Ba-rich celestite: 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 powderdata, single crystal X-ray refinement, thermal and chemical a!1alyses. The unit cell parameters (obtained bysingle 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 withan occupancy of Sr 87% and Ba 13%. Individual bond lengths (Sr,Ba)-O and bond strength calculationsconfirm that all twelve interactions are significant and define an irregular array around the cation.