230 ECONOMIC GEOLOGY 



tration. (6) Cost of grinding and packing in barrels. (7) Other 

 minor expenses. 



Chromium: Its Properties, Occurrence and Uses 



Properties. Chromium, symbol Cr, is a hard steel-gray 

 metal permanent in dry atmosphere, even in minute quantities. 

 Unlike iron it is non-magnetic. The magnetism sometimes 

 feebly present in the chief ore, chromite, is due to the ferrous 

 oxide present in the mineral. Chromium is soluble in HC1. It 

 imparts great hardness and tenacity to steel. Its specific 

 gravity is 6.8. Its melting point is 1515 C. Its atomic weight 

 is 52.1. 



Ores of Chromium. The metal does not occur free and un- 

 combined in nature on account of its affinity for oxygen. The 

 chromium minerals are therefore oxygenated compounds. 



Chromite, Cr 2 O 3 ,FeO, 68 per cent. Cr. The ferrous iron may 

 be replaced by manganese and the chromium by ferric iron and 

 aluminum. 



Chrome ocher, Cr 2 3 . A bright green mineral bearing some 

 clay. 



Crocoite, PbCrO4. A rare mineral in varying shades of hya- 

 cinth red. 



The first mentioned mineral, chromite, is the sole source of the 

 metal of commerce. It is widely diffused in the basic igneous 

 rocks rich in magnesium. It has been found in placers derived 

 from their decomposition. It has been observed adherent to 

 an interpenetrating platinum magnet. The rare mineral, 

 daubreelite, Cr 2 S 3 ,FeS, has been observed in several meteorites. 



Origin of the Ores. S. Meunier has suggested that whenever an 

 alloy of chromium and iron is brought up from the zone of 

 flowage it oxidizes as it nears the surface. The theory is 

 objectionable because no such alloy is known to exist in nature. 

 Chromite is essentially a primary magmatic mineral. It is one 

 of the first minerals to segregate from an ultra-basic magma 

 like peridotite. Its associate in this differentiation is magnetite. 

 Microscopic slides of the magnetites and chromites of northern 

 Vermont and Megantic County, Quebec, have been prepared to 

 ascertain which of these ores solidified first but the problem is 

 still unsolved. Ore bodies sufficiently large to be of commercial 

 significance are definitely recognized as a product of magmatic 



