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Nesosilicate Olivine (Mg,Fe)2Sio4 Series: Forsterite Mg2sio4

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Nesosilicate Olivine (Mg,Fe)2Sio4 Series: Forsterite Mg2sio4 Nesosilicate Garnet group A3B2Si3O12 (nesosilicate) Olivine (Mg,Fe)2SiO4 Diagnostic features: high hardness (6.5-7.5, usually harder than a knife), equant (isometric) habit, vitreous luster, no distinct cleavage. series: Forsterite Mg2SiO4 - Fayalite Fe2SiO4 NOTE: The use of parenthesis within a chemical formula indicates elements which may be found in variable Habit: equant, consisting of a dodecahedron, a trapezohedron or a combination of these (shown below). Faces proportions within a given mineral structure. In olivine, the silicate tetrahedra are connected by either Mg2+ or may be striated. Fe2+ cations. Colour: highly variable with composition. Many garnets have compositions intermediate between the end- Diagnostic features: hard (6.5-7: harder than knife) but grains are easily detached from the rock; no cleavage but members listed below. (For the test, simply identify any member of this group as "garnet".) pronounced conchoidal fracture; colour is green but can tend towards yellow or brown. This mineral is found mostly in dark-coloured igneous rocks, like the Hawaiian basalts. Pyrope, Mg3Al2Si3O12 is usually deep red (typical of igneous rocks formed at great depths, under high pressure). 2+ Colour: A perfectly pure forsterite would be colourless. The presence of some Fe in most olivine is Almandine, Fe3Al2Si3O12 is red to black. responsible for its green colour. Fe-poor varieties can be yellow, and Fe-rich varieties can be brown. The Spessartine, Mn3Al2Si3O12 is brownish red to pink. yellow-green, gem-quality variety of olivine is known as PERIDOT. Grossular, Ca3Mg2Si3O12 is colourless to yellow, green or brown (common, often associated with calcite). Andradite, Ca3Fe2Si3O12 is found in shades of yellow, green or brown to black. Habit: its stubby prismatic crystals usually look like small grains disseminated through the rock. Uvarovite Ca3Cr2Si3O12 has an emerald-green colour. Look alikes: epidote, diopside. Unlike epidote, green olivine is not found with quartz. It lacks the twinning and prismatic cleavage that are commonly seen in diopside. Right: Garnet often occurs in schists, i.e. metamorphic rocks showing a strong foliation caused by the alignment of platy mica crystals or prismatic amphiboles. Nesosilicate Nesosilicate Topaz Al2SiO4(F, OH)2 Staurolite Fe2Al9O6(SiO4)4(OH) 2 Diagnostic features: high hardness (8), perfect basal {001} cleavage, and prismatic crystals with a rhombic cross-section. Diagnostic features: characteristic cross-shaped twins and crystal form, high hardness (7-7.5 is harder than a knife) and occurrence in metamorphic schists and gneisses. Habit: commonly euhedral (i.e. well-developed faces), in prismatic crystals terminated by dipyramids, {0kl} and {h0l} prisms and basal pinacoids. The faces of the vertical prism give each crystal its rhombic cross Habit: the {110} prism (see below) gives it a characteristic rhombic outline. Cruciform twins are common. section. These faces are commonly striated vertically. There is no distinct cleavage. Colour: very variable, from colourless to pink, yellow, green or blue. Rarely dark. Colour: red brown to brownish black. Luster: vitreous to resinous when fresh, dull to earthy when altered or impure. Luster: vitreous, especially on a freshly broken cleavage face. Often found with garnet and mica in metamorphic rocks. Topaz is a mineral formed by the fluorine-rich vapours given off during the last stages of crystallization of siliceous igneous rocks. It is found in cavities of silica-rich lavas (rhyolites) and granites, and in pegmatites (very coarse-grained igneous rock). Associated with quartz, mica and feldspar and commonly found with tourmaline, apatite and fluorite. Nesosilicate Nesosilicate Sillimanite Al2SiO5 Andalusite 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. Andalusite is an indicator of a medium- to high-grade of metamorphism. low-pressure metamorphism. Diagnostic features: slender crystals with one direction of cleavage. Can be difficult to tell apart from tremolite. Diagnostic features: hard (7.5, harder than knife), prismatic crystals with nearly square cross-sections. Colour: A perfectly pure sillimanite would be colourless. The mineral is typically light-coloured, brown, pale- Colour: A perfectly pure andalusite would be colourless but the natural mineral can be anything from reddish green or white. Weathering tends to turn the surface rusty. 