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Nesosilicate Staurolite Fe2al9o6(Sio4)4(OH)2

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Nesosilicate Staurolite Fe2al9o6(Sio4)4(OH)2 Nesosilicate Nesosilicate Staurolite Fe Al O (SiO ) (OH) 2 9 6 4 4 2 Andalusite Al2SiO5 Diagnostic features: characteristic cross-shaped twins and crystal form, high hardness (7-7.5 is harder than a This is one of three geologically important aluminosilicate polymorphs. Andalusite, sillimanite and kyanite are knife) and occurrence in metamorphic schists and gneisses. identical in composition but the geometry of their internal arrangement is different. Andalusite is an indicator of low-pressure metamorphism. Habit: the {110} prism (see below) gives it a characteristic rhombic outline. Cruciform twins are common. There is no distinct cleavage. Diagnostic features: hard (7.5, harder than knife), prismatic crystals with nearly square cross-sections. Colour: red brown to brownish black. Luster: vitreous to resinous when fresh, dull to earthy when altered or Colour: A perfectly pure andalusite would be colourless but the natural mineral can be anything from reddish impure. brown to olive-green. The variety chiastolite includes carbon-rich material on some faces during its growth, giving rise to a cross-like pattern visible on cut and polished crystals (see below, on the left). Often found with garnet and mica in metamorphic rocks. Habit: usually forms distinct prismatic crystals, with nearly square cross-sections, terminated by {001}. The prismatic {110} cleavage is good but not perfect (not as obvious as in hornblende, for example). Look-alikes: Orthoclase. Nesosilicate (SHOWN IN CLASS BUT WILL NOT BE ON THE TEST) Nesosilicate Sillimanite Al2SiO5 Kyanite Al2SiO5 This is one of three geologically important aluminosilicate polymorphs. Andalusite, sillimanite and kyanite are This is one of three geologically important aluminosilicate polymorphs. Andalusite, sillimanite and kyanite are identical in composition but the geometry of their internal arrangement is different. Sillimanite is an indicator of identical in composition but the geometry of their internal arrangement is different. Kyanite is generally an a medium- to high-grade of metamorphism. indicator of high-pressure metamorphism. Diagnostic features: slender crystals with one direction of cleavage. Can be difficult to tell apart from tremolite. Diagnostic features: blue colour, bladed crystals, good cleavage and different hardness in different directions. Colour: A perfectly pure sillimanite would be colourless. The mineral is typically light-coloured, brown, pale- Habit: long, tabular or bladed crystals with perfect {100} cleavage. green or white. Weathering tends to turn the surface rusty. Hardness: 5 parallel to the length of the crystals, 7 at right angle to the Habit: long, slender crystals without clear terminations. Often in parallel groups, frequently fibrous. length. Hardness: 6-7 (scratches the knife sometimes). Colour: usually blue, with different shades from rim to center. Luster is vitreous to pearly. Look-alikes: Wollastonite, tremolite, anthophyllite can all crystallize as fibrous masses. Wollastonite and tremolite are frequently associated with calcite. Look-alikes: none. (below: sillimanite-garnet schist) Inosilicate Cyclosilicate (ring silicate) Amphiboles (double-chain silicates) Tourmaline (Na,Ca)(Li, Mg, Al)3(Al, Fe, Mn)6(BO3)3Si6O18(OH)4 Anthophyllite (Mg, Fe)7Si8O22 (OH) 2 Diagnostic features: "clove-brown" colour (i.e. beige brown like clove, the spice) and Diagnostic features: high hardness (7-7.5, can be harder than quartz but always harder than the knife), trigonal often softer than its "official" hardness (5.5-6: barely scratched by a knife) because it prismatic habit with a slightly rounded triangular section, conchoidal fracture. alters so easily to talc,. Prismatic to fibrous habit (fairly similar to those of actinolite and tremolite). The perfect prismatic cleavage of amphibole (two planes at angles of Habit: trigonal prisms, often terminated by trigonal pyramids. The prism is usually vertically striated. 124° and 56°) gives a “splintery” look to broken surfaces. Colour: can take a wide range of colours. One of the most common variety is schorl, coloured black by Fe. Habit: prismatic to fibrous. The crystals are generally too small to recognize the cleavage angle or rhombic Pink tourmaline is elbaite, coloured by Li. There are many more. Tourmaline sometimes varies in colour from cross section. one end of the prism to another, or from the core to the rim. Crystals that are pink inside with a green outer rim are sometimes called “watermelon” tourmaline. Colour: beige to brown, less commonly shades of green. Luster should be vitreous but is often pearly or silky because the mineral is partly altered to talc. Look-alikes: beryl, apatite. As in all amphiboles, the