being deposited at a given time are sometimes derived from areas
in which iron predominates, sometimes from areas in which iron and
manganese are both abundant, and sometimes, though rarely, on account
of the scarcity of such regions, from areas in which manganese
largely predominates over iron. If iron and manganese were always
precipitated from these waters in similar chemical forms and under
the same conditions, it would be expected that the strata deriving
their iron and manganese from surface waters would contain those
substances in the same relative proportions as they had existed in
the rocks from which they were derived, and that they would be in an
intimately mixed condition. Such is doubtless often the case, or at
least approximately so; but it is also often the case that iron and
manganese occur in separate deposits, yet in close proximity to each
other and often alternating along the same horizon. Besides this, the
two substances frequently form parts of the same deposit and yet are
distinctly separate from each other. In such cases the question arises
as to why the iron and manganese are not intimately mixed in the form
of a manganiferous iron ore, as would be expected if they had been
precipitated together. Moreover, deposits sometimes occur which are
composed largely of manganese ore, with little or almost no iron, and
when the source of the manganese is looked for, we often find that the
rocks which probably supplied it contained both manganese and iron, and
that the iron was present in a much larger proportion as regards the
manganese than in the new deposit. Here again the question arises as to
why the iron and manganese are not in the same relative proportions in
the new deposit as they were in the rocks from which they were derived.
Four principal causes suggest themselves in explanation of this
separation:
(1) It might be supposed that the deposits containing mostly iron and
those containing mostly manganese received these constituents from
waters derived from different sources, and carrying iron and manganese
only in the proportions in which they deposited them. Under some
conditions this explanation might suffice, but in many cases, such as
when iron and manganese alternate along the same geologic horizon, and
yet in close proximity with each other, the explanation is entirely
inadequate, for the deposits are too close to each other to have been
formed from different supplies of surface waters.
(2) It might be supposed that the iron or the manganese had been
leached out of a deposit of the mixed ores, leaving one free from
the other and depositing the dissolved ore somewhere else. This
explanation, except in special cases, also appears inadequate, because
the reagents in surface waters, which dissolve iron and manganese, seem
to affect both about equally, so that if one were dissolved, the other
should be taken up in the same way. Doubtless small differences could
be found in the behavior of the organic and inorganic compounds in
surface waters towards iron and manganese minerals, but they would be
small as compared with the more active reactions which go on.
(3) It might be supposed that a separation could be produced by
secondary concentration such as segregation, replacement, etc. This
has doubtless sometimes been the case, but where the concentrating
action is not assisted by a difference in the chemical behavior of the
two substances, the separation would only be on a small scale. Even
in the case of concentration by replacement of limestone, if iron and
manganese both acted in the same way during the replacement, it would
be expected to find them deposited in an intimate mixture. Though this
secondary concentration, therefore, unassisted by other agencies, would
not produce all the results found in nature, yet, when it is thus
assisted, it often plays an important part.
(4) The fourth, and what seems the most important, factor in the
separation of iron and manganese, is that, though very often they are
precipitated in the same form from the same solution, yet sometimes
they are precipitated in different forms; and even when precipitated in
the same form, the precipitation of one sometimes requires different
conditions from the precipitation of the other. This fact will explain
the alternate association and separation of iron and manganese, not
only when no secondary concentration has gone on, but also in cases
where 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.[18] 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.[19]
(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 solution as
the hydrous sesquioxide, the carbonate, the sulphide or the hydrous
silicate of iron and potash known as glauconite. Manganese under
similar conditions is deposited as the hydrous oxide[20] or as the
carbonate, and possibly sometimes, though very rarely, as sulphide.
When solutions of organic or inorganic salts of iron and manganese are
freely exposed to the action of air, as in shallow or rapidly moving
streams, or in lakes and some bogs, they are quickly oxidized and
both may be deposited as more or less hydrous oxides. In many bogs,
however, the metals may be precipitated as hydrous oxide on the surface
where oxidizing agencies predominate, but when these oxides sink and
come into contact with decaying organic matter, free from the active
oxidizing influences of the air, they may be reduced to carbonates.
The carbonates of iron and manganese may be precipitated when the
solutions containing them are protected from oxidation by a reducing
agent, such as decaying organic matter, or by being far removed from
the air. Carbonate of manganese, however, is a much more stable
compound than carbonate of iron, and the oxidizing conditions are
often sufficiently strong to cause the deposition of iron as hydrous
sesquioxide and not strong enough to change the manganese from its
carbonate form. It is not uncommon, therefore, to have iron deposited
in one place as hydrous sesquioxide, and manganese carried further on
and deposited as carbonate, or even under special conditions deposited
as carbonate with the hydrous sesquioxide of iron. Fresenius[21] has
shown that the warm springs of Wiesbaden, which contain iron and
manganese among their other mineral constituents, deposit iron in the
form of hydrous sesquioxide, while manganese is carried on further
in solution and deposited as carbonate. In this behavior, therefore,
we have the first striking difference in the deposition of iron and
manganese, and it will be further discussed later on.
