Redox and nonredox reactions of magnetite and hematite in rocks

Abstract

Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization [replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization [replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe2+ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe2+ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical.

Full text

Redox and nonredox reactions of iron oxides in rocks
Arno Mücke and Alexandre Raphael Cabral
With 2 plates

MÜCKE, A. & CABRAL, A. R. (200x): Redox and nonredox reactions of iron oxides in
rocks.- N. Jb. Miner. Mh.
Abstract: Redox and nonredox reactions causing pseudomorphic replacement of hematite by
magnetite and magnetite by hematite were compared.
Pseudomorphic replacements resulting from redox reactions are known as martitization
(replacement of magnetite by hematite due to oxidation; reaction 1) and mushketovitization (=
replacement of hematite by magnetite due to reduction; reaction 2). These two replacements
cause characteristic ore textures and volume changes (reaction 1: increase of 1.66%; reaction
2: decrease of 1.64%). These small volume changes are the reason that martitization and
mushketovitization are widespread in many rocks under condition, however, that oxidizing or
reducing fluids (solutions) are present.
The same initial and end products may also be involved in nonredox reactions. The first
reaction is the replacement of magnetite by hematite (1) due to leaching of Fe2+ atoms under
acidic conditions. The second and reverse reaction causes the replacement of hematite by
magnetite (2) due to simple addition of Fe2+atoms under basic conditions. Reaction (1) causes
a volume decrease of 32.2% and reaction (2) an increase of 47.6%. From these volume
changes it is concluded that the first replacement may occur in many rock types, whereas the
second is restricted to unlithified sediments only. Ore textures caused by nonredox reactions
are unknown and therefore their occurrence in rocks is hypothetical.
Key-words: redox/nonredox reactions, replacement of magnetite by hematite and vice versa,
volume changes, ore textures.
Introduction
Stimulated by the recent paper of OHMOTO (2003) who discussed nonredox reactions for the
conversion of iron oxides, a critical and brief review and comparison of nonredox and redox
reactions in rocks will be undertaken. The evaluation will be based on volume changes caused
by the conversions of iron oxides and microscopic studies of the resulting ore textures.
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In nearly all rock types where iron oxides occur, mutual replacement of magnetite and
hematite are widespread. In banded iron-formations the age relationships of hematite and
magnetite and their mutual replacement have intensively been debated. The reason for this
discussion is the existence of two concepts about the development of iron-formations. One
concept supports the idea that the iron oxides of iron-formations originated from direct
precipitation or that they are of early diagenetic origin resulting from precipitated ferric
hydroxide (designated as precipitation model by MÜCKE et al. 1996). In this model hematite
is the early mineral followed by magnetite which replaced the first (e. g. HUBER, 1959;
FLORAN & PAPIKE, 1978; MORRIS, 1980, 1993, 2002; HAN, 1982; BAUR et al. 1985;
HARMSWORTH et al., 1990; JAMES, 1992; KLEIN & BEUKES, 1993; TOMPKINS &
COWAN, 2001). The other concept favours the idea that magnetite is the early iron oxide (e.
g. HACKSPACHER, 1979; QUADE, 1988; ROSIÈRE et al., 2001) formed from mudstone-
like and iron-rich sedimentary protoliths under metamorphic conditions (MÜCKE &
ANNOR, 1993; MÜCKE et al., 1996, referred to as metamorphic model; MÜCKE, 2003).
Hematite is a younger mineral and was formed at the expence of magnetite due to oxidation.
Nonredox reactions
OHMOTO,s reaction is the following: Fe3+2O3 (= hematite) + Fe2+ + H2O = Fe2+Fe3+2O4 (=
magnetite) + 2H+. The forward reaction is the conversion of hematite to magnetite by simple
addition of Fe2+ ions, whereas the reverse reaction is the conversion of magnetite to hematite
by leaching of Fe2+ in an acidic environment. From the explanation of the two reactions, it can
be concluded that they are necessarily pseudomorphic replacements. According to OHMOTO
(2003), the forward reaction (= replacement of hematite by magnetite) may occur in many (if
not all) iron-formations. Hematite, being of early precipitation origin and constituent of the
sedimentary protoliths, converted subsequently to magnetite during diagenesis using Fe2+
originated from submarine exhalations. The reverse reaction (= replacement of magnetite by
hematite) was considered by OHMOTO in the deeply burried iron- formation of the
Australian Hamersley Basin.
The reactions of OHMOTO are not volume-constant. Magnetite has a cell volume of 592.704
Å3 which contains 8 formula units Fe2+Fe3+2O4. Hematite, on the other hand, has a cell volume
of 301.279 Å3 containing 6 formula units Fe3+2O3. From these data and the above-mentioned
reverse reaction (having an iron ratio of the initial and the final products of 3 : 2), it can be
calculated that the replacement of magnetite by hematite causes a volume decrease of 32.22%.
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This value was also discussed by OHMOTO (2003) and from this he concluded a significant
increase of the rock permeability, thus facilitating water-rock interaction. The forward