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). Habit: long, slender crystals without clear terminations. Often in parallel groups, frequently fibrous. Habit: usually forms distinct prismatic crystals, with nearly square cross-sections, terminated by {001}. The Hardness: 6-7 (scratches the knife sometimes). prismatic {110} cleavage is good but not perfect (not as obvious as in hornblende, for example). Look-alikes: Wollastonite, tremolite, anthophyllite can all crystallize as fibrous masses. Look-alikes: Orthoclase. Wollastonite and tremolite are frequently associated with calcite. (below: sillimanite-garnet schist) Nesosilicate + Mixed nesosilicate-sorosilicate Kyanite Al2SiO5 Epidote Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) 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. Kyanite is generally an Diagnostic features: moderately high hardness (6-7: harder than knife); perfect platy cleavage parallel to the b indicator of high-pressure metamorphism. axis; peculiar green colour. Diagnostic features: blue colour, bladed crystals, good cleavage and different hardness in different directions. Colour: a peculiar yellow-green or pistachio-green that often distinguishes from other minerals that it may resemble. Habit: long, tabular or bladed crystals with perfect {100} cleavage. Habit: tabular or prismatic monoclinic crystals are commonly elongated and striated parallel to the b axis. Hardness: 5 parallel to the length of the crystals, 7 at right angle to the length. Look-alikes: Olivine, augite. Unlike green olivine, epidote can be found with quartz. The many edges of faces give the epidote crystals the appearance of being striated along their long axis. Colour: usually blue, with different shades from rim to center. Luster is vitreous to pearly. Epidote is an abundant and widespread metamorphic mineral, occurring in calcium-rich rocks derived from impure limestone or in igneous rocks. Clinozoisite is a pale coloured variety of epidote that has very little Fe; it Look-alikes: none. occurs as a secondary mineral in metamorphosed igneous rocks. Cyclosilicate (ring silicate) Cyclosilicate (ring silicate) Beryl, Be Al Si O 3 2 6 18 Tourmaline (Na,Ca)(Li, Mg, Al) (Al, Fe, Mn) (BO ) Si O (OH) 3 6 3 3 6 18 4 Diagnostic features: high hardness (7.5-8, harder than quartz and knife), hexagonal prismatic habit, and a poor platy cleavage. Diagnostic features: high hardness (7-7.5, can be harder than quartz but always harder than the knife), trigonal Habit: hexagonal prisms terminated by a pinacoid (less commonly by dipyramidal faces) are common. prismatic habit with a slightly rounded triangular section, conchoidal fracture. Striations, when present, are parallel to the long axis of the prism. Habit: trigonal prisms, often terminated by trigonal pyramids. The prism is usually vertically striated. Colour: usually bluish green or light yellow, but it may be emerald green, pink, white or colourless. Colour: can take a wide range of colours. One of the most common variety is schorl, coloured black by Fe. Various names are given to gem-quality crystals: AQUAMARINE is the clear blue-green variety, coloured by Pink tourmaline is elbaite, coloured by Li. There are many more. Tourmaline sometimes varies in colour from small amounts of Fe, EMERALD is coloured deep green by chromium or vanadium, MORGANITE is pale pink one end of the prism to another, or from the core to the rim. Crystals that are pink inside with a green outer rim to deep rose, and GOLDEN BERYL is clear yellow. (You do not need to remember these variety names for the are sometimes called “watermelon” tourmaline. test.) Look-alikes: beryl, apatite. Inosilicate Inosilicate pyroxenes (single-chain silicates) pyroxenes (single-chain silicates) Augite: (Ca,Na)(Fe,Mg,Al)(Al,Si)2O6 series: Diopside CaMgSi2O6 - Hedenbergite CaFeSi2O6 Diagnostic features: stubby prismatic habit with an octogonal cross-section, moderate hardness (5.5, about same as a steel blade) and dark colour (deep green to black). Cleavage is imperfect, at angles of 87° and 93°. Diagnostic features: prismatic habit, octogonal section, moderate hardness (5.5, about same as knife) and colour (white to green). Cleavage is imperfect, at angles of 87° and 93°. A well-developed parting parallel to the basal Habit: stubby prismatic crystals very similar to those of diopside. The cross section seen perpendicular to the pinacoid c is sometimes visible. prism is often eight-sided, because the faces m, a and b in the prism zone
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