The sulphides of iron and manganese differ very much in their nature
and mode of occurrence. Iron is frequently deposited as sulphide, but
manganese rarely occurs in that form, and when it does it is always in
very small quantities. Iron forms several sulphides in nature: pyrite
(FeS₂), marcasite (FeS₂),[22] pyrrhotite (Fe₁₁S₁₂), troilite (FeS) and
numerous other more complex compounds unnecessary to enumerate here.
Pyrite is the commonest form of iron sulphide, and occurs in rocks of
all ages, from Archean to Recent. It is formed in nature by the action
of soluble sulphides or sulphuretted hydrogen on soluble salts of iron,
and also by the reduction of sulphate of iron by organic matter or
other reducing agents. Manganese forms two[23] sulphides, alabandite
(MnS) and hauerite (MnS₂). Both minerals are very rare, and so unstable
that they rapidly oxidize on exposure. Alabandite is the less rare
form, and usually occurs as a subordinate constituent of certain
metalliferous veins or allied deposits.
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.
CAUSES OF THE SEPARATION OF IRON AND MANGANESE.
When iron and manganese ores occur in more or less separate deposits,
it becomes necessary to suppose the action of influences different from
those which cause the deposition of both together, and such influences
are to be found in the different modes of precipitation, under certain
conditions, of the two metals. It has been shown by Fresenius[28] that
certain warm springs, on reaching the surface, first deposit hydrous
sesquioxide of iron, and farther on carbonate of manganese. This not
only points to the well-known fact that carbonate of iron is more
easily oxidized than carbonate of manganese, but it also leads to the
belief that the bicarbonate or other salt of iron in the water is more
easily oxidized than the manganese salt.
An action somewhat similar to that described by Fresenius readily
explains the occurrence of manganese sometimes in entirely separate
deposits, sometimes in distinct but closely alternating deposits.[29]
Under certain conditions, if the waters from which the precipitation
took place were moving, the iron and manganese, owing to the difference
in oxidability, as stated above, would be laid down in different
places, resulting in the formation of deposits of iron ore free from
manganese, and manganese ore free from iron in different positions
along the plane of the same geologic horizon. Such occurrences are
often seen in the iron regions of the Appalachian Valley, where there
are often found, in different places along the same belt, deposits
of iron ore and deposits of manganese ore in positions similar with
relation to the enclosing rocks.
These conditions of moving water might also cause the occurrence of
the two ores in interstratified layers, as is sometimes the case. Such
a condition would result if iron were deposited in a certain place at
one time, and if, later, on account of some increased facility for
oxidation, iron was deposited before it reached that place, and the
manganese, being less easily precipitated, were carried on and laid
down upon the first deposit of iron.
Suppose the metalliferous solutions to be confined in a shallow
basin, or, at least, to pass through it so slowly that they become
thoroughly oxidized. Under such conditions the deposition of iron and
manganese would go on continuously, and so nearly on the same spot that
a comparatively homogeneous manganiferous iron ore would be formed.
If the supply of metalliferous solutions were not continuous, but
were intermittent, as is sometimes the case in local basins, such as
coastal lagoons, which are often dependent for their supply of water
on the changes of season and the sudden fluctuations of weather, then
interstratified layers of iron and manganese ore might be produced. The
iron, becoming oxidized on the surface, sinks to the bottom, possibly
in some cases to be converted there to the simple carbonate by organic
matter. Further oxidation precipitates hydrous oxide or carbonate of
manganese on top of the iron. A renewed supply of surface waters brings
more solutions of iron and manganese, or else the evaporation of the
water in the closed basin concentrates the materials which have not
yet been precipitated. In either case there is a further alternate
deposition of the two ores.[30]
Another process of separation of iron and manganese in nature might
take place by the formation of sulphide of iron. It has already been
shown that iron is sometimes deposited as sulphide and later oxidized
in the same manner as the carbonate. Manganese, on the other hand,
is rarely found as sulphide, and there is reason to think that the
sulphide never represented the original form of any large sedimentary
deposits of manganese ore (see pages 364 to 365). It seems probable,
therefore, that from a solution of iron and manganese in surface waters
the iron might, where the conditions are favorable, be precipitated
as sulphide (FeS₂) and the manganese might be carried on in solution
to be deposited somewhere else as oxide or carbonate. Subsequently
the oxidation of the ores would give rise to oxide of iron from the
sulphide and oxide of manganese from the carbonate; and the two ores,
though occurring at the same horizon, would be separated by a greater
or less distance.
