686 APPENDIX 



Iron Minerals 



Magnetite (magnetic iron ore), Fe 3 4 . — Color, black. A black 

 streak is made when the mineral is scratched on a hard white surface. 

 The hardness is slightly greater than steel (H= 6). It is always at- 

 tracted by a magnet and is sometimes capable itself of lifting particles 

 of iron and steel. It is a valuable ore of iron. The Adirondack iron 

 ore is largely magnetite. 



Hematite (red iron ore), Fe 2 03. — The color is black to brick red. 

 Streak, red. Slightly harder than steel (H= 6). Occurs in compact 

 masses composed of micalike flakes, in an earthy form, and in thin 

 crystals set on edge. It is the most widely used iron ore in North 

 America, the most famous localities of which are in the Lake Superior 

 region and in Alabama. 



Limonite (brown hematite, iron hydroxide), 2 Fe 2 C>3'3 H 2 0. — The 

 color is usually dark brown, but is sometimes yellow. The streak is 

 yellow. The hardness of compact kinds is slightly less than steel 

 (H= 5). The ocher which occurs with limonite is composed of clay 

 and limonite in a finely divided condition. Limonite is really iron 

 rust and is formed from the hydration of many iron minerals, and con- 

 sequently occurs in many situations and is widespread. It is an ore 

 of excellent quality, but is little used in this country because of the 

 more abundant and more easily mined hematite. It is common in 

 New England and the Appalachians. 



Siderite (spathic ore), FeC0 3 . — The color is gray on freshly 

 broken surfaces ; surfaces exposed to the weather, even for a few 

 weeks, are brown. The streak is white, or nearly so. It can be 

 easily scratched with a knife (H = 4). It occurs commonly in 

 masses which show shiny, bent, cleavage surfaces. Siderite effer- 

 vesces with warm hydrochloric acid, giving off carbon dioxide. 

 It is an ore of iron which, however, is little used in the United 

 States. 



Pyrite or Iron Pyrites (fool's gold), FeS 2 . — The color is brass-yellow 

 when fresh, but oxidizes on the outside to brown limonite. It is 

 harder than steel (H = 6.5). It occurs in veins and is disseminated 

 throughout many igneous and sedimentary rocks. It often occurs 

 in cubical crystals or in crystalline masses. The yellow stains on 

 rocks are often due to the weathering of grains of pyrite. Pyrite 

 is a common mineral. It is not used as an ore of iron, but is used 

 in the manufacture of sulphuric acid. 



