The crystal structures of two kupletskite [K2NaMn7Ti2Si8O26(OH)4F] polytypes, (1) triclinic, space group P1̄, 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: amonoclinic = atriclinic, bmonoclinic = 2(bsinγ)triclinic, and cmonoclinic spans two HOH layers. These results represent the first single-crystal X-ray structure determination of polytypes in the astrophyllite group.

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