THE CHICAGO ACADEMY OF SCIENCES 



eign substances crystallization is retarded and the less stable 

 form, that of marcasite, is produced. Cleavage indistinct; 

 fracture conchoidal; brittle; hardness 6; specific gravity 4.9 to 

 5.2. By mixture with quartz or marcasite in microscopic inclu- 

 sions or because of incipient decomposition the specific grav- 

 ity sinks as low as 4.5, which is the minimum presented by 

 pure marcasite. This variation in the specific gravity increases 

 the difficulty of distinguishing between the two species. Its 

 lustre is metallic, splendent to glistening; color, pale brass 

 yellow, nearly uniform, often bronze, rarely brightly tarnished 

 because of oxidation. The streak is greenish black or brown- 

 ish, opaque even in the thinnest sections. Very slightly mag- 

 netic (paramagnetic). Offers strong resistance to electric cur- 

 rent; shows two kinds of thermo-electricity. Its chemistry, 

 association and history are identical with marcasite, and as mar- 

 casite is the most abundant form in the region we shall con- 

 sider the points under marcasite. 



Fig. 5. Fig. 6. Fig. 7. 



Marcasite. FeS 2 . Orthorhombic. 



Figs. 5, 6 and 7 represent the type forms of marcasite, 

 which occurs both in columnar and in tabular crystals. The 

 prism (no), macrodome (101), clinodome (oil), pyramid (in) 

 and the base (ooi) are often developed while the planes 

 most commonly appearing in Cook Co. marcasites are dif- 

 ferent brachydomes (oil), (012), (013), etc. Isolated crystals 

 extremely rare. Usually closely crowded, interfering and poly- 

 synthetic twins, globular, botryoidal, stalactitic, radiated. The 

 botryoidal masses are often composed of clusters of prisms 

 ending in various brachydome planes that are almost invari- 

 ably striated, as is shown in figure 6. The end planes may be 

 so nearly flat and may so nearly approach a square as to be 

 mistaken for cubic planes, until the diagonal striations are 

 observed. The striations are parallel to the brachydiagonal. 

 Fig. 7 presents a common flat form which is usually striated 



