Kupletskite polytypes from the Lovozero massif, Kola Peninsula, Russia: Kupletskite-1A and kupletskite-Ma2b2c
Piilonen, Paula C.; McDonald, Andrew M.; Lalonde, André e E.
European Journal of Mineralogy Volume 13 Number 5 (2001), p. 973 - 984
published: Sep 27, 2001
ArtNo. ESP147051305016, Price: 29.00 €
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: amonoclinic = atriclinic, bmonoclinic = 2(bsinγ)triclinic, and cmonoclinic spans two HOH layers. These results represent the first single-crystal Xmonoclinic triclinic monoclinic ray structure determination of polytypes in the astrophyllite group.