210 DESCRIPTIONS OF MINERALS. 
Gypsum.—Hydrous Calcium Sulphate. 
Monoclinic. JA J=143° 42’; 2¢A27+111° 42’. Figure 
6 represents a common twin (or arrow-head) crystal. Cleav- 
age parallel to 7-7 very easy, map 
thin pearly lamine ; parallel to O, 1 
perfect, giving a vitreous surface ; Re 
allel to fe fibrous. Occurs also in lam- | 
inated masses, often of large size; in | 
fibrous masses, with a satin lustre; in 
stellated or radiating forms consisting of 
narrow lamine; also granular and com- 
pact. 
When pure and crystallized it is as clear and pellucid as 
glass, and has a pearly lustre. Other varieties are gray, yel- 
low, reddish, brownish, and even black, and opaque. H.= 
1 5-2, orso soft as to be seratched by the finger-nail. G.=2°33. 
The plates bend in one direction and are brittle in another. 
Composition. CaO,8+2aq=Sulphur trioxide 465, lime 
32°6, water 20°9=100. B.B. becomes instantly white and 
opaque and exfohates, and then fuses to a globule, which 
when placed upon moistened turmeric paper shows an alka- 
line reaction. In a closed tube much water is given off. 
Dissolves quietly in hydrochloric acid, and the solution gives 
a heavy precipitate with barium chloride. 
The principal varieties are as follows : 
Selenite, including the transparent crystallized gypsum, 
so called in allusion to its color and lustre from selene, the 
Greek word for moon. 
Radiated and Plumose gypsum, having a radiated struc- 
ture. 
Fibrous gypsum or satin spar, white and delicately fibrous. 
Snowy gypsum and Alabaster, including the white or light- 
colored compact gypsum having a very fine grain. 
Diff. The foliated gypsum resembles some varieties of 
heulandite, stilbite, tale, and mica; and the fibrous looks 
like fibrous carbonate of lime, asbestus and some of the 
fibrous zeolites; but gypsum in all its varieties is readily 
distinguished by its softness ; its becoming an opaque white 
powder immediately and without fusion before the blow- 
pipe, and by not effervescing or gelatinizing with acids. 
Obs. Gypsum: forms extensive beds in certain limestones 
and clay beds, and also occurs in volcanic regions. New 
York, near Lockport, affords beautiful selenite and snowy 


