540 MINERALOGY 



on exposure. Chemically it may contain some magnesium, as 

 epsomite is very closely related. 



Pisanite is a variety containing copper. 



Melanterite is the result of the oxidation of such sulphides as 

 pyrite, marcasite, pyrrhotite, or other sulphides containing iron; 

 the soluble ferrous sulphate is carried off in solution, to be deposited 

 by evaporation, as copperas if the conditions are favorable. All 

 mine waters in sulphide regions contain ferrous sulphate in solution, 

 which according to the conditions may form several iron sulphates, 

 as coquimbite, Fe 2 (SO 4 )3 . 9 H 2 0, the ferric sulphate, or the ferrous 

 ferric sulphate roemerite, FeFe 2 (SO 4 ) 4 . 12 H 2 0. 



Iron sulphates are not common in nature, as owing to their 

 solubility they may occur only under very restricted conditions. 



The commercial copperas is a by-product produced in the pre- 

 cipitation of copper sulphate with scrap iron. It is used as a disin- 

 fectant, in dyeing, in the manufacture of ink and pigments. 



CHALCANTHITE 



Chalcanthite. Blue vitriol; Copper sulphate, CuSO 4 .5H 2 0; 

 CuO = 31.8 ; SO 3 = 32.1 ; H 2 O = 36.1 ; Triclinic; Type, Centro- 

 symmetric; &: b: c = 0.5721 : 1 : .5554; a = 82 5'; |3 = 107 

 8'; <y = 102 41/; 100 A 010 = 79 6'; 100,110 = 26 7'; 010 A 011 = 

 6458'; 011,011 =56 59'; 100,011 = 69 50'; Commonforms, 

 a (100), b (010), m(110), M (110), p (111), Cleavage, M, m, and p 

 imperfect; Brittle; Fracture, conchoidal ; H. = 2.5; G. = 2.12- 

 2.3; Color, shades of blue; Streak, white; Luster, vitreous; 

 Translucent; a = 1.514; p = 1.537; y = 1.543; -y - a = .029; 

 Optically (-) ; 2E = 93. The normal to the optic plane makes 

 an angle of 53 30' with the normal to 110, with 110 an angle of 

 12 30', and with the normal to ill, 67. 



B.B. Fuses easily on coal and blackens; reduced with soda, 

 yields copper, and a sulphur reaction on silver. In the closed tube 

 yields water. Soluble in water, and has a metallic taste. 



General description. Crystals stout, tabular parallel to 111 

 with the zone c, b striated parallel to their intersection. More 

 often massive, granular, stalactitic or in crusts. 



Chalcanthite occurs in many copper mines, where it is deposited 

 from the mine waters on evaporation. It is derived from the oxi- 

 dation of sulphides in the upper levels of the ore deposits and is 



