WARREN. — ALKALI-GRANITES AND PORPHYRIES. 319 



habit of the anorthoclase commonly differs from ordinary feldspars. 

 The crystals are often curiously distorted, acute terminations being 

 common. These may be due to the physical conditions of the solu- 

 tion under which they grew, but one cannot but recall that such 

 abnormalities are characteristic of certain mixed-crystals among 

 laboratory salts. 



There seems, however, to be more direct evidence of the complete 

 isomorphism of the soda and potash feldspars. P. L. Barbier and 

 A. Prost ^^ of Lyon, France, have recently described monoclinic feld- 

 spars in which the soda is present in a molecularly greater amount 

 than the potash. This has been confirmed by Dr. W. T. Schaller ^^ 

 of Washington, who has suggested for the new monoclinic modification 

 of soda feldspar, thus shown to exist, the name "Barbierite" in honor 

 of its discoverer. F. Angel ^* has also described a " Soda bearing, 

 monoclinic sanidine containing 4.95% Na20 and 6.75% K2O from 

 Mitrowitza. Although anorthoclase appears to be triclinic at ordi- 

 nary temperatures it is worthy of note that according to Forstner ^^ 

 it becomes monoclinic at higher temperatures. 



It appears to the writer that at the temperature of crystallization 

 we may assume that the potash and soda-feldspars form mixed- 

 crystals of Type 1, and that this condition of equilibrium continues to 

 hold for an interval, probably a short one, below this temperature. 

 With lowering temperatures both of the components may pass through 

 an inversion point, going over into other crystalline modifications, 

 albite and microcline. This produces a radical change in the equili- 

 brium of the system, with the result that there is an unmixing of the 

 original mixed-crystal phase, and the formation of two new mixed- 

 crystal phases having a eutectic point betw^een them — that is, they 

 pass from Type 1 of Roosebooms' to Type V. One of these phases 

 is albite, or a highly sodic feldspar of the albite type, holding still in 

 solution some of the other component, and the other is a microcline 

 likewise with some albite dissolved in it. If this l^e so, it would appear 

 that the alkali feldspars exist in two modifications each, an a and a (3 

 form. Vogt ^^ has discussed this question for microcline and arrived 

 at the conclusion that orthoclase and microcline stand to each other 

 in the relation of an a and /3 form, and he compares the polysynthetic 



82 Bull. Soc. Chem., p. Ill (1908). 



83 Bull. No. 509, U. 8. Geological Survev (1912). 



84 Neues Jahrb. fur Min., Biel-Bami, 30 (1910). 



85 Zeit. f. Kryst., etc., 9, p. 333 (1884). 



86 op. cit., p. 540. 



