Though the sulphides of manganese are easily oxidized, they are not so unstable that, had they ever been formed in considerable quantities in sedimentary deposits, they would, even at considerable depths, have left no trace of their former presence. Moreover, the sulphide of manganese, as produced artificially,[24] is soluble in certain organic acids, notably acetic, and, as the conditions for the deposition of sulphides of metals in sedimentary deposits generally require the presence of organic matter, it is not improbable that some of the acids given off by such matter would be capable of dissolving sulphide of manganese. Here, then, is one reason why manganese might not be deposited as sulphide under some conditions which would cause the precipitation of sulphide of iron. Moreover, the artificial formation of sulphide of manganese (alabandite) in the laboratory is brought about most easily at high temperatures. It has also been noted that when manganese, in the form of the alloys spiegeleisen and ferro-manganese, is added to molten steel, it bodily removes a part of the sulphur; and it is thought by some metallurgists, that sulphide of manganese is formed and carried into the slag.
These and other indications of the more easy transition of manganese into the form of sulphide at high rather than at low temperatures afford another cause which might prevent sulphide of manganese from being formed in sedimentary deposits, for such deposits are usually laid down at ordinary temperatures. On the other hand, they also afford a cause which might lead to the deposition of the sulphide of manganese in certain metalliferous veins and other deposits, where the temperature at the time of deposition may have been high.
In many of the silver and lead deposits of the Rocky Mountains manganese oxides occur with the superficial oxidation products of the sulphides of other metals, and it has often been suggested that the manganese also was originally in the form of sulphide. This may be true in some cases, for alabandite has been found in a few metalliferous deposits in Colorado, Mexico, Germany, Peru and elsewhere, but in most cases, at least in the Rocky Mountains, when the level is reached at which the oxidized forms of lead, zinc, iron and other metals pass into sulphides, the manganese passes into carbonate or silicate, and remains in one or both of those forms to all depths that have been reached.
In the deposition of iron and manganese as sulphide, therefore, there is a most marked difference of behavior, and here again is a good cause for the separation of the two substances in sedimentary rocks, as will be more fully explained below.
Iron is often deposited in sedimentary formations as the hydrous silicate of iron and potash known as glauconite, and composes the mass of the large greensand beds common in Cretaceous and Tertiary strata; but manganese is not found in an exactly similar condition.[25] Here again, therefore, is an important difference in the modes of deposition of iron and manganese, which also will be mentioned again.
It will thus be seen that while some of the forms in which iron and manganese are deposited are the same, others differ very widely, and even similar forms are often deposited under different conditions. It is doubtless to these various forms and conditions of deposition that the alternate association and separation of iron and manganese in nature are due.
CAUSES OF THE ASSOCIATION OF IRON AND MANGANESE.
The very frequent intimate association of iron and manganese in sedimentary rocks is what would be expected from a deposition as oxide or carbonate in basins such as coastal lagoons or bogs, where the waters moved very slowly, or not at all, for under such conditions, they are often deposited together.[26] Moreover, it is a well-known fact that isomorphous substances have a strong tendency to combine in a homogeneous mass, and to crystallize together in different proportions. Carbonate of iron and of manganese are isomorphous with each other, and this is hence a possible cause of the frequent intimacy of their association, such as is seen in almost all manganiferous spathic iron ores, whether these ores are formed by direct precipitation or by replacement of carbonate of lime. The oxidation of such a mixture would give the common form of an intimately combined iron and manganese ore.
Since there is usually more iron than manganese in the rocks from which both metals were originally derived, the surface waters draining from areas of such rocks usually contain the metals in a similar proportion. Hence, in cases where the deposition of the carbonates of both occurs at the same spot, the isomorphous carbonates derived from the solutions have a larger percentage of carbonate of iron than of carbonate of manganese, and the resulting oxides contain the two metals in the same proportion, thus giving rise to the common low-manganese iron ores.
The hydrous oxides of iron and manganese, however, are not isomorphous,[27] and, therefore, when they are precipitated together, as in bog-deposits, the association is often much less intimate than in the cases just mentioned, and is simply due to the fact that, under certain conditions, the oxides of both metals are precipitated in the same place.