3 polymorphs: same composition Al2SiO5 Additions to earlier silicate subclasses: but different atomic patterns (and therefore crystal classes) nesosilicate: andalusite-sillimanite-kyanite staurolite inosilicates: anthophyllite cyclosilicate: tourmaline phyllosilicate: biotite, lepidolite, chlorite chrysotile, antigorite tectosilicate: labradorite, albite Andalusite forms nearly square Sillimanite is fibrous, and could be easily prisms, often with a dark confused with tremolite. The absence cross... of calcite and the presence of other aluminous silicates can be helpful. Often in schists (foliated Kyanite is often blue, metamorphic always bladed, with rocks). different hardnesses across its width and NOT on length. test. Staurolite has an awful formula but a very Cyclosilicate of the week is tourmaline. cute growth habit. prism is It is prone to twinning, i.e. a pair of crystals striated grow together in a cross or an “X”. cross-section is triangular. often rounded. •trigonal pyramid •very variable colours. INOSILICATES INOSILICATES Pyroxenes: Pyroxenes: single chains single chains of tetrahedra of tetrahedra ... SiO or Si O 3 2 6 ... SiO3 or Si2O6 diopside, augite Amphiboles: the tetrahedral chains (right, in blue) are twice as wide... Si4O11 Amphiboles = double-width chain, hydrous Anthophyllite Mg7Si8O22(OH)2 -prismatic/acicular/fibrous like actinolite -softer than it should be, because it alters easily to talc Si2O6 + Si2O5 (view under microscope) - color is “clove-brown”, unlike Cleavage actinolite (green) and tremolite (white parallel to a or gray) rhombic prism Phyllosilicates: hydrous silicate minerals Muscovite: a clear mica... lift a thin sheet where the tetrahedra share three of to check its colour. A thick crystal (left) their oxygen ions. is darker. biotite: rich in Fe, greenish-black lepidolite: rich in Li, Al (rarer), lilac to pinkish grey... while muscovite was nearly transparent. Chlorite is a phyllosilicate that usually forms by alteration of ferromagnesian silicate minerals in the presence of water. It has the typical greenish color of most ferromagnesian minerals. The phyllosilicates chrysotile and antigorite/lizardite have nearly the same chlorite: alteration mineral composition. Older texts considered them formed from ferromagnesian true polymorphs. All three minerals form by silicates that come in contact hydrothermal alteration of olivine and/or with hot water. magnesian pyroxenes. You can’t tell antigorite from lizardite in hand specimen. The original shape may be retained (pseudomorph) but the None of these 3 minerals shows the good mineral will be softer, and platy cleavage of most phyllosilicates. Their scratches will look pale green. habit is either asbestiform (sheets curled into long thin fibers) or their crystal size is too small (antigorite/lizardite). Chrysotile is one of the Tectosilicates: silicate minerals where the minerals called tetrahedra share all four oxygen ions. “asbestos”. In this phyllosilicate, the sheets If all the are curled into rolls that tetrahedra are we see as thin fibers. SiO2, the Antigorite/lizardite mineral should occurs as very fine- be hard (Si-O grained, massive bonds are aggregates that are strong). greenish to yellow and waxy-looking. Three feldspars: different colours and composition, but same hardness and same cleavage. The whiter veins (almost vertical) in this perthitic microcline are Na-rich, the rest is light gray and K-rich. The Na-rich lamella may show the polysynthetic twinning. albite (NaAlSi3O8) labradorite Ab45An55 look for striations: polysynthetic twinning.