Structures of some tetragonal sheet silicates

The phases CaCuSitO10 ('Egyptian blue'), SrCuSi4010 and BaCuSi4010 have been grown in single crystals. Their structure, in the space group P4/ncc, is similar to that of gillespite, BaFeSi4010, previously established (Pabst, 1943). The parameters of the structure have been determined for CaCuSi4010 by means of repeated electron-density projections onto (100) and (110) followed by further adjustments. Gillespite is not known in good crystals. Each of the three artificial materials was obtained in measurable crystals showing the forms {001}, {101}, and {110} in the order of prominence predicted by the rule of Donnay & tIarker (1937). Gillespite is easily attacked by mineral acids. The copper analogues are impervious to attack. This may be attributed to the superior stability of copper in 4-fold planar coordination. Such coordination is characteristic of divalent copper but exceptional for ferrous iron. It seems probable that the imperfectly described mineral cuprorivaite is the natural analogue of 'Egyptian blue'.