532 MINERALOGY 



macroaxis occurs on crystals from Stassfurt. It is usually granu- 

 lar, massive, lamellar, or fibrous. While it is classed with the 

 barite group of sulphates, it differs from other members of the 

 group widely in its cleavage and axial ratio. Anhydrite is also 

 the most soluble of the series, and is therefore carried in solution 

 longer and farther than the others, and is often deposited by the 

 evaporation of these natural waters. 



It occurs in beds connected with salt deposits ; in the evapora- 

 tion and deposition of the soluble salts from saturated solutions, 

 anhydrite is normally on the bottom of such deposits, with the more 

 soluble magnesium, sodium, and potassium compounds above it. 



In the normal concentration of a calcium sulphate solution at 

 ordinary temperatures, gypsum is deposited, but when the solution 

 contains sodium chloride or magnesium salts, anhydrite is deposited 

 at temperatures as low as 25 C. Gypsum in a concentrated solution 

 of sodium chloride at 30 passes over to anhydrite, and, while gypsum 

 is the first mineral to separate on concentration, very often when 

 concentration has advanced the sulphate first crystallized as gypsum 

 forms anhydrite. Under heat and pressure gypsum may lose its 

 water and be transformed to anhydrite, or the reverse of this is 

 also possible and anhydrite by hydration in many cases forms 

 gypsum. The two minerals are often mixed, areas of anhydrite 

 occur containing considerable gypsum, which is probably secondary 

 to the anhydrous sulphate. 



Anhydrite occurs at the borax lakes and salt deposits of California 

 and Nevada, at the salt deposits of Michigan ; at Lockport, New 

 York; in Nova Scotia, at the mouth of the Avon and St. Croix 

 rivers. Commercially anhydrite is of little value ; when attrac- 

 tively colored and veined it is polished as an ornamental stone. 

 Artificial crystals may be formed by fusing calcium sulphate and 

 sodium chloride. 



ANGLESITE 



Angles! te. Sulphate of lead, PbSO 4 ; PbO = 73.6; S0 3 = 

 26.4; Orthorhombic ; Type, Didigonal Equatorial; a:b:c = 

 .7852 : 1 : 1.2894 ; 100 A 110 = 38 8' ; 001 A 101 = 58 40' ; 001 A 

 Oil = 52 12' ; 102 A 102 = 78 47' ; 001 A 111 = 64 24' ; Common 

 forms, a (100), b(010), c(001), m(110), d(102), z(lll); Cleav- 

 age, basal and prismatic distinct ; Fracture, conchoidal ; Brittle ; 

 H. = 2.75-3; G. = 6.1-6.4; Color, white, gray or pale yellow, or 

 blue ; Streak, white ; Luster, adamantine to resinous ; Trans- 



