Silicates (Minerals)

Kaolinite (33)

Chemical Formula Al2Si2O5(OH)4 Composition Basic aluminum silicate Color White, gray, yellow, beige. May also be darker colored brown, orange, or reddish-brown from iron oxide impurities. Streak White Hardness 2 - 2.5 Crystal System Monoclinic Crystal Forms and Aggregates Most often as unshaped compact masses. Crystals are microcrytalline as tiny grains and plates. Crystals are rarely visible to the naked eye. Transparency Opaque. Rarely translucent. Specific Gravity 2.6 Luster Dull Cleavage 1,1 Fracture Earthy Tenacity Brittle, sectile Other ID Marks Kaolinite is very friable, and can be cut and molded, especially when wet. May also give off a clay-like odor when wet or when breathed upon. In Group Silicates; Phyllosilicates Striking Features Friable habit and clay-like texture Environment As a secondary altering mineral, usually from weathering, in igneous, metamorphic, and sedimentary environments. Kaolinite is most often found near the oxidized surface where it has altered from aluminum silicates such as feldspar. Rock Type Igneous, Sedimentary, Metamorphic

Topaz (56)

Chemical Formula Al2SiO4(F,OH)2 Composition Aluminum fluoro-hydroxyl-silicate Color Colorless, white, yellow, orange, brown, pink, light purple, gray, light blue, greenish blue, green. Occasionally multicolored. Streak Colorless Hardness 8 Crystal System Orthorhombic Crystal Forms and Aggregates Prismatic, tabular, and stubby crystals, usually striated and sometimes quite large. Crystals may contain numerous faces, and often have complex terminations. Also occurs columnar, massive, grainy, radiating, and as rounded, waterworn pebbles. Topaz may also be in the form of feldspar as a pseudomorph. Transparency Transparent to opaque Specific Gravity 3.4 - 3.6 Luster Vitreous Cleavage 1,3 - basal Fracture Subconchoidal Tenacity Brittle In Group Silicates; Nesosilicates Striking Features Great hardness and perfect basal cleavage Environment In igneous environments in granite pegmatites and in rhyolite. Occasionally in sedimentary alluvial deposits. Rock Type Igneous

Beryl (10)

Chemical Formula Be3Al2Si6O18 Composition Beryllium aluminum silicate, occasionally with some sodium, lithium, and cesium Variable Formula (Be,Na,Li,Ce)3Al2Si6O18 Color Light to emerald green, light to deep sky-blue, blue-green, yellow, pink, purple, red, orange, brown, colorless, white, and gray. May also be multicolored blue, green, yellow, or white, and may also have deeper color highlights on one crystal end. Streak Colorless Hardness 7.5 - 8 Crystal System Hexagonal Crystal Forms and Aggregates Beryl often crystallizes in perfect, six-sided hexagons. Crystals are usually as individual prismatic hexagons. Crystals may be enormous in size; some 30 foot long (8 meter), well-crystallized examples have been found. Beryl may also be short, stubby crystals, and occasionally in tabular crystals and plates. The bases of Beryl crystals are usually flat; pyramidal terminations are rare. Also occurs in columnar aggregates, in distorted etched crystals, and massive. Occasionally in drusy or platy aggregates and as bundles of thin, long crystals. Crystals may be striated lengthwise. Transparency Transparent to opaque Specific Gravity 2.6 - 2.9 Luster Vitreous, waxy Cleavage 3,1 - basal Fracture Uneven to conchoidal Tenacity Brittle Other ID Marks Occasionally fluorescent yellow, light blue, purple, pink, or red. Complex Tests Insoluble in acids. In Group Silicates; Cyclosilicates Striking Features Crystal form and hardness Environment Beryl is most well-known from granite pegmatites. It can also be found in metamorphosed mica schists and in igneous rhyolite deposits. Rock Type Igneous, Metamorphic

Epidote (20)

