Crystal chemistry and nomenclature of the lovozerite group

摘要

The paper summarizes crystal-chemical data and describes the IMA-accepted nomenclature of lovozerite-group minerals (LGM). The lovozerite group includes nine zeolite-like cyclosilicates with the general formula A(3)B(3)C(2)MSi(6)O(12)O(6-x)(OH)(x)center dot nH(2)O, with species-defining M = Zr, Ti, Fe3+, Ca; C = Ca, Mn2+, Na, square; A = Na, Ca; B = Na, square; 0 <= x <= 6; n = 0-1. Their structures are based upon a heteropolyhedral framework consisting of rings of Si-centred tetrahedra and M-centred octahedra forming a 3D system of channels that host A, B, and C cations. The structures can be also considered as based upon pseudocubic modules centred at the midpoint of the Si tetrahedral ring. The M, A, and B cations are located at the borders of the module, whereas C cations are inside the module. The modules are stacked in three different arrangements in LGM allowing distinction of three subgroups: (1) zirsinalite-lovozerite subgroup (includes cation-saturated combeite, kapustinite, kazakovite and zirsinalite, and cation-deficient litvinskite, lovozerite and tisinalite), (2) koashvite subgroup (incl. koashvite) and (3) imandrite subgroup (incl. imandrite). The nature of cation-deficient LGM is discussed. The calculation scheme for empirical formulae of LGM and the criteria of definition of a mineral species (end-members) in the group are given.