SILICATES Introduction most abundant class of minerals (40%
SILICATES Introduction most abundant class of minerals (40% of all common minerals) of the common rocks, the essential and accessory minerals of all igneous rocks, many sedimentary rocks, and all but one metamorphic rock are comprised of silicate minerals polymerization occurs in silicates because of the presence of the mesodesmic bond typethis bond allows linkages of the basic building block, the silicon tetrahedron and results in the formation of the silicate subclasses
General Chemical Formula XmYn(ZpOq)wr X = cations with valence of 1 or 2 and CN of 6, 8 or 12 with O Y = cations with valence of 2-4 and CN of 6 with O Z = Al+3, Si+4 w = usually OH-1, F-1, Cl-1 p, q, m, n ,r = used for electroneutrality and p:q defines subclasses yH2O present in silicates is not shown in formula many minerals may not have all symbols represented in their formula Common elements representing symbols are: X = K+1, Na+1, Ca+2
Y = Mn+2, Fe+2, Mg+2, Fe+3, Ti+4, Al+3 Z = Al+3, Si+4 (Structural) Subclasses Kind of subclass based on the kind or degree of polmerization involving the tetrahedron linkage Nesosilicate units of independent tetrahedra in which all 4 O in each tetrahedron are free to directly connect to other cation polyhedra which in turn will connect with other independent silicate tetrahedra
p:q = 1:4 (where p=1 or 3 and q=4 or 12 in SipOq) crystal habit of minerals is equidemensional and pronounced cleavage is absent Sorosilicates unit of 2 silica tetrahedra sharing one common O the other 6 O are free to connect with other cation polyhedra and in turn more of the soro- units can connect with these p:q = 2:7 (where p=2 and q=7 in SipOq) Cyclosilicates
closed rings of of tetrahedra each sharing 2 O the remaining 12 O in the unit are free to connect with cations of other polyhedra which in turn can connect with more cyclo-units p:q = 1:3 (where p=6 and q=18 in SipOq) atoms can be physically trapped in the open spaces Inosilicates 1. continuous chain unit, each sharing 2 O free O in each tetrahedron available to connect to other cation polyhedra which in turn will connect with other ino-single chain units
p:q = 1:3 (where p=1or 2 and q=3 or 6 in SipOq) cleavage along connected polyhedra results in a typical 90 degree 2 directional cleavage Inosilicate 2. double chain unit of tetrahedra each sharing 2 and 3 O alternately free O in tetrahedra available to connect to other cation polyhedra which in turn connect to other ino-double chain units p:q = 4:11 (where p=8 and q=22 in SipOq) cleavage along connected polyhedra forms a 124-56
degree 2 directional cleavage Phyllosilicates continuous sheet units of tetrahedra each sharing 3 of its O free O are available to connect to other cation polyhedra which in turn connect to other phylounits much Al substitutes for Si one directional sheet cleavage results in minerals and atoms and ions can be trapped in open spaces p:q = 2:5 (where p=2 or 4 and q=5 or 10
in SipOq) Tectosilicates continuous framework of tetrahedra all sharing 4 of its O large amounts of Al for Si substitutions allows other polyhedra to connect to the tecto-units p:q = 1:2 ( where p=1, 2, 4 or 6 and q=2, 4, 8 or 12 in SipOq) Mineral Groups and Series in Subclasses Nesosilicates
olivine (solid solution series)--important rock forming mineral garnet (isomorphic group) minerals often occur in dodecahedron crystal form form abundantly in metamorphic rocks pyrope, almandine, grossularite zircon can form metamict structure kyanite (belongs to a polymorphic group) topaz
a mohs hardness scale mineral very hard8 staurolite commonly shows cross twinning Sorosilicates hemimorphite often found as clear hard bladed crystals epidote belongs to an isomorphic group an important rock forming mineral allanite black and no cleavagecan have metamict
form Cyclosilicates minerals have high hardness, there are many examples of gemstones, and cleavage is poor examples are: beryl emerald ( green transparent), aquamarine (pale green-blue transparent) and morganite (rose transparent) cordierite often displays dichroism
tourmaline Shorl (black) variety, rubellite (red-pinkish) variety, indicolite (blue) Inosilicates includes the pyroxene and pyroxenoid groups which are single chain silicates and amphibole group which are double chain silicates pyroxene and pyroxenoid minerals lack (OH)x and display 2 directional 90 degree cleavage and the pyroxenes are very important rock forming minerals
pyroxenes enstatite-ferrosilite (solid solution series) diopside-hendenbergite (solid solution series) augite--most common pyroxene spodumene important source of Li-kunzite=gemstone pyroxenoid group minerals commonly display splintery cleavage an example is: wollastonite can resemble albite but has no striation twinning
amphibole group double chain minerals with (OH)x present minerals display 2 directional cleavage intersecting at 124 and 56 degrees important rock forming minerals examples are: actinolite-tremolite(solid solution series) hornblendemost common amphibole Phylosilicates includes the clay and mica minerals one directional sheet cleavage
very important rock forming minerals Serpentine group (polymorphic group) little or no ionic substitution of Al for Si serpentine (massive) greenish crysotile fibrous or asbestos variety of serpentine Clay group hydrous Al layered silicates and little Al for Si kaolinite talc Tectosilicates
framework structure and much substitution of Al for Si very important rock forming minerals SiO2 polymorphic group quartz varieties include smoky quartz, amethyst, rose quartz, tigers eye, jasper quartz, chalcedony, opal, flint-chert K-feldspar polymorphic group orthoclase pinkish-reddish to beige in color occurring often with perthite form microcloine blue-green in color resulting from an omission solid solution effect caused
by 1Pb+2 ionic substitution for every 2K+1 Plagioclase feldspar solid solution series have twinning striations albitesodic variety white or light in color labradoritelight-dark gray commonly displaying labradorescence anorthitecalcic variety and black in color feldspathoid group minerals containing about 2/3 the amount of silica resulting from a silica deficient magma leucite(2)KAlSi2O6 = K2O + Al2O3 + 4SiO2
compared to orthoclase (2)KAlSi3O8 = K2O + Al2O3 + 6SiO2 sodalite Zeolite group hydrous silicates displaying ionic exchange and absorptive properties acting as water softeners by exchanging Na+1 for Ca+2 Na2Al2Si3O10.2H2O(natrolite) forming CaAl2Si3O10.2H2O stilbite occurs often in tabular or sheaflike aggregates
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