In mineralogy, the term ”accessory” refers to a mineral that occurs in a rock in minute quantities, and does not affect the way the rock is named or classified.
In an igneous rock: those minerals occurring in such small quantities that their presence or absence is not significant when considering the mineral composition of the rock for classification purposes. Contrasted with the ‘essential’ minerals for such purposes. ‘The original minerals of the crystalline rocks may be conveniently divided into two groups: those which are always present in every sample of the particular rock [rock-type], which may be called the “essential” constituents; and those which occur with greater or less frequency in addition to the essential ones, which may be called “accessory”.
In a sedimentary rock: for those discrete minerals that occur in very small amounts compared with the few, perhaps one only, that make up almost the entire bulk of the rock.
The accessory minerals (in whatever kind of rock) happen to be mostly heavier than the main constituents and thus provide the ‘heavy minerals’.
Related terms: