Systematic Classification of Silicate Minerals Dr. James Martin-Hayden, EEES-1010 Silicate Silica Content Structure (%SiO2) Isolated Silicates Single Chain Silicates Double Chain Silicates Sheet Silicates Example Mineral Olivine Amphibole◊ Group (e.g., Hornblende) Mica Group (e.g., Biotite◊) (e.g., Muscovite◊) content ‡ 25-50% 18-26% 0-16% 50-60% ‡ 15-22% 0-9% 39% 39% ‡ 18-33% 0% 6% 20% 0% 21% 0% 0% 10-19% 10% † 0% (†) † † † Clay Group (e.g., Kaolinite) Framework Silicates Fe/Mg‡ Content ‡ ~50% ~16% Pyroxene# Group (e.g., Augite) 46% † Feldspar Group, ◊ ‡Ferromagnesians Al† # Plagioclase(Na -Ca ) Orthoclase (K◊) Quartz (pure SiO2◊) 43-69% 76% 100% 0% 0% Cleavage Density †Alluminosilicates Hardness Color Occurrence none Comprises ~4 g/cm3 most of the green Mantle 6½ 2 (90o) ~3.3 Found in basalt Oceanic Curst 5½ black with Ca-Plag.# o o 2 (120 -60 ) ~3.3 Found in Continental 5½ black Crust◊ ~3.0 Found in 1 (perfect) black Continental ◊ 2½ 2.8 slvr Crust Microscopic 2.6 From chemical platelets green weathering of or gray silicates* 2 (90o) 2.6 Ca Plag.# in wht-blk Oceanic Crust 6 2.7 pink Others Contin◊. Concentrated none (fracture) 2.6 white in Continental 7 to gray Environments Isolated Silicates Single Chain Silicates Double Chain Silicates Sheet Silicates General trends toward bottom: Increasing SiO2, Decreasing Fe/Mg, Lighter in density, Lighter in Color Other Trends #Oceanic crust (basalt) is mostly pyroxene and Ca-plagioclase (Ca-Feldspar) and thus more dense and dark. ◊Continental crust (e.g., granite) contains minerals rich in silica, low in iron, and thus less dense and light in color. Melting points (and crystallization temperature) decrease toward bottom. Framework Silicates (e.g., quartz melts first and is the last to crystallize, ~700oC. Pyroxene and Ca-plagioclase ~1000oC) Rate of chemical weathering decreases toward bottom of the chart or Resistance to chemical weathering increases toward bottom. Chemical (e.g., quartz is extremely resistant to chemical weathering) Weathering *All silicates (except quartz) are converted to clay minerals by chemical weathering.