 4 3 1⁄2 m 2.6–2.7 n None o Hackly p White q Dull to vitreous Red coral r Mostly CaCO3 The use of red coral dates back to the Iron Age. This specimen from CORAL the Mediterranean has a wood- grain pattern on its branches. According to Greek legend, coral came from the drops VARIANTS of blood shed when the mythic hero Perseus cut off the Blue coral head of the monster Medusa. Coral is actually the skeletal A type of coral used in artifacts material generated by marine animals also known as and jewelry coral polyps. In most corals, this material is calcium Black coral A variant that carbonate, but in black and golden corals it is a hornlike is polished to make jewelry substance called conchiolin. Coral has a dull luster when Brain recovered, but it can be polished coral An elaborate and brightened. It is sensitive even confection of organic aragonite to mild acids and can become dull with extensive wear. Coral is variable in color. Red and pink precious coral is found in the warm seas around Japan and Malaysia, in African coastal waters, and in the Mediterranean Sea. Coral necklace Black coral comes from the West This triple-stranded Indies, Australia, and from around necklace from Morocco the Pacific Islands. is made of coral, silver, and turquoise.

248 MINERALS | ORGANICS PROFILE conchiolin covering Orthorhombic 43 m 2.7 n None o Uneven, brittle p White q Pearly black pearl iridescent mother-of-pearl Black mother-of-pearl The black-lip shell, a mollusk that produces black mother-of-pearl, also produces black cultured pearls. VARIANTS r Mostly CaCO3 blister pearl PEARL Blister pearls Attached to A concretion formed by a mollusk, a pearl consists the shell, these are flat on one side mainly of aragonite (p.115), the same material as the Freshwater pearls These animal’s shell (p.249). The shell-secreting cells are located have the same luster as marine pearls in a layer of the mollusk’s body tissue called the mantle. When a foreign particle enters the mantle, the cells build up concentric layers of pearl around it. Colors vary with the mollusk and its environment and can be any delicate shade from black to white, cream, gray, blue, yellow, green, lavender, or mauve. The finest pearls are produced by limited species of saltwater oysters and freshwater clams. A pearl is valued by its translucence, luster, color, and shape. The most valuable pearls are spherical or droplike, with Pearl bracelet a deep luster and good play of This Cartier Art Deco color. Rose-tinted Indian pearls bracelet has five strands of are highly prized. cultured pearls with a gold and oxidized-silver clasp.