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GNEISS
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A medium to coarse grained metamorphic rock which is usually grey or pink in colour. It's alternating dark and light banding makes it useful as a gemstone. It may also used to make decorative slabs. Gneiss gets its name from the German word gneista which means to give off sparks.
It is principally made up of feldspars, mica and quartz but may also contain some accessory minerals depending on the conditions under which it is formed e.g. apatite, epidote, garnet, hornblende, pyrite, magnetite and others. Gneisses may be named after their parent material (e.g. granite gneiss), an accessory mineral (e.g. staurolite gneiss) or their appearance (e.g. pencil gneiss).
It is formed by the metamorphism of sedimentary rock at very high pressures and temperatures (approx. 700 degrees centigrade). At even higher temperatures and pressures melting might occur and the rock would be classed as an igneous formation. Although gneiss is the product of metamorphism some deposits are primary formations resulting from crystallized magma.
It can be found in France, Italy, Sri Lanka, US and the Black Forest (Germany).
The most common intrusive igneous rock. It's essential components include quartz, feldspars, hornblende and biotite.
It is coarse grained and has chemical characteristics between granite and diorite, hence it's name. Since these rocks are so similar in appearance chemical analysis may be required to determine the difference. Grandiorite contains more silica than diorite but less than granite (granite consists of more that 65% silica). Grandiorite also contains less alkali feldspar than granite, which is why it is usually grey or white rather than pink.
Grandiorite contains higher concentrations of calcium and magnesium bearing minerals such as biotite and hornblende than granite. It may also contain some accessory minerals (e.g. apatite, magnetite or zircon) and some inclusions of muscovite or pyroxenes.
It may be used as a building material and for decorative slabs.
Grandiorite may be found in Leicestershire (England), Ireland, Scotland and Japan.
