Abstract:
The crystal structures of two kupletskite [K2NaMn7Ti2Si8O26(OH)(4)F] polytypes, (1) triclinic, space group PT, 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-1A are as follows: a(monoclinic) = a(triclinic), b(monoclinic) = 2(bsin gamma)(triclinic), and c(monoclinic) spans two HOH layers. These results represent the first single-crystal Xray structure determination of polytypes in the astrophyllite group.
