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Minerals, Silicate Structures
A mineral is naturally occurring inorganic homogeneous solid with a definite (though variable) chemical composition an ordered atomic arrangement
Mineral Classes - Chemical Classification of Minerals (textbook section 2.5) there are about 90 naturally occurring elements on Earth ! you could make a lot of compounds from the multitude of combinations there are presently around 3800 described and named minerals how to learn/understand them all??? Mineral Classes: minerals are grouped into classes with the same anion ! e.g., oxides (O-2^ ), sulfides (S -2^ ), halides (Cl -^ , F-^ , Br-^ , I-^ ), carbonates (CO 3 -2^ ) ! minerals in a class have similar properties (specific gravity, hardness, luster...) however, the minerals in most classes are not abundant the most abundant class, by far, are the silicates (based on silica: cmpd of Si & O) the reason is that Si & O are, by far, the most abundant elements in the crust
The Eight Most Common Chemical Elements in the Crust
! (Klein and Hurlbut, 1985)
Element Chemical Symbol Weight Percent
Volume Percent Oxygen O 46.60 93. Silicon Si 27.72 0. Aluminum Al 8.13 0. Iron Fe 5.00 0. Calcium Ca 3.63 1. Sodium Na 2.83 1. Potassium K 2.59 1. Magnesium Mg 2.09 0.
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Silicate Structures (textbook section 2.5.1, 5.4) the basic building block for silicate minerals is the silica tetrahedron ! 4 oxygen atoms surrounding a silicon atom ! Si-O bond is relatively covalent every silicate mineral, but quartz, is some combination of silica plus metal cations ! (silica plus Al+3, Fe+2, Mg+2, Ca+2, Na+, K+^ …) ! Al replaces a fraction of the Si atoms in the silica tetrahedra in some silicates !! (in which case there is some alumina in addition to silica) !! these silicates (e.g., feldspars) are referred to as aluminosilicates isolated tetrahedra: single SiO 4 tetrahedra bond with metal cations all around ! i.e., covalent tetrahedra are connected to one another with metal cations !! via ionic bonds ! e.g., olivine (Fe,Mg)SiO 4 single chain silicates: SiO 4 tetrahedra form long chains by sharing corner oxygens ! chains are connected to one another with intervening metal cations (ionic bonds) ! e.g., pyroxenes (augite) double chain silicates: SiO 4 tetrahedra form long double chains; more sharing ! double chains are connected to one another with intervening metal cations ! e.g., amphibloles (hornblende) sheet silicates: SiO 4 tetrahedra link together into sheets, sharing corner oxygens ! sheets are connected to one another by intervening layers of cations ! e.g., micas, clays framework silicates: SiO 4 tetrahedra link together into a 3-dimensional framework ! of interconnected tetrahedra with oxygens at all 4 corners shared ! no connecting cations !! (but cations may be placed inside structure with coupled substitution)
Summary of Trends! felsic silicate rocks are rich in silica ! (felsic stands for fel dspar and si lica) mafic silicate rocks are rich in magnesium & iron ! (mafic stands for ma gnesium and f errum, latin for iron) ions (ionic bonds) are more easily weathered than covalently-bonded silica ! more metals (cations) = chemically weathers more rapidly ! more silica = chemically weathers more slowly
isolated tetrahedra - single chains - double chains, sheets framework more cations! < ----------------------------------- >! more silica ! (esp. Fe+2, Mg+2) more ionic! < ----------------------------------- >! more covalent
less resistant to weathering! < ------------ >! more resistant to weathering
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