536 MINERALOGY 



centration mirabilite separates from the saturated solution when 

 the temperature is below 32 ; when the temperature is higher, the 

 anhydrous salt, thenardite, is separated. Its solubility varies 

 greatly with the temperature, thus at Great Salt Lake, Utah, on 

 cold days in winter large amounts of sodium sulphate are thrown 

 up on the shore by the waves, only to be redissolved when the 

 temperature rises. In a dry atmosphere it loses its water of 

 crystallization, falling down as a fine white powder. It also occurs 

 as an efflorescence on rocks or near springs, where much water is 

 quietly evaporating. 



GYPSUM 



Gypsum. CaS0 4 . 2 H 2 O ; Hydrous calcium sulphate ; CaO = 

 32.5, SO 3 = 46.6, H 2 O = 20.9; Monoclinic; Type, Digonal 

 Equatorial ; a : b : c = .6899 : 1 : .4124 ; p = 80 42' = 001 A 

 100; 100 A 110 = 34 15'; 001,101 -28 17'; 001,011 =22 9'; 

 111-111=36 12'; Common forms, b(010), c (001), m (110), 

 1(111), n(lll); Twinning plane, 100, contact and crossed pene- 

 trating, also 101 less common ; Cleavage, clinopinacoidal perfect, 

 a less so, and 111 fibrous; laminae flexible parallel to the fibrous 

 cleavage; H. = 1.5-2; G. = 2.3-2.33, when pure; Color, white, 

 pale yellow, red, brown to black when organic matter is present; 

 Streak, white ; Luster, silky, pearly, vitreous to dull ; Transparent 

 to. opaque; a = 1.5204; (3 = 1.5229; y = 1.5296; -y - a = 

 .009 ; Optically ( + ) ; Axial plane = 010 ; Bx a A c = 52 30' in 

 front; 2V = 58 8'. 



B.B. Whitens and fuses to an opaque white mass at 3 ; colors 

 the flame yellowish red ; after ignition reacts alkaline with turmeric 

 paper. In the closed tube yields water. Fused with soda and a 

 little coal dust in the R. F. reacts for sulphur with silver. Soluble 

 in HC1. 



General description. Crystals usually simple combinations of 

 the plus and minus unit pyramids with the unit prism and the 

 clino- and basal pinacoids. Crystals six feet in length have been 

 found in Wayne County, Utah. The faces m and b often striated 

 parallel to their intersection with the base. Parallel growths and 

 rounded stellate aggregates are common, as at St. Mary's River, 

 Maryland, and Postelberg, Bohemia. Twinning in which the 

 composition plane is parallel to the orthopinacoid, forming the 

 well-known swallow-tail twins, and, when the crystals are rounded, 



