254 N. L. Bowen — The Ternary System : 



investigation has shown to be characteristic of the intermediate 

 pyroxenes. 



Natural Pyroxenes with Analogous Optical Relations. — 

 In the ordinary text-book descriptions of the pyroxenes there 

 is commonly no mention of a series analogous to the pyroxenes 

 prepared in the course of the present work. Occasionally, 

 however, there is mention of pyroxenes poor in lime which 

 show certain similarities with the magnesia-rich artificial 

 varieties. * In 1906 Wahl made a systematic study of such 

 pyroxenes from a number of localities. Wahl's studies, both 

 optical and chemical, show that there are natural, magnesia- 

 rich, monoclinic pyroxenes and they appear to form a series 

 closely similar to the diopside-clino-enstatite series. Wahl 

 terms those described by him, enstatite-augites.f The natural, 

 minerals contain considerable iron and, therefore, differ from 

 the iron-free varieties in many of their properties, but they 

 show the same behavior of the optic axial angle, viz., in the 

 varieties relatively poor in magnesia the axial plane is //010 

 and the axial angle large ; with increasing magnesia, as in the 

 artificial varieties, the angle grows smaller, passes through 

 zero and opens again in the plane normal to 010. Wahl was 

 able to show that the axial angle for blue light passed through 

 zero before that for red light in the ferriferous varieties 

 (p. 39), an observation which the present writer was unable to 

 duplicate in the non-ferriferous pyroxene on account of the 

 extremely small dispersion. Wahl found moreover, that as 

 the axial-angle decreased (magnesia increased) the extinction 

 angle also decreased (p. 33). 



The Relation of Clino-enstatite to Enstatite. 



It was no part of the plan of the present work to study the 

 relation between clino-enstatite and enstatite, but some of the 

 results obtained have a bearing on the problem and seem 

 worthy of attention. 



It has been demonstrated by Wright, Wahl and others that 

 clino-enstatite is monoclinic in symmetry. In all other properties 

 it is very similar to enstatite. The universal prevalence of twin- 

 ning //100 in clino-enstatite has led Zambonini and others to 

 conclude that enstatite is clino-enstatite in which the twinning 

 is sub-microscopic ; that the relation is one of polysymmetry. 

 Some of those who support this view have pointed out that 

 in meteorites and artificial melts clino-enstatite is found, in 

 terrestrial rocks, almost universally, enstatite, and they have 

 considered that the determining factor is the rate of cooling. 



*Cf. Pigeonite-Wmchell, Optical Mineralogy, p. 331. 

 fDie Enstatitaugite, T. If. P. M. xxvi, 1-131, 1907. 



