MINERAL SPECIES, VARIETIES AND MODIFICATIONS
There are various chemical compounds which can crystallise into more than one type of lattice. These substances thus give rise to more than one mineral. Calcium carbonate, for instance, forms the two distinct minerals calcite and aragonite, and is thus said to be dimorphous. Another well-known dimorphous substance is iron sulphide, which can crystallise either as pyrite or marcasite (Plate 44). Titanium oxide forms three minerals: rutile, anatase and brookite; and another trimorphous mineral is the aluminium silicate which forms the minerals andalusite (Plate 65), kyanite and sillimanite. So the actual mineral species is determined by the atomic structure as well as by the chemical composition of any given substance. In some crystal lattices certain atoms of a given element can be partially or completely replaced by atoms of another element which are of similar ionic size and have the same electric charge. The substitution of atoms does not greatly alter the structure of the crystal lattice, but changes the chemical composition of the mineral.