Chemical Formula Ca2(Al,Fe)2(SiO4)3(OH) Composition Basic calcium aluminum iron silicate. Color Light to dark-green, olive-green, brownish-green, yellowish-green, yellow, brown, black. Transparent forms can be strongly pleochroic with a greenish color on one angle and brownish color on the other angle. Streak White Hardness 6 - 7 Crystal System Monoclinic Crystal Forms and Aggregates Usually in long slender prismatic crystals; also in thick tabular crystals. Crystals are sometimes striated and may have interesting wedge-shaped terminations. They also may have etchings or growth layers, and may contain late-growth small crystals layers growing upon a larger crystal. Also columnar reticulated, acicular, radiating, in fan-shaped and wheat sheaf crystal groups, and in long, slender fragile interconnected crystal groupings. May also form as a thin microcrystal crusting and may be massive. Transparency Transparent to nearly opaque Specific Gravity 3.3 - 3.6 Luster Vitreous Cleavage 1,1 Fracture Uneven Tenacity Brittle In Group Silicates; Sorosilicates Striking Features Greenish color and common crystal habits Environment Epidote occurs in several different environments. It is found in hornfels and skarns in contact metamorphic rocks, and in metamorphosed limestone and in schists in regional metamorphic rock. It is also found in igneous rock in basalt and diabase, and is occasionally found in granite pegmatites. Rock Type Igneous, Sedimentary, Metamorphic

Staurolite (53)

Chemical Formula Fe2Al9Si4O22(OH)2 Composition Basic aluminum iron silicate, often with magnesium and sometimes with zinc and lithium. Variable Formula (Fe,Mg,Zn,Li)2Al9Si4O22(OH)2 Color Brown, grayish brown, gray, yellowish-brown, reddish brown Streak White Hardness 7 - 7.5 Crystal System Monoclinic Crystal Forms and Aggregates In pseudo-orthorhombic crystals, usually in rectangular form with a hexagonally-shaped cross section. Crystals often have a triangular formation on the termination. Crystals are usually prismatic, but may also be short and stubby, as well as tabular. Most often in penetration twins, in the form of perfect perpendicular crosses (sometimes known as staurolite twins), 60° penetration twins, and occasionally even in triple-twinned, star-shaped combinations. Also in groupings of several crystals, and in twins where a smaller twinned crystals appears to be swallowed by a larger one. Transparency Translucent to opaque. Rarely transparent. Specific Gravity 3.7 - 3.8 Luster Vitreous, dull Cleavage 3,1 Fracture Uneven to subconchoidal Tenacity Brittle Other ID Marks Crystal texture is commonly pitted and rough. In Group Silicates; Nesosilicates Striking Features Crystal habits, color, and habits Environment In metamorphosed schists and gneisses. Rock Type Metamorphic

Talc (55)

Chemical Formula Mg3Si4O10(OH)2 Composition Basic magnesium silicate Color White, beige, gray, yellow, brown, pink, purple, blue, green. Rarely colorless. Streak White Hardness 1 Crystal System Monoclinic Crystal Forms and Aggregates Most often as large distorted masses and foliated sheets and plates. Also micaceous, radiating, botryoidal, and in fibrous masses. Crystallized examples, which include flat tabular crystals, are rare and are almost always microscopic. Talc very commonly pseudomorphs after many minerals, assuming their original shape. Some minerals commonly pseudomorphed are Quartz, Calcite, Dolomite, and Pyroxenes. Transparency Transparent to opaque Specific Gravity 2.7 - 2.8 Luster Greasy, waxy, pearly Cleavage 1,1 Fracture Uneven Tenacity Sectile Other ID Marks 1) Has a greasy feel. 2) May be lightly fluorescent. In Group Silicates; Phyllosilicates Striking Features Low hardness and greasy feel Environment In metamorphic rocks, especially Serpentine deposits. Rock Type Metamorphic

Sodalite (51)

Chemical Formula Na8Al6Si6O24Cl2 Composition Sodium aluminum silicate with chlorine, and occasionally sulfur. Variable Formula Na8Al6Si6O24(Cl2,S) Color Most often blue, less commonly purple, pink, gray, green, brown, white, and colorless. May also be multicolored blue with white and gray veins. Streak White Hardness 5.5 - 6 Crystal System Isometric Crystal Forms and Aggregates Crystals, which are dodecahedral, are rare and will usually have complex or rounded terminations. Most often massive, grainy, and nodular. Transparency Transparent to opaque Specific Gravity 2.2 - 2.3 Luster Vitreous to greasy; may also be dull. Cleavage 3,6 Fracture Uneven Tenacity Brittle Other ID Marks Often fluorescent a creamy-white, yellow, or orange color in shortwave ultraviolet light. The variety Hackmanite fluoresces orange to red in shortwave, and orange in longwave. In Group Silicates; Tectosilicates; Feldspathoid Group Striking Features Habitual deep blue color Environment Sodalites occur in a variety of different environments, including alkaline nepheline syenite pegmatites, metamorphosed marbles and skarns, and both intrusive and extrusive igneous rock. Rock Type Igneous, Metamorphic

