410 K T. Allen, F. K Wright, and J. K. Clement— 



show the cleavage plainly, still in a number of cases where 

 fibers were tilted to a vertical position under the microscope 

 an indistinct cleavage of about 120° was observed. Their 

 hardness is 6. In transmitted light the aggregates do not 

 appear so clear and transparent as the pyroxenes, but are 

 usually of a pale brown color. The refractive indices were 

 measured by the method of refractive liquids : 7=1'591±'003, 

 a — 1*578±'003, 13 — l-585±'004r. Birefringence is not strong, 

 *013 approximately, and the interference colors are conhned to 

 the bright tints of the first and second orders. The extinction 

 is usually- parallel to the direction of elongation, although in 

 a few sections a small extinction angle, c : c = 3°-6°, was 

 observed, indicating intergrowths of orthorhombic and mono- 

 clinic amphiboles similar to those observed in nature. The 

 plane of the optic axes lies parallel to the long direction ; the 

 optic axial angle is large and the optical character apparently 

 positive, determined on a section nearly perpendicular to an 

 optic axis. The pleochroism is c = brown, and b lighter brown ; 

 absorption, c > b. 



Lines of growth are often distinct within the crystallites 

 and exhibit a plumose arrangement. 



Although the above optical data clo not suffice to prove 

 definitely that this form of magnesium metasilicate is an 

 amphibole, they do agree closely with the properties of the 

 orthorhombic amphibole of similar composition found in 

 nature. Unfortunately, the definite solution of this question 

 must be deferred until after means have been devised to pro- 

 duce measurable crystals. 



The specific gravity was determined on several different 

 preparations, two of which (I and II) were very carefully 

 examined microscopically and found to be practically pure; 

 the third, having been observed by the same method to contain 

 small quantities of glass and the monoclinic form, was separated 

 with great care by heavy solutions (methylene iodide and ben- 

 zene), the separation of each portion being twice repeated and 

 controlled by the microscope. 



Specific gravity at 25° 0. 

 Prep. I. 2-858 Prep. II. 2'860 Prep. III. 2'855 



2-856 2-860 2'853 



2-857 2-860 2*854 



Average, 2-857 



'to' 



Transformation of Orthorhombic Amphibole by Heat. — 

 The orthorhombic amphibole changes, like enstatite, and some- 

 what more rapidly, into the monoclinic pyroxene, when heated 

 to a sufficient temperature. Here, too, the change is sluggish, 

 as the following table indicates : 



