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Kupletskite polytypes from the Lovozero massif, Kola Peninsula, Russia

¹ Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, University of Ottawa, Ottawa, Ontario KIN 6N5, Canada
² Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada

e-mail: ppiilon{at}science.uottawa.ca

The crystal structures of two kupletskite [K₂NaMn₇Ti₂Si₈O₂₆(OH)₄F] polytypes, (1) triclinic, space group P, 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 slacking 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 +HOHI/+HOHI/+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 +HOHI/–HOHI/+HOH stacking sequence with a two-layer period. The resultant relationship between the unit cell parameters of monoclinic kupletskite-Ma2b2c and triclinic kupletskite-LA are as follows: a_monoclinic = a_triclinic, b_monoclinic = 2(bsin)_triclinic, and c_monoclinic spans two HOH layers. These results represent the first single-crystal X-ray structure determination of polytypes in the astrophyllite group.

Key-words: Key-words: astrophyllite group, kupletskite, polytype, Lovozero, HOH layer.

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