Olivine (38)

Chemical Formula The Olivine group is composed of the following primary members: Forsterite: Mg2SiO4 Olivine (Chrysolite): (Mg,Fe)2SiO4 Fayalite: Fe2SiO4 The intermediary variety, Olivine, is not scientifically recognized as a separate mineral, but is nevertheless well-established. The mineral Tephroite (Mn2SiO4), which many consider a member of the Olivine group, forms a series with Forsterite. There are other rarer members of the Olive group such as Tephroite. Composition Magnesium iron silicate. The series ranges from the magnesium end member, Forsterite (Magnesium silicate), through the intermediary member, Olivine (also known as Chrysolite), to the iron end member, Fayalite (Iron silicate). Color Forsterite and Olivine can be olive-green, light green, dark green, yellow-green, yellow-brown, and brown. Rarely white, gray, or orange. Pure Forsterite is colorless, but this is extremely rare. Fayalite is usually yellow-brown to brown. Streak Colorless Hardness 6.5 - 7 Crystal System Orthorhombic Crystal Forms and Aggregates Most often as rounded grains, in dense aggregates of grainy crystals, as fractured masses, and as rounded waterworn pebbles and grains. Large crystals, which are prismatic and stubby, are uncommon except at a few select localities. Crystals often have rounded faces. Transparency Transparent to translucent Specific Gravity 3.2 - 3.4 Luster Vitreous Cleavage 2,1 ; 3,1- forming a 90º angle Fracture Conchoidal Tenacity Brittle Complex Tests Soluble in hydrochloric acid. In Group Silicates; Nesosilicates Striking Features Color, localities, and hardness Environment Olivine occurs in mafic and ultramafic igneous rocks. It is also found in metamorphic rocks and Serpentine deposits as a primary mineral. Olivine may also occur in meteorites. Rock Type Igneous, Metamorphic, Meteoric

Tourmaline Group (57)

Chemical Formula The simple chemical formula, which covers the main forms of Tourmaline (Elbaite, Schorl, and Dravite), is as follows: (Na,Ca)(Mg,Li,Al,Fe2+)3Al6(BO3)3Si6O18(OH)4 The expanded formula, which additionally covers Uvite, Liddicoatite, and Buergerite, is as follows: (Na,Ca)(Mg,Li,Al,Fe2+,Fe3+)3(Al,Mg)6(BO3)3Si6O18(OH,O,F)4 The formula for the Tourmaline group is very complex. See The chemical formula of Tourmaline for more details. Composition See The chemical formula of Tourmaline. Color Tourmaline is extremely varied in color. Colors include black, brown, green, red, pink, blue, and gray. White, colorless, yellow, orange, and purple colors are less common. Crystals are frequently multicolored, containing two or more distinct colors. Some specimens are pleochroic. Streak White Hardness 7 - 7.5 Crystal System Hexagonal Crystal Forms and Aggregates Usually as elongated prismatic crystals that are heavily striated. Also as short, stubby, prismatic crystals. Most Tourmaline crystals have a rounded, triangular cross-section. Seldom in tabular crystals. Aggregates include columnar, radiating, botryoidal, stalactitic, in dense groups of tiny, elongated needles, and in compact masses. Transparency Transparent to opaque Specific Gravity 2.9 - 3.3 Luster Vitreous. Some black and brown specimens may be dull. Cleavage 3,2 Fracture Conchoidal to uneven Tenacity Brittle Other ID Marks 1) Strongly pyroelectric. 2) Piezoelectric. 3) A few forms of Tourmaline fluoresce yellow in shortwave ultraviolet light. In Group Silicates; Cyclosilicates; Tourmaline Group Striking Features Color, crystal form, hardness, and deep vertical striations. Environment Elbaite, Schorl, and Liddicoatite are almost exclusively from granite pegmatites, while Dravite and Uvite or mostly from metamorphic environments such as marbles. Buergerite is from igneous rhyolite deposits. Rock Type Igneous, Metamorphic