206 Canfield, Ilillehrand, and Schaller — Mosesite, 



crystal, the four octahedra faces were seen to lie in the posi- 

 tions required for the form (111). 



From the above data, meager and poor as they are, the 

 isometric character of the crystals is deduced, especially as 

 this is supported by the investigation of the optical properties. 



Under tlie microscope mosesite is pale 3'ellow, non-pleochroic 

 and seems to possess a rather low double refraction. When a 

 fragment is crushed many of the resultant pieces show the octa- 

 hedral cleavage, either in their triangular or rhombic shape 

 (when only two directions of cleavage are developed) or in three 

 series of cleavage lines, parallel to the octahedral faces. A 

 fragment of fiuorite was crushed and examined under the 

 microscope, when similar effects were seen, though in this case 

 the cleavage was much better developed. , 



The crystal lographic investigation has shown that the crys- 

 tals are in all probability isometric octahedra and should, 

 therefore, be isotropic when examined under the microscope 

 with crossed nicols. Such an examination, however, shows the 

 mineral not to be isotropic but doubly refracting. On heating 

 the mineral to 186°, the crystals lose their double refraction 

 and become isotropic. Mosesite is therefore dimorphic, the 

 isometric optical condition that agrees with the geometrical 

 form being stable only above 186°, the mineral changing to 

 the dimorphous doubly refracting condition at temperatures 

 below 186°. 



As examined at ordinary temperatures, i. e., as the mineral 

 now occurs, the sections show great similarity to kleinite. 

 Much of the material does not extinguish at all, other parts 

 do so four times during a complete revolution of the stage and 

 the interference-colors are fairly brilliant. On crushing a frag- 

 ment between two glass slides and examining the very minute 

 particles, they are seen to be nearly colorless and, unlike the 

 larger pieces, isoti'opic. It seems as if the local heat developed 

 by the crushing is sufficient to heat these small particles to at 

 least 186° and cause their reversion to the isotropic state. 



Several different fragments were heated on glass slides, in 

 an air oven to the temperature given, with the results shown in 

 the following table : 



Effect of heating Mosesite. 



Probable temp. 

 EfEect. of change. 



No cliange, mineral still doubly 



refracting. 

 Brown and isotropic. 150-200° 



Brown and doubly refracting. 

 Still doubly refracting. 



K ie a 



Isotropic. 180-189° 



Slide with 





mosesite. 



Temp. 





r 150° 



First 



i 





[ 200° 





f 160° 



Second . . _ 



^ 167° 



180° 





189° 



