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Crystal chemistry and nomenclature of the lovozerite group

¹ Faculty of Geology, Moscow State University, Vorobievy Gory, 119992 Moscow, Russia
² Department of Crystallography, Faculty of Geology, St. Petersburg State University, University emb. 7/9, 199034 St. Petersburg, Russia
³ Geological Institute of Kola Science Center of Russian Academy of Sciences, Fersman str. 14, 184209 Apatity, Russia
⁴ Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestr.3, 3012 Bern, Switzerland

^* Corresponding author, e-mail: igorpekov{at}mail.ru

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 A₃B₃C₂MSi₆O₁₂O_6x(OH)_x·nH₂O, with species-defining M = Zr, Ti, Fe³⁺, Ca; C = Ca, Mn²⁺, Na, ; A = Na, Ca; B = Na, ;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.

Key-words: lovozerite group, cyclosilicate, combeite, imandrite, kapustinite, kazakovite, koashvite, litvinskite, lovozerite, tisinalite, zirsinalite, crystal structure, nomenclature of minerals.