reaction (hematite to magnetite; having an iron ratio of the initial and the final products of 2 :
3), causes an increase in volume of 47.55%. Microscopic characteristics that originated from
nonredox reactions were not described by OHMOTO (2003).
Evaluating his own reactions which are indirectly based on experimental evidence, OHMOTO
(2003) stated that the replacements of hematite by magnetite, and magnetite by hematite, are
not restricted to iron-formations only. As an example, OHMOTO mentioned that many
mangnetite-skarn deposits show the conversion of early hematite to magnetite and magnetite
to late hematite; and he continues that these conversions may also have occurred without the
involvement of an oxidant or reductant. OHMOTO concluded that nonredox reactions may
have been the principle mechanism for the transformation of iron oxides in nature, especially
in hydrothermal environments. Therefore, he put forward the reactions as a model.
Redox reactions
Pseudomorphic replacements of magnetite to hematite and vice versa resulting from redox
reactions are:
1. 2Fe2+Fe3+2O4 (= magnetite) + 0.5O2 = 3Fe3+2O3 (= hematite); and
2. 3Fe3+2O3 (= hematite) + H2 = 2Fe2+Fe3+2O4 (= magnetite) + H2O
The first reaction involves the pseudomorphic replacement of magnetite to hematite due to
oxidation which is known as martitization. The oxidation reaction follows the (111)-planes of
primary magnetite and the resulting arrangement of the newly-formed hematite is known as
martite-textured hematite (Plate 1A). Incomplete martitization of magnetite leads to the same
texture, but newly-formed hematite contains relics of magnetite (Plate 1B). Martitization may
also be developed along grain boundaries of, or patchy-like within, magnetite. This
conversion is known as strain-martitization (Plate 1C; BAUMANN & LEEDER, 1991). The
above-mentioned textures are often obliterated, particularly in iron-formations, due to
subsequent metamorphism which causes the recrystallization of martite-textured hematite into
coarse-grained hematite or specularite. As an indication that they originated from early
magnetite, both coarse-grained hematite and specularite may contain relics of martite-textured
hematite (Plates 1D and 2A) and subordinately magnetite. In rare cases, magnetite may also
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be directly replaced by hematite without martitization (Plate 2B). The second reaction is the
pseudomorphic replacement of primary hematite by magnetite under reducing conditions.
This replacement is known as mushketovitization. Incomplete reduction leads to magnetite
pseudomorphs after hematite in which remnants of hematite are preserved (Plate 2C).
In both redox reactions the amount of the iron atoms remains constant, there is only an
addition or removal of oxygen. From the cell volumes of magnetite (592.704 Å3) and hematite
(301.279 Å3) and the amount of Fe atoms in them (24 in magnetite and 12 in hematite), it can
be calculated that the replacement of magnetite by hematite causes a volume increase of
1.66% and that of hematite by magnetite a decrease of 1.64%.
Discussion
Generally, the volumes of the initial and the final products of pseudomorphic replacements
should have nearly the same or slightly smaller volumes. However, the volumes of the final
products may also be larger, but the enlargment cannot exceed a defined value which may be
definitely lower than 5%. Theoretically, smaller volumes of the final products may have no
limitation, but practically the decrease should not be bigger than about 30%. This value was
deduced from the reaction 2FeOOH = Fe2O3 + H2O. The forward reaction concerns the
abundantly distributed pseudomorphic replacement of goethite by hematite due to dehydration
and causes a volume decrease of 27.42%. The reaction is irreversible because the reverse
reaction, the rehydration of hematite causing an increase of 37.78% is never observed in rocks
(PALACHE et al., 1944; MÜCKE, 1994, page 369-370; Plate 2D). In agreement with the
observation, LANGMUIR (1971) demonstrated the impossibility of the rehydration of
hematite on theoretical grounds.
The volume changes caused by the widely accepted redox reactions, showing only small
differences in the volumes of the initial and final products (not higher than 2 %), may explain
the abundance of martitization and the formation of mushketovite under the assumption that
the environmental conditions for the reactions 1 and 2 are fulfilled. In contrast to redox
reactions, nonredox reactions are connected with considerable volume changes. Under the
assumption that the reaction of magnetite to hematite took place, from the considerable
volume decrease of 32.2 % it is concluded that the newly-formed hematite should have a
porous appearance and/or abundant shrinkage cracks.
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The other nonredox reaction causes the replacement of hematite by magnetite and an increase
of the volume of 47.55%. This value shows that the proposed reaction is unrealistic. In order
to explain the impossibility of this reaction, a rock that contains 50 vol% hematite should be
considered. After conversion of the preexisting hematite to magnetite, 1 m3 of this rock
increases to a volume of 1.24 m3. However, in sediments, like the protolith of iron-formations
(which was considered by OHMOTO) such a reaction may possibly occur, because these
rocks are unlithified. If such rocks increased their volumes, the excess volume has simply to
displace seawater.
However, a widely distributed pseudomorphic replacement is known in many rocks including
