VARIETY OF CRYSTAL FORM
The number of crystal forms which any one mineral can produce varies enormously. Some minerals never produce any crystal with shaped faces, and some ateonly rarely found in crystalline form or produce only simple crystals. There are others, again, such as topaz, garnet, potash-feldspar, galena, tetrahedrite, pyrite and copper pyrites, which can give rise to a large number of crystalline shapes. The minerals of hydrothermal origin, that is, those which crystallised from dilute solutions at moderately high temperatures, can occur in a particularly large variety of crystal shapes. Barytes, for example, has over 200 distinct crystal forms; the mineral with the largest number is calcite. Several hundred distinct face positions relative to the crystallographic axes have been observed in this mineral, and the combinations of these faces can produce several thousand separate forms. It is quite common to find one mineral in the same deposit, or even specimen, occurring in several crystal forms.
The actual shape of a mineral is determined by its environment during growth. As in the case of a living being, whose inherited characteristics are to some extent modified by its environment, the shape of a crystal depends not solely on its atomic structure but also on such environmental factors as the temperature, confining pressure and concentration of the crystalline fluid, interference from adjacent crystals and, last but not least, the speed of crystallisation.