OCCURRENCE OF HEMATITE. 225 



especially the belt of weathering - . It is almost equally certain that the 

 passage of corundum into spinel, sillimanite, and cyanite is characteristic 

 of the zone of anamorphism. 



The case, however, is not clear in reference to the muscovite, margarite, 

 and zoisite. The equations as written are those of silioifiation and slight 

 hydration. If these equations be correct, they should occur in the lower 

 part of the belt of cementation or in the zone of anamorphism. It is 

 tolerably certain that margarite, zoisite, and muscovite form in the lower 

 part of the belt of cementation; but the zone in which muscovite charac- 

 teristically develops is that of anamorphism. It is not at all impossible 

 that the potassium carbonate, and perhaps the calcium carbonate, or even 

 the silica, are added in solution for the margarite and zoisite. In this case 

 there would be a considerable volume increase. Whether the same may 

 be assumed for the muscovite is uncertain. Very likely the materials 

 added to the corundum are in some cases carried in by the solutions, in 

 others are derived from adjacent minerals, and in still others partly from 

 both. Where the lime and potash are derived from minerals adjacent, they 

 may come from other compounds than carbonates, and the silica may have 

 been previously united with other bases. So far as this is so, in considering 

 the variations in volume the minerals from which the elements added to the 

 corundum to produce the muscovite, margarite, and zoisite were derived 

 must be taken into account. It is clearly impracticable in the present state 

 of knowledg-e to give definite statements as to the volume changes for these 

 minerals. 



HEMATITE. 



occurrence. — Hematite is a pyrogenic constituent in igneous rocks and is 

 an abundant metamorphic mineral. Its most abundant source in the 

 metamorphic rocks is by the dehydration of limonite, a reaction occurring 

 with the absorption of heat and reduction of volume. A second important 

 source of hematite is from iron carbonate by loss of carbon dioxide and by 

 oxidation, a reaction occurring with the liberation of heat and reduction of 

 volume. Hematite may also be produced by the oxidation of magnetite, a 

 reaction resulting in liberation of heat and expansion of volume. Fre- 

 quently after this change the hematite has the isometric form of the original 

 magnetite and is called martite. A fourth source of hematite is by the 

 oxidation of the ferrous iron of silicates at the time of their decomposition. 

 mon xlvii — 04 15 



