Ba-rich celestine: new data and crystal structure refinement

A Ba-rich celestine (SrOX7BaO13S04) filling cavities of volcanoclastic rocks from Montecchio Maggiore (Vicenza, lIaly) was studied. The role of the Ba content in the mineral was determined using X-ray powder data, single crystal X-ray refinement, thermal and chemical a!1alyses. The unit cell parameters (obtained by single crystal diffraction) are a = 8.408, b = 5.372, c = 6.897 A, and the refinement in the space group Pnma (Z 4) gives the final R value of 0.039. The average (Sr,Ba)-oxygen bond length is 2.842 A and agrees with an occupancy of Sr 87 % and Ba 13%. Individual bond lengths (Sr,Ba)-O and bond strength calculations confirm that all twelve interactions are significant and define an irregular array around the cation. KEYWORDS:celestine, barium, crystal structure refinement, Vicenza, Italy. occurs as euhedral to subhedral crystals, as crystal fragments and as granular aggregates. It is white or light blue in colour and closely associated with analcite, natrolite, celadonite and apophyllite. Recently, a number of samples or celestine hav