72 NEW YORK STATE MUSEUM 



Hematite occurs in rocks of all geologic horizons. It is 

 widely distributed and the numerous foreign localities afford 

 beautifully crystallized specimens. In the United States vast 

 beds of hematite are found in the Archaean rocks of northern 

 Michigan, northern Wisconsin, Minnesota, Missouri, and in Jef- 

 ferson and St Lawrence counties of northern New York; also in 

 the Clinton group of the Upper Silurian in New York and 

 Pennsylvania. 1 



Hematite constitutes the chief source of iron; the earthy 

 variety is ground for paint. 



Ilmenite or menaccanite (titanic iron ore) (Fe,Ti) 2 3 

 Umenite is an oxid of iron and titanium containing 31.6$ 

 oxygen, 31.6$ titanium and 36.8$ iron. The crystals, which are 

 trirhombohedral, somewhat resemblesthose 

 of hematite in habit (fig. 189). Ilmenite 

 commonly occurs in iron-black plates and 

 masses of submetallic luster, also in em- 

 bedded grains or as loose sand. 



Ilmenite is found in many igneous rocks 

 notably in gabbros and diorites; it is some- 

 times altered to limonite and titanite. In addition to several 

 European localities ilmenite is found in the town of Warwick, 

 Orange co. N. Y. and at Litchfield Ct. 



The large amount of fuel required to reduce this mineral 

 renders it, in most cases, undesirable as an ore of iron. It is, 

 however, used as a lining in furnaces. 



Spinel MgO.Al 2 3 



Spinel, the magnesium aluminate, contains 

 71.8$ alumina and 28.2$ magnesia. These two 

 components may be replaced in part by ferrous 

 and ferric iron, manganese and chromium. 



The crystals of spinel are isometric, usually 

 the octahedron or the octahedron modified, and are frequently 

 twinned (fig. 190). The luster is vitreous to dull and the color 

 varies from red to blue, green, yellow and black. A transparent 

 variety called ruby spinel is transparent to translucent and 

 often of a rich red color. This constitutes the gem known as 



a N. Y. state mus. Bui. 7. 18S9. 



