CRYSTAL-STRUCTURE OF KUPLETSKITE FROM THE KANGERDLUGSSUAQ INTRUSION, EAST GREENLAND

Kupletskite is the Mn-rich member of the astrophyllite group with the generalized formula (K,Na)(3)(Mn, Fe)(7)Ti2Si8O26(OH, F)(5). It occurs as anhedral to subhedral aggregates in the alkaline pegmatitic bodies cutting the pulaskites in the Kangerdlugssuaq intrusion, East Greenla nd. The mineral is transparent, orange-brown and shows a perfect cleav age after {001}. Kupletskite is triclinic with space group <P(1)over b ar>, a = 5.4035(11), b = 11.958(2), c = 11.724(2) Angstrom, alpha = 11 3.14(3)degrees, beta = 94.74(3)degrees, gamma = 103.10(3)degrees, Z = 1. Microprobe analysis gave (K1.74Na1.04Ca0.36)(Sigma 3.14) (Mn3.67Fe2 .29Mg0.70)(Sigma 6.66) (Ti1.82Zr0.17Nb0.06Ta0.01)(Sigma 2.06) (Si7.78A l0.27)(Sigma 8.05)O-26 (OH4.17F0.83)(Sigma 5). Single-crystal X-ray da ta were collected, and the structure was refined to R(F) = 4.7%. Kuple tskite is isostructural with astrophyllite, consisting of an octahedra l sheet sandwiched between two heterogeneous silicate-sheets. The Mn s ubstitution for Fe gives rise to minor differences in the misfit betwe en the octahedral sheet and in the heterogeneous sheet.