Crystal structure of kapitsaite- (Y), a new borosilicate isotypic with hyalotekite - Crystal chemistry of the related isomorphous series

摘要

Kapitsaite- (Y), (Ba,K,Pb~(2+) ,Na)_4(Y,Ca,REE)_2[Si_8B_2(B, Si)_2O_(28)F], is the Y dominant equivalent of hyalotekite and occurs in peralkaline pegmatites of the Dara-i-Pioz alkaline massif, Tajikistan. The crystal structure of kapitsaite-(Y) has been refined from single-crystal and powder X-ray diffraction data [MoK#alpha# radia-tion; a = 11.181(4), b = 10.850(7), c = 10.252(4) A, x = 90.64(6), 13 = 90.05(4), y= 89.97(7) deg, V 1,243.6(9)A~3, Z = 2; space group I1]. Kapitsaite-(Y) is isotypic with hyalotekite but with substantial differences in the cation composition: (i) Y (+REE) dominates over Ca in the [81-coordinated M site; (ii) Ba is nearly the only cation filling two split sites; (iii) B fills fully a tetrahedral site and 85 percent of a second site. The structure of hyalotekite has been described as a low-temperature/low-symmetry derivative stabilized by a stereoactive lone-pair effect of Pb~(2+), but the occurrence of the same structure in kapitsaite-(Y), which has a very low content of Pb~(2+), shows that this type of structure is independent from a lone-pair effect. Kapitsaite - (Y) and hyalotekite are members of a complex isomorphous series, which involves 5 inde-pendent crystallographic sites and at least 10 major cations (Ba, Pb~(2+), K, Na, Y, Ca, REE, Si, B, Be). In principle, other members could exist, both with reference to the content of the M site, which discriminates the two known species, and to other sites.