CRYSTAL-CHEMISTRY OF LEAD ALUMINOSILICATE HOLLANDITE - A NEW HIGH-PRESSURE SYNTHETIC PHASE WITH OCTAHEDRAL SI

Single crystals of lead aluminosilicate hollandite, with composition Pb0.8Al1.6Si2.4O8, have been synthesized at 16.5 GPa and 1450 degrees C. Crystals are tetragonal [space group I4, Z = 2, a = 9.414(3), c = 2.750(3) Angstrom, V = 243.7(3) Angstrom(3)]. Si and Al are disordered on the octahedral site. Pb is best modeled by two atom sites, one on the fourfold axis and the other split, lying off the fourfold axis. This feature was predicted by Post and Burnham (1986) but not previously observed.