318 MINERALOGY 



composition, in the ferrous and ferric states. Marcasite oxidizes 

 more readily than pyrite, yielding ferrous sulphate and sul- 

 phuric acid; the ferrous sulphate appears as a white efflores- 

 cence, and the specimen swells, cracks, and falls to pieces. Marca- 

 site is the less stable form and passes into pyrite. Their 

 ultimate oxidation products are the same. At 450 marcasite 

 passes over to pyrite with an evolution of heat; this probably 

 explains the absence of marcasite as a primary constituent of 

 igneous rocks. 



Marcasite has been formed artificially by the slow action of 

 H 2 S on ferric sulphate or chloride at several temperatures up to 

 300. Acidity favors the formation of marcasite, for when the 

 solution is neutral, pyrite is formed. The amount of pyrite mixed 

 with marcasite formed under these conditions decreases with the 

 acidity. The favorable conditions for the formation of nearly 

 pure marcasite is 100 and 1.18 per cent, of free H 2 SO 4 . 



ARSENOPYRITE 



Arsenopyrite. Mispickel, FeSAs; Sulpharsenide of iron; 

 Fe = 34.3, As ==46, S = 19.7 ; Orthorhombic ; Type, Didigonal 

 Equatorial ; a : b : c = .6773 : 1 : 1.1882 ; 100 A 110 = 34 6', 001 A 

 101 = 60 19', 001 A 011 = 49 54'; Common forms, m (110) A 

 u (014), e (101) q (Oil) ; Twinning plane, 110, also 101, both contact 

 and interpenetrating ; Cleavage, m distinct, c faint traces ; Brittle, 

 fracture uneven ; H. = 5.5-6 ; G. = 5.9-6.2 ; Color, silver-white 

 to steel-gray ; Streak, grayish black ; Luster, metallic. 



B.B. On coal fuses easily to a brittle globule and yields an 

 arsenic odor and sulphur dioxide fumes ; roasted and treated in the 

 borax bead yields an iron reaction ; in the closed tube yields first 

 an arsenious sulphide sublimate and then a metallic mirror of 

 arsenic. 



General description. Crystals are prismatic combinations of 

 the unit prism with the unit domes, the prism zone striated ; at 

 times the prism is very short when the crystal is pyramidal in ap- 

 pearance. Twinning is of two classes, those in which the composition 

 face is m, often repeated, yielding pseudo-hexagonal forms, as in 

 marcasite, or the dome e is the composition face ; as the angle e A e' 

 is 120 38', the individuals when repeated form a six-armed star. 

 Arsenopyrite is very often massive, granular, or compact; some 