After the deposition of the sulphide of iron, the conditions might
change and permit the deposition, in the same place, of the carbonates
of iron and manganese together. This is an easy case to imagine, and
where such a deposit was exposed to surface influences, the resulting
product would be oxide of iron from the underlying sulphide and a
manganiferous iron oxide from the overlying isomorphous carbonates.
Hence another possible cause of the frequent association of pure iron
ores and manganiferous iron ores. It is possible also that after the
solution of iron and manganese had been freed from the former by
precipitation as sulphide, the manganese might be carried on and laid
down as carbonate on a previous deposit of iron sulphide, and when such
a combination was oxidized, the result would be oxide of iron and oxide
of manganese in beds closely associated but yet distinct.
By supposing the iron sometimes to be deposited in sea water as
glauconite, a manner in which manganese is not laid down (see page
365), a further means of separation of the two metals would result.
Thus by alternating the conditions of the deposition of iron and
manganese in different forms, a great variety of methods of association
and separation of the two metals can be produced.
The above discussion refers not only to the deposits of iron and
manganese ores of notable size, but also to the iron and manganese
frequently found disseminated through shales, sandstones etc. In these
rocks they usually form a small but often a very important part, for in
many cases the iron and manganese is taken into solution from the rocks
and redeposited by a process of replacement with carbonate of lime
in neighboring beds of limestone, or more rarely by replacement with
other rocks, thus giving rise to important ore deposits. The question
of the association and separation of the iron and manganese in these
replacement deposits depends on a number of conditions, the principal
of which are, just as in the class of deposits that has been discussed,
the conditions during deposition and the forms in which the iron and
manganese are precipitated. The processes by which association and
separation occur in replacement deposits differ somewhat in detail from
the processes just discussed, but are based on the same principles.
Many of the iron and manganese deposits of the Appalachian region are
supposed by many to be replacement deposits. N. S. Shaler[31] in 1877
suggested that some of the iron deposits of Kentucky and Ohio were
formed by the solution of iron from certain rocks, and its deposition
in the form of carbonates by replacement with underlying limestone.
Subsequently it was changed by oxidation to brown hematite. A notable
case of replacement has also been shown by R. D. Irving and C. R. Van
Hise[32] in the iron deposits of the Penokee series of Michigan and
Wisconsin. Here the ore is supposed to be partly a replacement of chert
in a trough between quartzite and igneous rocks. The solution that
contained the iron was derived from strata in the same series of rocks
in which the iron was re-deposited and contained a certain amount of
manganese. It is shown how the iron and manganese were more or less
separated in the replacement process and that the separation was due
to the difference in the oxidability of the carbonates as explained on
page 363.
R. A. F. PENROSE, JR.
FOOTNOTES
[13] The exact age of the iron and manganese deposits here referred
to is, in some cases, a little uncertain. Some may be Cambrian,
others Silurian, but the exact determination of the age of the
horizon is not a part of the present discussion. The matter has
been discussed by the writer in Geological Survey of Arkansas,
1890, Vol. I., pp. 376–379.
[14] See Geological Survey of Arkansas, 1890, Vol. I., pp. 320–325.
[15] See Geological Survey of Arkansas, 1890, Vol. I., pp. 456–457.
[16] R. D. Irving and C. R. Van Hise, U. S. Geol. Survey, Tenth Ann.
Report, 1888–1889, Vol. I, pp. 409–422.
[17] Geol. Survey of Penn., Vol. II, 1858, p. 739.
[18] 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).
[19] 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.
[20] This oxide is generally in the form of the peroxide or the
sesquioxide in a more or less hydrous condition.
[21] Jahrb. des Vereins f. Naturkunde in Herz. Nassau, Vol. VI., p.
160 (Bischof).
[22] Marcasite has the same composition as pyrite, but differs in
crystalline form.
[23] Manganese also occurs in the mineral youngite, which contains
lead, zinc, iron, manganese and sulphur, but the mineral is
considered of doubtful homogeneity. (See System of Mineralogy,
E. S. Dana, 1892).
[24] When manganese is precipitated artificially as sulphide it is
usually in the form of the monosulphide (MnS), in either a
hydrous or an anhydrous form.
[25] Manganese occurs in various hydro-silicates, but they do not
appear to be deposited as sedimentary strata in the same manner
as glauconite.
[26] If the water moved very slowly, the deposition would probably
take place approximately in the same spot; if the waters moved
more rapidly, the iron might be deposited in one place and the
carbonate in another, in the way explained on page 363.
[27] The hydrous oxides of iron are not crystalline.
[28] See p. 363.