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+4sharing 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 = 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
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
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

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

C. (Structural) Subclasses of Silicates 
     -the divisions of subclasses are based on the degree or kind of polymerization---the following describes tetrahedra linkages of the basic unit for each subclass in the silicate class 

  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 SiO4 tetrahedra are independent, the crystal habit of minerals  
   is equidimensional and pronounced cleavage is absent
silineso.jpg (6455 bytes)

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  
silisoro.jpg (2410 bytes)

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
silicyclo.jpg (6273 bytes)

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
siliino1.jpg (4972 bytes)

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
siliino2.jpg (9453 bytes)

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  
siliphyllo.jpg (11209 bytes)

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
silitecto.jpg (7085 bytes)


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)
           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)
               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)x and amphibole
           group--double chain with (OH)x  present)--- minerals in both groups both are very important 
            rock forming minerals
            pyroxene group displays essentially 2 directional 90 degree cleavage
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

           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
         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 quartzamethyst (gemstone),  rose quartztiger'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
                  labradorite commonly displays labradorescence
         feldspathoid group minerals similar to feldspars but contain about 2/3 the amount of silica
                                        and form a silica deficient magma
                     (2)KAlSi2O6 = K2O + Al2O3 + 4SiO2 (leucite)
                                         compared to
                     (2)KAlSi3O8 = K2O + Al2O3 + 6SiO2 (orthoclase)
         zeolite group hydrous silicates displaying ionic exchange and absorption properties--these
                               can act as water softeners by exchanging Na+1 for 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

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