The crystal structures of orthoclase and sanidinized orthoclase

Abstract

Orthoclase upon heating for long periods at about 1000°C. is transformed irreversibly into sanidine. Explanations for the phenomenon have been sought in structural changes, but have never been confirmed by X-ray analysis, although work by Chao, Hargreaves & Taylor on the structure of unheated orthoclase suggested that disordering of Al atoms might be responsible for the transformation. In the present paper an account is given of a highly accurate structure determination of the structure of an orthoclase after sanidinization, the sample examined being the same as that investigated as orthoclase by the above-mentioned authors. A change from an ordered distribution of aluminium atoms in orthoclase to a disordered distribution in sanidine should produce, as one observable effect, changes in the size of the tetrahedral groups around (Si, Al) in the structure. The present examination of sanidine indicates uniformity in the size of these groups, as would be expected from a random arrangement of atoms of either kind in the (Si, Al) positions. A re-examination of the earlier determination of orthoclase, however, shows that its accuracy was not sufficient to establish significant differences in the size of the tetrahedral groups, and so establish an ordered arrangement of Al atoms for that material. A redetermination of its structure, of the same accuracy as the present work, would be required to settle this point finally. Available evidence indicates that the position of the cation does not change during sanidinization.