Silicate Minerals: Introduction ESS 103 May 2, 2006 Anastasia Chopelas
Use of structural units • Tetrahedra (Silicon, Aluminum) • Octahedron (divalent Fe, Mg, Ca, Mn) • Dodecahedron (divalent Fe, Mg, Ca, Mn in certain structures such as garnets)
Silicate Mineral Classification Silicates are classified on the basis of Si­O polymerism: how does the silica tetrahedron appear in the mineral
Neso­, Soro­ and Ring Silicates Isolated tetrahedra, dimers and rings of SiO 4 ’s [SiO 4 ] 4­ Independent tetrahedra Nesosilicates Examples: olivine garnet [Si 2 O 7 ] 6­ Double tetrahedra Sorosilicates Examples: tanzanite n[SiO 3 ] 2­ n = 3, 4, 6 Cyclosilicates Examples: benitoite BaTi[Si 3 O 9 ] axinite Ca 3 Al 2 BO 3 [Si 4 O 12 ]OH beryl Be 3 Al 2 [Si 6 O 18 ]
Pyroxenes and Amphiboles Single chains and double chains of SiO 4 ’s [SiO 3 ] 2­ single chains tetrahedra pryoxenes pyroxenoids Inosilicates [Si 4 O 11 ] 4­ Double amphiboles
Phyllosilicates Sheets of SiO 4 ’s linked together as hexagonal units [Si 2 O 5 ] 2­ Sheets of tetrahedra micas talc clay minerals serpentine
Phyllosilicates Tectosilicates 3 dimensional frameworks of SiO 4 units low­ quartz
low ­quartz [SiO 2 ] 3­D frameworks of tetrahedra: fully polymerized Tectosilicates quartz and the silica minerals feldspars feldspathoids zeolites Nesosilicates • Several important gem minerals are in this class including: – Olivines (Peridot) – Garnets – Zircon – Topaz – Kyanite – Andalusite
Olivine Mg 2 SiO 4 = 2MgO + SiO 2 Contains isolated silica tetrahedra plus edge linked magnesia octahedra:
Nesosilicates: independent SiO 4 tetrahedra b c projection
projection Olivine (100) view blue = M1 yellow = M2 How does the microscopic translate to the macroscopic crystal? Not often seen in
this form usually:
Gemstone Peridot Birthstone for August or Libra Emerald was Cleopatra’s favorite stone, but it turned out to be Peridot. Famous locations for Peridot is Bamble Norway and San Carlos Arizona, also well studied scientifically because of its nearness to mantle composition of 88% Mg and 12% iron. Also found in Burma and islands in the Red Sea. Found in meteorites and moon samples. Mentioned in the Bible as Chrysolite.
Related minerals: • Monticellite – when calcium substitutes for half of the magnesium. Usually white: • Fayalite: iron end member. Iron and magnesium mixes readily because of their similar sizes. Black.
Nesosilicate occurrences: Olivine (Mg,Fe) 2 SiO 4: – Principally in mafic and ultramafic igneous and meta­igneous rocks – Fayalite in meta­ironstones and in some alkalic granitoids – Forsterite in some siliceous dolomitic marbles – Meteorites, moon Monticellite CaMgSiO 4 Ca ® M2 (larger ion, larger site) High grade metamorphic siliceous carbonates
Garnets, Nesosilicates Occur in all terraines. Will crystallize with clear crystal faces even in solid rock, particularly metamorphic terraines where it is surrounded by micas. General formula is X 3 Y 2 Si 3 O 12 X is Mg, Fe, Mn or Ca (divalent) There is little intermixing between Ca and the other three elements in the X site Y is Al, Fe, Cr (trivalent)
Structure of Garnet Consists of • Corner linked Si­ tetrahedra (red) + • Y 3+ Octahedra (green), Y is Al, Fe, Cr • Interstices have 8 fold coordinated X 2+ (dodecahedra ­ blue) where X is Mg, Fe, Mn, Ca
Garnet Polyhedra
Polyhedral view of garnet structure Garnet: A 2+ 3 B 3+ 2 [SiO 4 ] 3 Occurrence: Mostly metamorphic Pyralspites in meta ­shales shales Pyralspites in meta­ Ugrandites in meta ­carbonates carbonates Ugrandites in meta­ Some high
Some high ­­ Al igneous Also in some mantle peridotites
peridotites Garnet (001) view blue = Si purple = A turquoise = B Garnet forms crystals readily, can you see its cubic form? Uvarovite Grossular almandine andradite pyrope spessartine
Garnets Pyralspite – (red) Pyrope Mg 3 Al 2 Si 3 O 12 Ugrandite (green) Grossular Ca 3 Al 2 Si 3 O 12 Almandine Fe 3 Al 2 Si 3 O 12 Andradite Ca 3 Fe 2 Si 3 O 12 Spessartine Mn 3 Al 2 Si 3 O 12 Uvarovite Ca 3 Cr 2 Si 3 O 12
Depending on where they appear in the periodic table, atoms and ions will have different sizes: General rule: Group I elements the largest to Group VII the smallest. General rule: The fewer electrons, the smaller.
Aluminosilicates • Group of 3 mineral polymorphs all with the formula Al 2 O 3 + SiO 2 = Al 2 SiO 5 • Generally not used as gems but two have very distinctive effects that are beautiful • The phase diagram that contains all of them help us determine the temperature and pressure of the environment • Usually found in metamorphic terraines.
Al 2 SiO 5 ­ Kyanite
Al 2 SiO 5 ­ Andalusite
Al 2 SiO 5 ­ Sillimanite
Why one rather than another?
Al 2 SiO 5 Comparison • Density A(3.13)<S(3.23)<K(3.53) • Refr Index A(1.62)<S(1.65)<K(1.71) • Bulk Modulus A(151)<S(171)<K(193) • S has tetrahedral Al • A has 5­coord Al • K has all octahedrally coord Al
Topaz • Edge sharing octahedral units that are bound together by the isolated tetrahedral units
Obviously hexagonal • This one inch long hexagonal prism was on the market for $200. Comes in many colors, brown, yellow, pink, blue. Here color is what makes the price. • Topaz was thought to be able to prevent sudden death, cure madness, and improve vision • Ancient gem like ruby
Zircon ZrSiO 4 • December Birthstone • Diamond similant, similar fire and index as diamond, can fool even experienced jewelers. • NOT cubic zirconia, which has no silicon. • Tetragonal, nearly cubic. • Comes in many colors, especially brown, green. • Heat treatment can form blue, golden, and colorless forms
Zircon Structure • Rotation of this structure can be found at http://socrates. berkeley.edu/~ eps2/wisc/geo3 60/zircon.mov