Topaz is a silicate mineral. It belongs to the orthosilicate group. These are silicates that have isolated silica tetrahedra (isolated from each other like islands in a three-dimensional body). It contains volatile components (F– and OH–) in addition to common chemical elements of most silicate minerals. The chemical formula is Al2SiO4(F,OH)2.
Crystals from Ouro Preto, Brazil. Width of view 30 mm.
Volatiles suggest that topaz must form in an environment that contains lots of them. Such an environment forms when granitic magma is almost completely crystallized. Residual fluids are rich in incompatible and volatile elements which favors the formation of minerals containing them. Such minerals which are frequently associated with topaz are cassiterite (contains tin which isn’t compatible in main magmatic minerals), tourmaline (contains boron and volatiles), apatite (volatiles), lepidolite (lithium), fluorite (fluorine), beryl (beryllium), etc. The rocks that host these sanctuaries for unpopular chemical elements are known as pegmatites. It may also form in cavities of rhyolite (volcanic equivalent of granite) and in greisen (granitic rock intruded by residual magmatic fluids). Common magmatic phases like quartz, feldspar, and mica are also usually present in topaz-bearing rocks.
Topaz is pretty famous mineral because it is extensively used as a gemstone. It could find use as an abrasive (its hardness in the Mohs scale is 8, it is one of the ten defining minerals of the scale) but there are enough much cheaper synthetic alternatives.
Topaz forms elongated prismatic crystals which may be striated (striations parallel to the elongation). It has one good cleavage perpendicular to the striations. It often determines the shape of the crystals which seem to be cut at right angles to the elongation of crystals. It is resistant to the weathering processes and may be therefore found in sand.