Silicate Mineral Class

Silicate Mineral Class

                            Lecture exam #5 will include concepts in A-D inclusive

A. Introduction
     -silicates are the most abundant class of minerals comprising 75% of all known minerals and 40%
      of all common minerals
     -essentially all igneous rocks, all but one metamorphic rock, and many sedimentary rocks
       are comprised of only silicate minerals (essential minerals)
     -the Si-O bond is mesodesmic allowing polymerization or linking of the building block of the
     silicates, the silicon oxygen tetrahedron--tetrahedra can be shared directly and are mostly
      shared at points not edges nor corners because of the strong repulsion of the Si⁺⁴atoms--if Al⁺³ ionically substitutes for Si⁺⁴, sharing of edges between the Si and Al tetrahedra may exist since
      there is a lesser repulsion between these ions--the different kinds of direct linkages of these
      tetrahedra units is the basis of the subdivision of silicates into subclasses--these will be
      discussed in detail below---tetrahedra units are in turn linked via other kinds of polyhedra in
      the mineral

B. General Chemical Formula of the Silicates

             X_mY_n(Z_pO_q)W_r,

                  X = cations with large ionic radii and small valence numbers (1 or 2), forming a
                         C.N. of 6, 8 or12 with O

Y = cations with medium size ionic radii and 2-4 valence numbers, forming a C.N.
of 6 with O

Z = cations with small ionic radii and large valence numbers (3 or 4), forming a
C.N. of 4 with O

w = usually is OH^-1, F^-1 or Cl^-1 or equivalent

p, q, m, n, r = subscript numbers used to maintain electroneutrality

                   p, q = numbers used to maintain electroneutrality, and the p:q (ratio) defines the subclass
                             of silicates below
                   note: even though some silicates have Y(H₂O) present, this is not shown in the
                  general chemical formula--also it should be noted, some silicates do not have all
                  symbols in the general formula present in their formulas

                     Below is a table of common elements in silicates with some of the aforementioned
                    symbols related to the general formula

Nesosilicates units of independent tetrahedra p:q = 1:4 or (3:12)--basic unit is (SiO₄)^-4 or (Si₃O₁₂)^-12 4 oxygens on each unit are free to connect with cations of other polyhedra and in turn more independent tetrahedra units can connect with these polyhedra because the SiO₄tetrahedra are independent, the crystal habit of minerals is equidimensional and pronounced cleavage is absent

Sorosilicates units of two tetrahedra sharing one common oxygen p:q = 2:7--basic unit is (Si₂O₇)^-6 6 oxygens on each unit are free to connect with cations of other polyhedra and in turn more of these soro- units can attach to these polyhedra

Cyclosilicates closed ring units of tetrahedra each sharing 2 oxygens p:q = 1:3 (6:18)--basic unit is (Si₆O₁₈)^-12 12 oxygens on each unit are free to connect with cations of other polyhedra and in turn more of these ringed units can attached to these polyhedra atoms and ions can be trapped within the open spaces of the rings

Inosilicates continuous single chain units of tetrahedra each sharing 2 oxygens p:q = 1:3 (1:3 or 2:6)--basic unit is (SiO₃)^-2 or (Si₂O₆)^-4 single chained units are connected by other polyhedra units--cleavage along the connected polyhedra forms the characteristic 90 degree 2 directional cleavage typical in this subclass

Inosilicates continuous double chain units of tetrahedra each sharing 2 and 3 oxygens alternately p:q = 4:11 (4:11 or 8:22)--basic unit is (Si₄O₁₁)^-6 or (Si₈O₂₂)^-12 double chain units are separated by connected polyhedra units--cleavage along connected polyhedra forms the characteristic 120--60 degree cleavage typical in this subclass ions and atoms can be trapped between open spaces between tetrahedra

Phyllosilicates continuous sheet units of tetrahedra each sharing 3 oxygens p:q = 2:5 (2:5 or 4:10)--basic unit (Si₂O₅)^-2 or (Al Si₃O₁₀)^-5 where Al substitutes for Si -sheet units are separated by connected polyhedra units--cleavage along connected polyhedra forms the characteristic perfect cleavage in one direction (sheet cleavage) typical in this subclass atoms and ions can be trapped in open spaces between tetrahedra units

