Kupletskite polytypes from the Lovozero massif, Kola Peninsula, Russia: Kupletskite-1A and kupletskite-Ma2b2c

Authors: Piilonen, Paula C.; McDonald, Andrew M.; Lalonde, Andrée E.

Source: European Journal of Mineralogy, Volume 13, Number 5, 1 September 2001 , pp. 973-984(12)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

The crystal structures of two kupletskite [K₂NaMn₇Ti₂Si₈O₂₆(OH)₄F] polytypes, (1) triclinic, space group P1, and (2) monoclinic, space group C2/c, from nepheline syenite pegmatites of the Lovozero massif, Russia, have been determined. The two structures differ in the stacking sequence of an identical HOH layer (where H = heterophyllosilicate and O = octahedral): in the triclinic structure, individual HOH layers are related by a centre of symmetry, resulting in a +HOH/+HOH/+HOH stacking sequence with a one-layer period; in monoclinic kupletskite, adjacent HOH layers are related by a two-fold axis parallel to b, resulting in a +HOH/-HOH/+HOH stacking sequence with a two-layer period. The resultant relationship between the unit cell parameters of monoclinic kupletskite-Ma2b2c and triclinic kupletskite-1 A are as follows: a_monoclinic = a_triclinic, b_monoclinic = 2(bsinγ)_triclinic, and c_monoclinic spans two HOH layers. These results represent the first single-crystal Xmonoclinic triclinic monoclinic ray structure determination of polytypes in the astrophyllite group.

Keywords: ASTROPHYLLITE GROUP; KUPLETSKITE; POLYTYPE; LOVOZERO; HOH LAYER

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2001/0013/0973

Publication date: 2001-09-01

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