prismatic cleavage is parallel to the direction of elongation of the crystal. phyllosilicate phyllosilicate chlorite group serpentine group Chlorite (Mg, Fe)3(Si, Al)4O10(OH) 2·(Mg, Fe) 3(OH) 6 Antigorite/lizardite Mg3Si2O5(OH) 4 Diagnostic features: pale green colour appears when scratched, micaceous habit and cleavage, folia are not elastic. Diagnostic features: variegated green colour (i.e. not uniform) and greasy to waxlike luster. Habit: Large crystals are rare. Found most commonly as small scales dispersed in metamorphic rocks, giving them a greenish colour. The perfect {001} cleavage often makes it easy to detach small platelets by scraping the Habit: usually massive and fine-grained (the individual crystals cannot be distinguished). rock with the knife. Chlorite is often found as pseudomorphs of other ferromagnesian silicate minerals, and the crystals may be submicroscopic. Scratching the specimen will leave a pale green groove. Colour: often variegated, with mottling in lighter and darker shades of green. Colour: various shades of green (pale to nearly black), depending on the relative amounts of Mg and Fe. Hardness: more variable than most minerals, 3-5 (usually 4) but softer than olivine (6.5-7). Other properties: low hardness (2-2.5: close to fingernail). May be hard to test if small chloride crystals are The text calls these minerals polymorphs of chrysotile (the fibrous mineral of this group), but crystallographers dispersed among harder minerals in a rock, or if a thin layer of chlorite covers a much harder mineral. would disagree. Recent techniques have shown that antigorite and lizardite differ slightly in composition and structure from chrysotile. However the differences are too subtle to be recognized in hand specimen. This is This mineral is a common product of the alteration of other ferromagnesian minerals (e.g., olivine, augite, why the more general term “serpentine” is used for hand specimens that contain either antigorite or lizardite. hornblende, actinolite and some garnets among those seen last week). The green colour of many igneous rocks is due to the alteration to chlorite of the original amphiboles (e.g. hornblende) and/or pyroxenes (e.g. augite) in Minerals of the serpentine group are a common alteration mineral of olivine and magnesian pyroxenes or these rocks. The green colour of many schists and slates (metamorphic rocks) is due to disseminated chlorite. amphiboles. phyllosilicate phyllosilicate serpentine group Lepidolite, K(Li, Al)2-3(AlSi3O10)(O, OH, F) 2 Chrysotile: Mg Si O (OH) 3 2 5 4 Diagnostic features: micaceous habit and cleavage; pink to lilac colour in thick books but colourless and transparent in thin sheets. Low hardness (2-2.5). BE CAREFUL when handling CHRYSOTILE or ANY FIBROUS MINERAL. Any fibrous mineral will release microscopic fibers if it is scratched or shaken. Breathing mineral fibers or dust irritates the lung tissues. Habit: monoclinic, but euhedral crystals are rare and their cross-section is nearly rhombic or pseudo-hexagonal. Found most commonly in coarse to fine-grained scaly aggregates. Diagnostic features: fibrous to asbestiform habit, greenish colour. Occurrence: lepidolite is a relatively rare mineral, found in pegmatites, and usually associated with other Habit: asbestiform (long, flexible fibers), usually interbedded with other massive minerals of the serpentine lithium-bearing minerals such as pink and green tourmaline and the rare pyroxene spodumene. group. (The fibers actually consists of layer-like crystals rolled up like miniature carpets.) Colour: shades of green, pearly luster. Hardness: 3-4, but hard to test on asbestiform specimens because the fibers separate so easily. This is a common alteration mineral of olivine and magnesian pyroxene or amphibole. It is one of the minerals called "asbestos" (a general term for fibrous silicate minerals used in industry for fire- and earthquake-proofing materials), but probably the least harmful of them all because the fibers can dissolve in the lung fluids. Tectosilicate phyllosilicate Albite NaAlSi3O8 sodic feldspar/plagioclase: Biotite, KMg3(AlSi3O10)(OH) 2 Diagnostic features: moderately high hardness (6: barely harder than knife), two directions of cleavage mutually Diagnostic features: micaceous habit and cleavage; dark green or brown to black, perpendicular (parallel to faces b and c), vitreous luster, striations due to polysynthetic twinning. even in thin sheets. Low hardness (2-2.5). Habit: euhedral crystals have a tabular to prismatic habit. Habit: monoclinic, the cross-section of euhedral crystals is nearly rhombic or pseudo-hexagonal. Also commonly found in coarse to fine-grained scaly aggregates (in metamorphic schists and gneisses). Colour: variable. Colour: varies with Fe content, from light yellow (rare) to deep green, brown or black. Thin sheets usually
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