SYNTHESIS AND CRYSTAL-STRUCTURE OF FELDSPAR ANALOGS IN BERYLLOSILICATE AND ALUMINUM-COBALT PHOSPHATE SYSTEMS

The hydrothermal syntheses and X-ray crystal structures of two three-d imensional framework structures encapsulating potassium and ammonium c ations are presented. Both K2BeSi7O16 and NH4AlCoP2O8 have a feldspar framework topology. The structure consists of cross-linked, double-cra nkshaft chains of Si4+ (or P5+) and Be2+ (or Co2+ and Al3+) tetrahedra with charge-compensating cations occupying small cavities in the fram ework. The Be2+ and Si4+ sites are indistinguishable, but the strict a lternation between metal-atom sites and phosphorous sites leads to a t wofold supercell for NH4AlCoP2O8. Despite the ubiquity of feldspars, t hese two phases have unprecedented chemical compositions and unique te trahedral atom charge combinations. Crystal data for K2BeSi7O16: M = 5 39.84, space group C2/m, alpha = 8.546(1) Angstrom, b = 12.833(2) Angs trom, c = 7.060(1) Angstrom, beta =116.128(3), V = 695.0(2) Angstrom(3 ), Z = 2, D-c = 2.579 g cm(-3), Mo K alpha, mu = 1.379 mm(-1), 2 theta (max) = 50 degrees, R(F) = 3.22% for 68 parameters and 574 reflections with I > 2 sigma(I). Crystal data for NH4AlCoP2O8: M = 293.89, space group I2/a, a = 8.9598(7) Angstrom, b = 13.257(1) Angstrom, c = 14.700 (1) Angstrom, beta = 115.844(1), V = 1571.5(2))Angstrom(3), Z = 8, D-c = 2.484 g cm(-3), Mo K alpha, mu = 2.719 mm(-1), 2 theta(max) = 50 de grees, R(F) = 4.28% for 123 parameters and 1302 reflections with I > 2 sigma(I). (C) 1998 Elsevier Science B.V. All rights reserved.