rocks of iron-formations, to which a nonredox reaction can be applied. This concerns the
replacement of magnetite by goethite (Plate 2D). The reaction was hitherto considered to be
the following: 2Fe2+Fe3+2O4 (= magnetite) + 3H2O + 0.5O2 = 6FeOOH (= goethite). The
postulated reaction is based on the oxidation and hydrolization of preexisting magnetite
without any removal or addition of Fe. Goethite has a cell-volume of 138.37 Å3 which
contains 4 Fe-atoms, whereas in the cell volume of magnetite, which is 592.70 Å3, 24 Fe
atoms are included. Therefore, six cell volumes of goethite must be compared to one cell
volume of magnetite, showing that the reaction causes an increase of the volume of 40.07%.
Due to this high volume increase, the reaction cannot be realized in rocks. Therefore the
correct reaction may be nonredoxic in the sense of OHMOTO as follows: Fe2+Fe3+2O4 + 2H+ =
2FeOOH + Fe2+. The reaction taking place in an acidic environment and by the leaching of
Fe2+-ions causes a decrease of the volume of 6.62%.
Conclusions
The ore textures of mutual pseudomorphic replacements of magnetite and hematite formed
under redox conditions are well-known and therefore they can easily be recognized in
reflected light. The volumes of the initial and final products are nearly of the same size
(deviations smaller than 2 vol%). The small deviations are the prerequisites for their
abundance in many rocks. Among all replacements of metallic minerals, martitization is the
most widespread one. The reducing reaction (= mushketovitization) causing the replacement
of hematite by magnetite is by far not as common as the first and occurs in contact with
intrusions, particularly if they are sulphide-bearing. In rocks of iron-formations,
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mushketovitization is generally absent. As exceptions, in small proportions it was recently
observed in some drill hole samples of the Hamersley Basin (Dales Gorge, Joffre and Mara
Mamba Units; TOMPKINS & COWAN, 2001).
Although both, redox and nonredox reactions involve the same initial and final products, the
reactions cannot be simply compared. They are not comparable insofar as they are realized
due to an addition or removal of either oxygen or Fe2+ ions only. These exchanges occur
under oxidizing/reducing (redox reactions) or under basic/acidic (nonredox reactions)
conditions and are, respectively reponsible for the volumes remaining nearly constant or
changing drastically. As a consequence of this, the results of the two reaction types have to
cause different ore textures that, of course, must be distinguishable in reflected light.
However, textures other than those originating from redox reactions are unknown.
Acknowledgement
We are indepted to Sharon WEBB for her careful reading of the manuscript.
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Received:
Authors, addresses:
A. MÜCKE, Experimentelle und Angewandte Mineralogie, Göttinger Zentrum
Geowissenschaften, Georg-August-Universität, Goldschmidtstrasse 1, 37077 Göttingen,
Germany; e-mail: amuecke@gwdg.de
A. R. CABRAL, Rua Coelho, 32/704, 22231-110 Rio de Janeiro-RJ, Brazil; e-mail:
cabral70tricolor@yahoo.com.br
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Captions
Plate 1
A: Completely martitized magnetite crystals which consist of spindle-like hematite crystals
arranged parallel to [111]-directions (three are visible) of the original magnetite crystals. –
Reflected light, oil immersion, crossed polars, longer edge 450 μm. – Kushaka iron-
formation, Nigeria.
B: Partially martitized magnetite crystals containing newly-formed and spindle-shaped
hematite crystals arranged parallel to [111]-directions of primary magnetite. - Reflected light,
oil immersion, longer edge 450 μm. – Iron-formation of Maraba Hill, Maru schist belt,
Nigeria.
C: Idiomorphic magnetite crystals being partially replaced by hematite (white arrows) due to
strain martitization along the rim. Associated minerals are goethite (black arrows) and quartz
(black). - Reflected light, oilimmersion, length of the longer edge 450 µm. – Iron-formation of
Daitari, Orissa, India.
D: Recrystallized, coarse-grained and twinned hematite crystals containing relics of martite-
textured hematite (in the center). - Reflected light, oil immersion, crossed polars, longer edge
450 μm. – Iron-formation of Tajimi, Nigeria.
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Plate 2
A: Remnants of martited-textured hematite which is replaced by elongated and twinned
hematite (= specularite). - Reflected light, oil immersion, longer edge 450 μm. – Iron-
formation of Muro, Muro schist belt, Nigeria.
B: Replacement of slightly martitized magnetite (note the irregular grain boundaries) by
hematite. - Reflected light, oil immersion, longer edge 450 μm. – Iron-deposit of Umm Nar,
Wadi Mubarak, Western Desert, Egypt.
C: Aggregate of idiomorphic elongated hematite crystals which are partially replaced by
magnetite (arrows). - Reflected light, oil immersion, longer edge 450 μm. - Pegmatite of
Hagendorf, Bavaria, Germany.
D: Primary idiomorphic and partially martitized magnetite crystals in quartz (black).
Subsequently magnetite, which is partially preserved (mt) was pseudomorphically replaced by
goethite (go). Late goethite does not affect hematite (he) which is martite-textured. - Reflected
light, oil immersion, longer edge 450 μm. – Iron-formation of Maraba Hill, Maru schist belt,
Nigeria.
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