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Framework silicate subclass

Cordierite

Hand Specimen identification: Prismatic cleavage, color, and association aid identi-fication, but if it is not blue, this mineral is often dif-ficult to distinguish without a microscope. It may be easily confused with quartz in hand specimen and with plagioclase in thin section. Chemical Composition : (Mg,Fe)2Al4Si5O18 Hardness : 7 to …

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Tridymite (low tridymite)

Hand Specimen identification: Tridymite is usually sufficiently fine grained that X-ray or optical measurements are needed for identification. It is sometimes confused with zeolites. Figure 4 shows a scanning electron microscope image of hexagonal tridymite crystals. Chemical Composition : Hardness : 6 to 7 Specific Gravity2.28 Cleavage/Fracture : none/conchoidal Luster/Transparency …

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Stilbite

Hand Specimen identification: Sheaflike aggregates of twinned crystals, one excel-lent cleavage, pearly luster, color, and association identify stilbite. See Figure 6b. Chemical Composition :CaAl2Si7O18 . 7H2O Hardness : 31 2 to 4 Specific Gravity2.15 Cleavage/Fracture : perfect (010)/subconchoidal Luster/Transparency : pearly/transparent to translucent Color : gray Streak : gray Optical …

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Coesite

Hand Specimen identification: Coesite cannot be identified in hand specimen. Chemical Composition : SiO2 Hardness : 7 to 8 Specific Gravity2.93 Cleavage/Fracture : none/conchoidal Luster/Transparency : vitreous/transparent Color : colorless Streak : white Optical Properties : Biaxial 1+ 2, a = 1.59, b = 1.60, g = 1.60,d = 0.01, …

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Sodalite

Hand Specimen identification: Color (if blue) and association are usually diagnostic for sodalite. If not blue, identification may require chemical tests to tell it from zeolites or analcime. It is sometimes confused with lazulite but has different associations. Chemical Composition : Na3Al3Si3O12.NaCl Hardness : 51 2 to 6 Specific Gravity2.3 …

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Stishovite

Hand Specimen identification: Stishovite only exists as tiny grains in rocks near meteorite impact craters. Chemical Composition : SiO2 Hardness : 7 to 8 Specific Gravity4.3 Cleavage/Fracture : {110}/conchoidal Luster/Transparency : vitreous/transparent Color : colorless Streak : white Optical Properties : Uniaxial 1+ 2, v = 1.799, e = 1.826, …

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Quartz (a-quartz)

Hand Specimen identification: Hardness, lack of cleavage, conchoidal fracture, and vitreous luster usually serve to identify quartz. When euhedral, its pseudohexagonal prismatic habit can be distinctive. Quartz is sometimes confused with calcite, beryl, cordierite, or feldspars. Plates 2.1 through 2.8 show color pictures of quartz. Chemical Composition : Hardness : …

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Beryl

Hand Specimen identification: Association, sometimes hexagonal crystal shape, very poor cleavage, color (if blue), and hardness identify beryl. See Figure 7. Chemical Composition : Be3Al2Si6O18 Hardness : 71 2 to 8 Specific Gravity2.7 to 2.9 Cleavage/Fracture : poor (001)/even Luster/Transparency : vitreous/transparent to translucent Color : blue, colorless, variable Streak …

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Anorthite

Hand Specimen identification: Cleavage, hardness, luster, and association help identify anorthite, but it can be extremely difficult to tell from other feldspars.Several kinds of twinning are common (see albite). Albite twins, if present, may be difficult to see in hand specimen. Chemical Composition : CaAl2Si2O8 Hardness : 6 to 61 …

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Albite

Hand Specimen identification: Cleavage, hardness, luster, association, and fine polysynthetic twinning help identify albite and other plagioclase feldspars. If not twinned, it may be diffi-cult to tell from K-feldspar. Distinguishing albite from other plagioclase feldspars cannot be done precisely without detailed X-ray or optical data. Plate 6.8 shows rare euhedral …

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