SPINEL GROUP. 229 



MAGNETITE. 



■■■ — Magnetite is a very abundant pyrogenic constituent in 

 igneous rocks. It is abundantly deposited from solutions, and especially 

 from solutions bearing iron carbonate, according to the reaction: 



3FeCO s +0=FeA+3C0 2 +k. 



Magnetite also extensively forms from siderite in situ. These changes 

 liberate heat and decrease the volume. A third source of magnetite is by 

 incomplete oxidation of pyrite and marcasite, reactions occurring with 

 liberation of heat and diminution of volume. Fourth, frequently siderite 

 and iron sulphide together pass into magnetite with decrease in volume. 

 (See pp. 244, 845.) A fifth way in which magnetite may be produced is 

 by the reduction of hematite by organic compounds, a reaction occurring 

 with the liberation of heat, because of the simultaneous oxidation of the 

 organic compounds, and with diminution of volume. A sixth way in which 

 magnetite is produced is by the incomplete oxidation of ferrous iron of 

 silicates; for instance, olivine and garnet. 



In summary, magnetite is derived from actinolite, ankerite, arfvedsonite, 

 augite, biotite, bronzite, diopside, garnet, greenalite, griinerite, hematite, 

 hornblende, hypersthene, ilmenite, marcasite, and pyrite. 



Alterations. — Magnetite alters into hematite (rhombohedral ; sp. gr. 5.225), • 

 limonite (amorphous; sp. gr. 3.80), and siderite (rhombohedral; sp. gr. 3.83- 

 3.88). The reactions are as follows: 



(1) 2FeA+0=3Fe 2 3 +k. 



(2) 4Fe 3 4 +20+9H,0=3(2Fe 2 3 .3H 2 0) +k. 



(3) FeA+CO+2cd 2 =3FeC0 3 +k. 



In the change the increase in volume is, for (1), 2.44 per cent; for (2), 

 64.63 per cent, and for (3), 101.30 per cent. The increase in volume in the 

 change from magnetite to siderite — over 100 per cent — is the greatest 

 volume change in which only two minerals are concerned which the calcu- 

 lations of Mr. Lincoln have given, with the exception of the alteration of 

 corundum into gibbsite. (See p. 224.) All of the above changes are well 

 known to occur in the zone of katamorphism, and corresponding with this 

 position the)' all take place with the liberation of heat, expansion of volume, 

 and decrease in symmetry. 



CHKOMITE. 



occurrence. — Chromite occurs in the igneous rocks, especially those rich 

 in magnesium. It also occurs in the metamorphic rocks, often in co.nnec- 



