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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

View larger version (23K): [in this window] [in a new window]   Fig. 1. Lovozerite-type ring [Si6O12(O, OH)6]; small circles are O atoms forming the bridges Si–O–Si and Si–O–M, large circles in "pendent" vertices of Si tetrahedra represent (O, OH), large circles connected with small circles are M cations.

View larger version (48K): [in this window] [in a new window]   Fig. 2. Structure of lovozerite (after Yamnova et al., 2001a): Si tetrahedra, M octahedra, C (small circles), A (medium circles) and B (large circles) sites are shown.

View larger version (52K): [in this window] [in a new window]   Fig. 3. Pseudocubic (a' 7.5 Å) module in the lovozerite structure-type. The arrow along the space diagonal in the cube represents a three-fold axis. A sites are on the cube edges, B sites centre the cube faces and C sites are on the three-fold axes.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Four types of arrangements of cubic modules in the lovozerite structure-types: (a) in trigonal members; (b) in imandrite; (c) in koashvite; (d) in synthetic Na5(Na0.5+xCa0.5–x)2(NdxCa1–x)2(Si6O18).

View larger version (15K): [in this window] [in a new window]   Fig. 5. Linkage of the lovozerite modules via half-faces (a) may lead either to the formation of the module with M cations at the vertices and at the midpoints of four vertical edges (b, c) or in "transition" (shown by arrows) of the M cations from the two vertices to the midpoint of edge between them (d, e). See text for details.

View larger version (12K): [in this window] [in a new window]   Fig. 6. Location of the A, B, and M cations relative to the outline of the cubic module in minerals of the lovozerite group: (a) the lovozerite-zirsinalite subgroup, (b) the koashvite subgroup and (c) the imandrite subgroup. The insets show the location of the M-type cations.

View larger version (26K): [in this window] [in a new window]   Fig. 7. Octahedral cluster MC2 in the lovozerite-type structure.