210 



DESCRIPTIONS OF MINERALS. 



Gypsum. — Hydrous Calcium Sulphate. 



Moiioclinic. IaJ=143°42'; 2iA2i + lll°42'. Figure 

 2 represents a common twin (or arrow -he ad) crystal. Cleav- 

 age parallel to i-i very easy, affording Q 

 thin pearly laminae ; parallel to 0, im- 

 perfect, giving a vitreous surface; par- 

 allel to I, fibrous. Occurs also in lam- 

 inated masses, often of large size : in 

 fibrous masses, with a satin lustre ; in I 

 stellated or radiating forms consisting of | 

 narrow lamina? ; 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.=r 

 1 o-2, or so soft as to be scratched by the finger-nail. &.=2*33. 

 The plates bend in one direction and are brittle in another. 

 Composition. Ca 4 S + 2 aq— Sulphur trioxide 40*5, lime 

 32 '6, water 20*9 = 100. B.B. becomes instantly white and 

 opaque and exfoliates, 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, talc, 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 



