Wells u3a: Classification of Minerals

Classification of Minerals

Outline lecture notes [JA]**

minerals classified by their anionic (-ve) component
divided into silicates and non-silicates
mantle and crust all made from silicates, so most important by volume
symbols used below: Si = Silicon, O = Oxygen, Ca = Calcium, Mg = Magnesium, Na = Sodium, K = Potassium, Al = Aluminium, Fe = Iron

Silicates

building block of silicates is silicon tetroxide (compound anion, with 4 -ve charges)
oxygen atoms at corners of a tetrahedron, silicon at centre. These form a framework, with cations acting as "glue"
only 6 important cations: Ca, Mg, Na, K, Al, Fe.
rock-forming silicates divided into ferromagnesian and non-ferromagnesian. Former contain Fe and Mg and so are denser, latter don't
ferromags divided into olivine, pyroxine, amphibole and mica groups
non-ferromags divided into feldspar, mica and quartz
cations can substitute for each other as long as roughly the same size (within 15%) - indicated by parentheses in chemical formula, where any combination is permissible.
also classified by structure: independent tetrahedral, single chain, double chain, sheet, 3-D or network. This determines their physical properties.
usually, need cations to neutralise silicate charge, but quartz is special case - charges neutralised internally, so no cations - unique properties result from this.

Non-silicates

only 10% by volume of earth's crust, but often important even so
haematite (iron oxide) got us out of the Stone Age
calcite (calcium carbonate) formed shells, left fossils. Nowadays, coral, clams, oysters, snails all have shells made of Ca carbonate. Limestone ground to fine powder is used in agriculture (lime) and construction (concrete).
halite (sodium chloride, or common salt) is required for cells to function
sulphide minerals important as ores: zinc for galvanising steel, lead for batteries.

**more detailed lecture notes available to members on request

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