Summary: A series of potash-soda-felspars of the orthoclase-microperthite type, selected with increasing soda-contents up to 60 %, have been examined and found to exhibit a progressive relationship between chemical composition and physical properties. Albite (Ab₉₇An₂Or₁) conforms in physical properties to the soda end-member of the series.

Adularia and microcline do not strictly belong to the series, the optic axial angle 2V and the birefringence γ - β being too high. On the other hand, the sanidines and anorthoclase felspars have lower refractive indices and a lower optic axial angle than orthoclase-microperthites of corresponding composition. This low optic axial angle is the result of relatively rapid cooling through the temperature range of 1050–900° C., and the ensuing modification of the internal structure enables the felspar to hold the soda-component in complete solid solution on further cooling through the perthite exsolution temperature range (850–350° C.), however slow the cooling through that range may be.

The microperthite of the orthoelase-microperthite series is of the exsolution type, and can be taken into solid solution again by heating for some time to 750–800° C followed by rapid cooling.

At the same time, the schiller is more or less destroyed, and the specific gravity and the refractive indices are reduced, the reduction being roughly proportional to the amount of microperthite taken into solution. Providing that the optic axial angle has not been reduced by prolonged heating at high temperatures, the microperthite may be in part reprecipitated by very slow cooling; the schiller and microperthite structure reappear and the drop in specific gravity and refractive index are in part restored.

In all cases where the optic axial angle is normal and microperthite exsolution complete, the angle remains unaltered during the refractive index changes which accompany the solution of the microperthite on heat-treatment. If the optic axial angle is sub-normal and exsolution of the microperthite is incomplete—as withKorea moonstone, etc.—the solution of the microperthite is accompanied by a slight rise in the axial angle.

Recent X-ray examination of felspar structures has led to the suggestion that microperthitie exsolution represents a local segregation of the sodium and potassium ions within the silica-alumina framework followed by a contraction and slewing of the framework in the areas rich in sodium ions. This view explains the relative ease with which the microperthitie changes can be brought about on heating.

The optic axial angles of potash-soda-felspars are reduced by prolonged heating at high temperatures. Specimens with less than 40 % of soda-felspar can be completely ‘sanidinized’ or converted into the abnormal type of felspar with the optic axial plane parallel to the plane of symmetry. The equilibrium value of the optic axial angle, for a temperature of 1075° C., depends on the chemical composition, the relationship being approximately linear between 2V 60° (0 % soda-felspar) and 2V 0° (40–45 % soda-felspar). Microcline can be converted to the abnormal form by heating at 1075° C. for a long period; at the same time the cross-hatch twinning is destroyed.

Attempts to reverse this process and to restore the original optic axial angles to ‘sanidinized’ felspars, by cooling them very slowly through the high-temperature range, have not given positive results, but the indications are that a part of the change may be reversible in laboratory cooling times.

In conclusion, I wish to thank Professor P. G. H. Boswell for his help and advice and Mr. R. Walls of the Imperial College of Science and Technology, London, for assistance in the preparation of this paper; also members of the Geological Survey of India, especially Sir Lewis Fermor and Dr. J. A. Dunn, for their kind help and criticism; and Messrs. K. B. Sen and B. P. Gupta, of Messrs. Bird & Co., Calcutta, for much help with the chemical analyses and physical determinations.