The structures of synthetic paracelsian analogs, SrGa₂Si₂O₈ and SrGa₂Ge₂O₈

The crystal structures of two synthetic paracelsian analogs, SrGa₂Si₂O₈ (SGSP)² and SrGa₂Ge₂O₈ (SGGP), have been refined by conventional methods. These compounds are pseudo-orthorhombic; completely ordered T³⁺, T⁴⁺ distributions in their respective tetrahedral frameworks result in P2₁/a symmetry for both. Pertinent crystal data are:

Mean Sr-O and T-O distances (in Å) are:

These structures are compared in detail with hurlbutite (CaBe₂P₂O₈), another paracelsian-type structure, and with danburite (CaB₂Si₂O₈) whose T³⁺, T⁴⁺ ordering is distinctly different. Onefourth of the T-O-T linkages in danburite are of the type T³⁺-O-T³⁺ or T⁴⁺-O-T⁴⁺; three fourths are of the type T³⁺-O-T⁴⁺. In paracelsian all linkages are of the latter type. Consideration of charges on bridging oxygen atoms in T₂O₇^n- groups (as calculated by extended Hückel molecular orbital approximations) indicates that the charges on oxygen atoms involved in T³⁺-O-T³⁺ linkages are substantially different from those in T⁴⁺-O-T⁴⁺ linkages except for the case T³⁺ = boron. This suggests that in the presence of Si (or Ge) any T³⁺-O-T³⁺ bonds involving Al or Ga (but not B) will result in a non-uniform charge distribution on the oxygen atoms. This is offered as an explanation for the absence of danburite analogs of ordered M²⁺T₂³⁺T₂⁴⁺O₈ compounds containing Al or Ga which have thus far been observed as only feldspar or paracelsian-type structures.