SILICATES     I. SILICATE MINERALS--General Information A. > 90% of all "rock forming" minerals B. Contain silicon, oxygen, and one or more cations C. Si, O are bonded in the simplest geometric form that has planar faces and completely encloses space D. Bonding forms include: cube, pyramid, tetrahedron II. SILICON-OXYGEN TETRAHEDRON (SiO4) A. One "small" Si ion surrounded by four "large" oxygen ions, forming a tetrahedron B. Strong covalent bonds C. Silicon tetrahedron (SiO4)-4 1. Si has valence of +4 (ionic state is +4) 2. Oxygen has valence of -2 3. Overall charge of tetrahedron is -4 1 Si+4 =  +4 4 O-2  =  -8                 -4 4. Because of -4 charge, the tetrahedron must combine with cations to satisfy negative charge imbalance a. Example 1 Mg+2 combines to form the stable mineral forsterite (Mg2SiO4)  2Mg+2 = +4 two cations  1(SiO4)-4 = -4 single anion      0   b. Example 2 Fe+2 has the same +2 charge and combines to form the stable mineral fayolite (Fe2SiO4)  2Fe+2 = +4 two cations  1(SiO4)-4 = -4 single anion      0   III. IONIC SUBSTUTION A. Ions with similar charges and sizes will freely substitute in a crystal structure 1. Transparency B. Solid solution 1. Single crystalline phase that may vary in composition within a specific range 2. Occurs commonly in nature 3. Example--the olivine series  Fosterite   Fayolite  Mg2SiO4 <--------X--------> Fe2SiO4 100% Mg, 0% Fe   0% Mg, 100% Fe a. The chemical formula for the olivine series is (Mg,Fe)2Si04. Parentheses mean it is a solid solution between iron and magnesion which can substitute for one another b. Can write the formula for olivine in terms of decimal places such an X, above = (Mg.25, Fe.75)2SiO4 c. Need a computer to balance mineral equations d. Physical properties of the mineral vary with chemical composition e.g. Density Fe = 7.86 gm/ml while density Mg = 1.74 g/ml Therefore, the more Fe in olivine, the more dense the mineral 4. Apatite--another solid solution 1. (Ca5(F,Cl,OH)PO4)3--what teeth are made of 2. A solid solution of (F-, Cl-, OH-), each of which will freely substitute. Floride toothpaste tries to force flouride into our teeth, making them harder. IV. COMMON TETRAHEDRAL ARRANGEMENTS  Drawing Si:0 Arrangement of tetrahedra Characteristics  Example    1:4  isolated  blocky, equidimensional, lacks cleavage Olivine (Mg, Fe)2SiO4    1:3 single chain  needlelike, cleavage at 90 o Pyroxene (Ca, Mg)Si2O6    4:11 double chain  bladed, good cleavage in two directions (60 and 120 degrees) Amphibole (Mg, Fe)7(Si8O22)(OH)2   2:5 continuous sheets  platy, perfect cleavage in 1 direction Kaolinite Al4(Si4O10)(OH)8    1:2 3D framework poor cleavage, blocky crystals quartz SiO2 V. COMMON ROCK FORMING SILICATE MINERALS A. Olivine Fe, Mg B. Pyroxene Fe, Mg, Ca C. Amphibole Fe, Mg, Ca, K, Al D. Biotite Fe, Mg, K, Al E. Muscovite K, Al F. Feldspars K, Al, Ca, Na G. Quartz VI. FERROMAGNESIAN MINERALS A. Minerals with Fe and Mg 1. Dense 2. Dark color B. Examples 1. Olivine Fe, Mg 2. Pyroxene Fe, Mg, Ca 3. Amphibole Fe, Mg, Ca, K 4. Biotite Fe, Mg, K, Al VII. NONFERROMAGNESIAN MINERALS A. No Fe or Mg 1. Light colored or transparent 2. Less dense than ferromags 3. Relatively complex silicate structures when compared to ferromags (Si:0 = 1:2) B. Examples 1. Muscovite K, Al 2. Quartz 3. Feldspars K, Al, Ca, Na a. Common mineral group in Earth's crust (two groups) b. Potassium feldspar (or orthoclase) = K(AlSi3O8) c. Plagioclase feldspar, which forms a special type of solid solution series known as a coupled solution of Na+ and Ca+2 with Si+4 and Al+3 between two end members Albite     <------------>    Anorthite      Na(AlSi3O8) <---------->   Ca(Al2Si208)