An X-ray examination of some potash-soda-felspars

Abstract

The systematic classification of the potash-soda-felspars is of the greatest interest and importance to the mineralogist, but problems demanding physical investigation are raised in work carried out recently by Dr. Edmondson Spencer, who has described the results of exhaustive chemical, optical, and thermal investigations of a representative collection of these minerals. Dr. Spencer has very kindly allowed us to examine by X-ray methods a large number of his specimens, so that for the first time it has been possible to make a fairly complete survey of potash-soda-felspars of known chemical composition and with accurately measured optical properties.This paper presents the results of our general survey of potash-soda-felspars with compositions ranging from nearly pure potash-felspar to approximately 50 % soda-felspar. The natural minerals, and the same minerals after specified heat-treatment, have been examined. We do not discuss in detail the interpretation of our results in terms of the atomic structure of felspars; the subject is extremely complicated, and it is necessary to await the results of other investigations now in progress in this laboratory before attempting a complete structural interpretation.

The lamellar structure of potash-soda felspars

“…(c) Our earlier survey revealed a slight but apparently real increase in the length of the principal (potash-felspar) axis in passing from F to M types (Chao et al 1939, figure 1). This is confirmed in the new examination by the figures given in § 3, para, (i), for the individual specimens F and M. I t is highly significant th a t the principal axial lengths of specimen P, which contains more soda-felspar than specimen M, are identical with those of specimen F, within our experimental accuracy.…”

“…Ito and Inuzuka (1936) examined powder photographs of various potash-soda felspars, and Ito (1938) has now described an extremely in teresting investigation of Korean moonstone; the second paper will be [ 57 ] discussed in a later section of this paper. In a paper read before the Mineralogical Society in January 1939, and published in th Magazine in June 1939, the present authors with D. L. Smare (1939) reported the results of a general survey by single-crystal methods of a number of speci mens of potash-soda felspars, mostly of the orthoclase-microperthite type, which were very kindly supplied by Dr Edmondson Spencer. The results of detailed chemical, optical, and thermal investigations of these materials have been described in three important papers by Spencer (1930Spencer ( , 1937Spencer ( , 1938.…”

“…The following axial lengths and directions were deduced from rotation photographs taken with Mo Ktxradiation (Chao et al 1939): ax = 8-65 a2 = 8* 12 A; axes b2c2 equal in length to b1c1 respectively; axes a2c2 parallel to a1c1 respectively, axis b2 inclined to at an angle ~ 3°. The new experi mental results are as follows:…”

“…From rotation photographs with Mo Koc radiation, and with rotation axis parallel to each of the principal axes a1b1c1 in turn, axial lengths were deduced as follows (Chao et al 1939): principal axes®! = 8*50 A, bx = 13-07 A, = 7-18 A; sub sidiary axis a2 = 8-12 A.…”

“…The general survey (Chao, Smare and Taylor 1939) of orthoclase-microperthites containing various amounts, up to 60 %, of soda-felspar suggested the existence of three types of structure. The first is a simple monoclinic structure, and is found when the amount of soda-felspar is very small-not exceeding 10 % by weight.…”