SUL1MIIDI S, AKSKNIDKS, ANTI M< >\ I I)KS 317 



General Description. Crystals from Sarrabus, Sardinia, arc 



combinations of the cube, rhombic dodecahedron, and the pyri- 

 tohcdron ; \vliile crystals from Lolling in Carinthia are tetrahedral 

 in habit, combinations of the tetragonal and trigonal tristetrahe- 

 In uis with the plus and minus tetrahedrons and the rhombic do- 

 decahedron. Chemically the two are alike and therefore must be 

 tctartohedral in symmetry, as in that type only can all these forms 

 occur. 



Ullmannite is a rare mineral and its principal interest is as a repre- 

 sentative of tetartohedral symmetry in the isometric system. 



MARCASITE 



Marcasite. Bisulphide of iron, FeSa ; Fe =_46.6, S = 53.4 ; 

 Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .7662 : 1 : 

 1.2342; 100 A 110 = 37 27', 001 A 101 = 58 10', 001 A Oil = 50 

 59', 111 A 111 =66 7'; Common forms, c(001), 1(011), e (101), 

 m(110); Twinning planes, 110 and 101; Cleavage, m distinct, 1 

 less so ; Brittle, fracture uneven ; H. = 6-6.5 ; G. = 4.85-4.9 ; 

 Color, light bronze yellow ; Streak, gray ; L i ;ter metallic. 



B.B. Fuses easily on coal and blackens, yielding a sulphur 

 dioxide odor in O. F. In R. F. becomes magnetic and reacts for 

 iron with the fluxes. 



General Description. Crystals when simple are tabular paral- 

 lel to the base ; it is more often twinned, with the prism face the 

 composition plane, which twinning is often repeated to fivelings with 

 a pentagonal outline and concentric striations; this method of 

 twinning forms the spearhead pyrites of the Dover Cliffs, England. 

 Twins with the macrodome as the composition face are not so com- 

 mon ; also, radiated, encrusted, stalactitic, and reniform. 



Marcasite is dimorphous with pyrite. It is always a secondary 

 mineral found in sedimentary rocks, or associated with coal beds 

 and organic matter ; how its molecule differs from that of pyrite 

 has not been determined ; marcasite has been represented as a 



Q _ O 



ferrous sulphide, Fe/g __ g/>Fe, and pyrite as a ferriferrous sul- 



/S -S\ 



phide, Fe\ S Fe S /Fe. This conclusion has been reached 



\S- -S/ 

 by a consideration of the amount of iron yielded by their de- 



