Crystal chemistry and nomenclature of the lovozerite group

Authors: Pekov, Igor V.; Krivovichev, Sergey V.; Zolotarev, Andrey A.; Yakovenchuk, Viktor N.; Armbruster, Thomas; Pakhomovsky, Yakov A.

Source: European Journal of Mineralogy, Volume 21, Number 5, October 2009 , pp. 1061-1071(11)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

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_6-x(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.

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Keywords: LOVOZERITE GROUP; CYCLOSILICATE; COMBEITE; IMANDRITE; KAPUSTINITE; KAZAKOVITE; KOASHVITE; LITVINSKITE; LOVOZERITE; TISINALITE; ZIRSINALITE; CRYSTAL STRUCTURE; NOMENCLATURE OF MINERALS