400 K T. Allen, F. K Wright and J. IT. Clement— 



gate of enstatite crystals in quantity, therefore, it suffices to 

 crystallize the glass somewhat below 1100°, but not below 

 1000°, if the preparation requires microscopic identification, 

 for the product then formed is cryptocrystalline. The ensta- 

 tite was distinguished from the monoclinic form solely by its 

 parallel extinction in the prism zone, the refractive indices of 

 the two forms not differing sufficiently to permit the use of 

 that method. By cooling mixtures of magnesium silicate with 

 10 per cent of its weight of albite, or the same amount of 

 sodium and potassium silicates, enstatite may be obtained in 

 long, well-developed prisms, the index of refraction of which 

 indicates only a small quantity of dissolved albite or other 

 foreign matter. Some experiments hi this direction are 

 described in detail farther on p. 433. 



Properties of Chemically Pure Enstatite. — Thus far, w T e 

 have obtained pure enstatite only in fibrous aggregates and 

 radial spherulites which do not admit of a precise deter- 

 mination of its morphologic constants. It is a remarkable 

 fact that the direction of elongation of the fibers in the small 

 radial spherulites is not the prism axis, but another, such that 

 the greater ellipsoidal axis a is parallel to it instead of c. 



Cleavage after 110 is well marked ; its angle approximates 90° 

 and was observed under the microscope by tilting on end the 

 individual fibers embedded in thick Canada balsam. The 

 extinction is parallel in the prism zone and the least ellipsoidal 

 axis c coincides with the prism axis. 



The optical properties are similar to those of the monoclinic 

 magnesium pyroxene, differing chiefly in the parallel instead 

 of inclined extinction, lower average refractive index and 

 smaller optic axial angle. The refractive indices were deter- 

 mined by the immersion method in liquids : 



a= l-640±'004, /?= 1-646 ±-004, y = V652±.004; 



birefringence not strong, about '01. The plane of optic axes 

 contains the prism axis (c = c) ; optical character, positive. 

 The optic axial angle was measured on a section nearly per- 

 pendicular to an optic axis by the graphical method recently 

 described by F. Becke.* This method is based on the differ- 

 ence in curvature of the black hyperbola bar which passes 

 through the optic axis, for different optic axial angles in the 

 diagonal position. In place of the type of revolving drawing 

 stage described by Becke, a somewhat simpler form was 

 devised which can be clamped directly to the microscope and 

 can be readily adjusted to suit the conditions. This method of 

 Becke is only an approximate one and should only be used when 



* Becke, F., Die Messung der optisctien Axen der Hyperbola-Kriimmung. 

 Tscherm. Min. petr. Mitth. xxiv, pp. 32-49, 1905. 



