a New Mercury Mineral from Terlingxia, Texas. 207 



Third .... 

 Fourth... 



Fifth .... 



( 169° 



f 169° 

 J 184° 

 ] 186° 

 [ 190° 

 f 184° 

 I 186° 

 188° 

 190° 



Doubly refracting, but section 

 too opaque for further study. Above 1( 

 Doubly refracting. 



{ 



Nearly isotropic. 

 Isotropic. 

 Doubly refracting. 

 Nearly isotropic. 



186-190° 



I 



Sixth .. 



186° 

 192° 



f 186' 



Seventh .. ■{ 



I 192° 



Isotropic, except for a few mi- 

 nute doubly refracting spots. 184-186° 



Doubly refracting. 



Still doubly refracting in places 

 but partly isotropic. This par- 

 ticular piece may have been 

 thicker than the others and 

 therefore did not revert so 

 readily, 186-192°-!- 



Isotropic on thin edges, but 

 doubly refracting in thicker 

 center. 



Isotropic. 186-192° 



From these data the temperature of reversion to the isotropic 

 isometric state probably lies between 180° and 190° and is taken 

 as 186°. If the " molecular inertia " of mosesite is consider- 

 able, as is the case with kleinite,* then this temperature, namely 

 186°, is probably a little high. On cooling, the isotropic min- 

 eral does not immediately again become doubly refracting. 

 Examination after 24 hours shows the mineral to be still iso- 

 tropic. The change back seems to be, like that of kleinite, a 

 very slow one, but doubtless it is gradually taking place. The 

 section of kleinite described f as almost entirely reverted after 

 thirty months is now, after four years, completely changed 

 back again to its original doubly refracting condition. 



The optical similarities, shown by these two minerals, coupled 

 with their evident chemical relationship, suggested trying the 

 effect of heating kleinite to the reversion temperature of moses- 

 ite (186°). When this was done, kleinite :t became optically 

 isotropic like mosesite. The results of the experiments are 

 shown in the table below. 



* Hillebrand, "W. P., and Schaller, W. T., The Mercury Minerals from 

 Terlingua, Texas. Bull. U. S. Geol. Survey, No. 405, 1909, p. 26. 



\ Log. cit., p. 25. 



X Fragments were used that were not parallel to the base as these would 

 become isotropic at 130°, kleinite being uniaxial, hexagonal. 



