The chevkinite group: underestimated accessory phases from a wide range of parageneses

Chevkinite-group minerals are widespread in a very wide range of igneous and metamorphic parageneses, forming important components of accessory mineral assemblages. Their presence in a rock may be difficult to establish by standard optical techniques, which has contributed to their importance being underestimated; a combination of SEM and EMPA is recommended here. Currently, there are eleven IMAapproved members of the group but undoubtedly several more will be described in the near future. There is considerable compositional variation in the group, which can be expressed as:

REE + M²⁺_C + M³⁺_C = Ca²⁺ _A + Sr + Ti⁴⁺_C + Zr⁴⁺_C

where A and C are structural sites. Chevkinite-group minerals strongly fractionate geochemically coherent pairs, such as LREE-HREE, Nb-Ta, Zr-Hf and Th-U, and thus play a critical role in geochemical modelling.