CRYSTAL CHEMISTRY OF SYNTHETIC 1:3 SILICATE AND GERMANTE PYROXENES M. Behruzi, E. Banerjea-Appel, F.D. Scherberich and Th. Hahn. Institut fuer Kristallographie, RWTH Aachen, D 52056 Aachen, Germany.

Binary systems of the type LiAlGe₂O₆-LiGaGe₂O₆ show peritectic melting and increase of the melting temperatures with increasing atomic number of the three valent cations (Fe, Sc, In). All 1:3 pyroxenes crystallize only in the monoclinic form with the exception of LiScGe₂O₆ and LiInGe₂O₆ which can be synthesized in the monoclinic and orthorhombic enstatite types. The orthorhombic forms are the low temperature phases and undergo a sluggish but complete transition to the monoclinic form; the reverse transition is much more sluggish.

The monoclinic germanates show a rapid and reversible phase transition from the low temperature form (mostly P21/c) to the high temperature form (C2/c). The phase transitions give rise to a discontinuous increase of the a, c and ß but decrease of the b axis. Furthermore, the symmetrically non-equivalent tetrahedral chains become equivalent and the tetrahedral chains stretch.

Special features of monoclinic pyroxenes: 1) The observed M³⁺-O bond lengths are 0.985 of the calc. bond lengths . 2) The Li-O and Na-O bond lengths differ for the corresponding silicates and germanates 3) They are a linear function of the M³⁺-O bond lengths. 4) The c axes of the Li and Na silicates are equal and a linear function of the M³⁺-O. 5) The c-axes of the germanates are larger than those of the corresponding silicates. They are also a linear function of M³⁺-O; for C2/c they are larger than for P21/c.