Tectosilicates continuous framework of tetrahedra each sharing all 4 oxygen atoms--large substitutions of Al⁺³ for Si⁺⁴ in the tetrahedron allows polyhedra to connect to oxygens in the tetrahedra with both Al and Si bonds since the sum of e.v.of Al and Si will not equal the valence of oxygen p:q = 1:2, 2:4, 4:8, 6:12--basic units can be (SiO₂), (AlSiO₄)^-1, (Al₂Si₂O₈)^-2, or (Al₂Si₄O₁₂)^-2

D. Some Mineral Groups and Series in the Silicate Subclasses -part of the material you will be tested on in lab # 5 will be the identification by name of silicate minerals underlined below using hand specimens--information given below for the minerals and additional information for the same treated in the Mineral Classification and Physical Properties sections will be material subject for examination in lecture exam #4 1. Nesosilicates olivine solid solution series--important in igneous rock forming processes garnet isomorphic group--often occur as dodecahedron crystals- pyrope, almandine, grossularite--abundantly found in metamorphic rocks zircon can form metamict structure kyanite belongs to a polymorphic group (Al₂OSiO₄) topaz staurolite can form cross twinning 2. Sorosilicates hemimorphite-- often found as bladed crystals epidote forms an isomorphic group--important rock forming mineral allanite can form metamict structure--characteristally black with no cleavage 3. Cyclosilicates ( hardness is high, comprises many gemstone varieties and poor cleavage is abundant) beryl gemstone varieties include: emerald (deep green and transparent); aquamarine (pale- greenish-blue transparent); morganite (rose transparent) cordierite often displays dichroism tourmaline (commonly shorl or the black variety) gemstone varieties include: rubellite (red-pink); indicolite (dark blue) 4. Inosilicates (includes the pyroxene group--single chain minerals lacking (OH)_xand amphibole group--double chain with (OH)_xpresent)--- minerals in both groups both are very important rock forming minerals pyroxene group displays essentially 2 directional 90 degree cleavage enstatite-ferrosilite solid solution series diopside-hedenbergite solid solution series augite common rock forming mineral spodumene is an important source of lithium and kunzite is the gemstone variety pyroxenoid group--minerals commonly display splintery cleavage wollastonite amphibole group displays essentially 2 directional cleavage at 124-56 degrees tremolite-actinolite solid solution series hornblende common rock forming mineral 5. Phyllosilicates (includes the clay and mica minerals-perfect 1 directional cleavage with sheet structure)--very important rock forming minerals *serpentine group* of polymorphs--little or no ionic substitution between Al and Si serpentine (massive) crysotile is the fibrous or asbestos variety of serpentine *clay group* consists of hydrous aluminum layered silicates--little or no substitution of Al-Si kaolinite talc mica group form in thin sheets and alot of ionic substitution of Al⁺³ and Si⁺⁴ muscovite light colored and an important rock forming mineral biotite black or dark colored and an important rock forming mineral lepidolite lilac to pink in color and a source of lithium chlorite green and soft 6. Tectosilicates ( framework structure in which there is great amounts of Al-Si substitution) very important rock forming minerals SiO2 polymorphic group consists of many varieties of quartz, a very important rock forming mineral smoky quartz, amethyst (gemstone), rose quartz, tiger's eye ( gemstone), jasper, chalcedony, opal (gemstone), flint-chert *K-feldspar polymorphic group* orthoclase is an important forming rock forming mineral microcline 1 Pb⁺² substitutes for every 2 K⁺¹ representing an omission solid solution causing a blue green color in the mineral plagioclase feldspar solid solution series have twinning striations and are important rock forming minerals albite labradorite commonly displays labradorescence anorthite *feldspathoid group* minerals similar to feldspars but contain about 2/3 the amount of silica and form a silica deficient magma leucite (2)KAlSi₂O₆ = K₂O + Al₂O₃ + 4SiO₂ (leucite) compared to (2)KAlSi₃O₈ = K₂O + Al₂O₃ + 6SiO₂ (orthoclase) sodalite zeolite group hydrous silicates displaying ionic exchange and absorption properties--these can act as water softeners by exchanging Na⁺¹for Ca⁺² in solution--the following is an example of water softening: Na₂Al₂Si₃O₁₀.2H₂O(natrolite) forming CaAl₂Si₃O₁₀.2H₂O stilbite commonly occurs in tabular or sheaf like aggregates CLICK HERE FOR MORE ON SILICATES

Symbol in formula	Ion	C.N. with O

X	K⁺¹	8-12
X	Na⁺¹, Ca⁺²	6-8

Y	Mn⁺², Fe⁺², Mg⁺²,	6
	Fe⁺³, Ti⁺⁴, Al⁺³	6

Z	Si⁺⁴, Al⁺³	4