Minerals from Terlingua, Texas. 



263 



c=l - 6642 (mean of Schaller's and Sachs' values). The habit is 

 short prismatic, rarely equidimensional. Single crystals seldom 

 exceed one millimeter in length, but masses of crystalline 

 material may exceed one centimeter. Five_ forms have been 

 observed, namely, c J0001}, m {1010}, a {1120},^ {1011}, and 

 x {1012} (new). Cleavage is good parallel to {0001} and imper- 

 fect parallel to {1010}. Brittle. Luster adamantine to greasy 

 on bright surfaces. Hardness apparently slightly over 3*5. Not 

 radio-active (Boltwood), but Nutting reported faint evidence 

 of helium on first warming the mineral in a vacuum. 



The mineral being geometrically hexagonal, a basal section 

 should remain dark under crossed nicols. But, as described 

 by Moses, such a section does not remain dark but shows 

 double refraction, and if thin enough will be seen to be composed 

 of innumerable individuals, none of which is large enough to 

 show interference figures. The double refraction is strong, the 

 colors being of the third and higher orders. At about 130° 

 the double refraction begins to decrease, as seen by the 

 descending colors, until finally it becomes zero and the min- 

 eral remains dark under crossed nicols. The section now 

 gives a uniaxial positive interference figure. After cooling, 

 the section remains dark but after the lapse of many months 

 is seen to be slowly returning to its doubly refracting condi- 

 tion. This phenomenon seems to show that kleinite is dimor- 

 phous and that the uniaxial optical state agreeing with its 

 outward hexagonal form is stable only above 130° approxi- 

 mately, while below that temperature its stable condition is 

 biaxial, probably triclinic. According to this the hexagonal 

 crystals of kleinite must have been formed at a temperature 

 not much if any below 130°. As is stated just below, it is at 

 a point but a few degrees higher than this that the first per- 

 manent browning of the mineral become visible when it is 

 heated, and that considerable loss of water has then taken 

 place. What connection, if any, there may be between these 

 two phenomena is not known. 



Pyrognostic behavior. — When carefully heated in a closed 

 tube, or better in one through which passes a slow current of 



