Crystal Structures of Ca sub 5 Cr sub 3 O sub 12 and Ca sub 5 Cr sub 2 SiO sub 12 , the Chromium Analogs of Silicocarnotite

The crystal structures Ca sub 5 Cr sub 2 SiO sub 12 and Ca sub 5 Cr sub 1.8 Si sub 1.2 O sub 12 , the Cr analogues of silicocarnotite, Ca sub 5 P sub 2 SiO sub 12 , have been determined. Both compounds were grown at 1250 deg C and analyzed by electron microprobe analysis. Diffraction data collection was done on spherically ground crystals which are both orthorhombic with space group Pnma. Charge-balance requirements and site-occupancy refinement of the Si-containing compound point strongly to the presence of both tetravalent and hexavalent Cr in tetrahedral sites. The Si is located together with tetravalent Cr in a general position, whereas the hexavalent Cr is situated on a mirror plane. The Ca atoms are located in seven-, eight-, and nine-coordinated sites. The presence of vacant channels of 3.5 A diameter perpendicular to (100) is a feature of this structural type. Its relation to the apatite and glaserite structures is also shown. This is part of a study of the fundamental chemistry of lime--chromite roasting reaction which is being considered as an alternative to the soda--ash process for the production of Cr chemicals.