Page:The Journal of geology (IA journalofgeology11893univ).pdf/378
- such concentration has taken place, such as in the replacement of limestone, etc.
It will now be attempted to show how the various degrees of association and separation of iron and manganese found in nature may be produced by different conditions during deposition.
THE FORMS OF IRON AND MANGANESE DEPOSITED AT ORDINARY TEMPERATURES.
The mineralogical forms in which iron and manganese are deposited from solution in nature at ordinary temperatures depend on the conditions of air and water, whether of an oxidizing or a reducing nature, and on the character of the associated organic and inorganic matter either in solution or on the floor of the sea, lagoon or bog in which the deposition occurs.[1] There are four principal methods by which iron and manganese are precipitated in nature from surface waters:
- (1) By oxidation, as in the case of the precipitation of hydrous oxides and in the precipitation of the carbonate by the partial oxidation of more complex organic salts.[2]
- (2) By reduction, as in the precipitation of sulphide of iron by the reduction of sulphate of iron.
- (3) By gaseous or soluble precipitants, as in the precipitation of sulphide of iron by the action of sulphuretted hydrogen or a soluble sulphide on a soluble salt of iron, and as in other cases to be mentioned later.
- (4) By replacement of carbonate of lime or some other substance. Different forms are precipitated by these different methods.
Iron at ordinary temperatures is usually deposited from solu-
- ↑ The solutions may be precipitated, as already shown, either with or without admixture with mechanical sediments; and there are in nature all gradations from almost pure deposits of iron and manganese ore to beds of shale, sandstone, etc. stained with iron or manganese. Subsequent concentration frequently causes decided changes in the latter deposits (see p. 370).
- ↑ It has been suggested by A. A. Julien (Proceed. Amer. Assoc. Adv. Sci., Vol. XXVIII., 1879, p. 356) that in some cases the carbonates of iron and manganese may be only the fixed residue of organic compounds of more complex form once in solution in surface waters.