Chromium: its Physicochemical Behavior and Petrologic Significance. Papers from a Carnegie Institution of Washington Conference, Geophysical Laboratory

304 Pages·1976·9.732 MB·English

by  T. N. Irvine (Eds.)

CHROMIUM: ITS PHYSICOCHEMICAL
BEHAVIOR AND PETROLOGIC SIGNIFICANCE
Papers from a Carnegie Institution of Washington Conference,
Geophysical Laboratory
Edited by
T. N. IRVINE
PERGAMON PRESS
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Library of Congress Cataloging in Publication Data
Main entry under title:
Chromium.
"First appeared in Geochimica et cosmochimica acta, volume 39, number 6/7."
1. Chromium—Congresses. I. Irvine, T. N. II. Carnegie Institution of Washington. Geophysical
Laboratory. III. Geochimica et cosmochimica acta. QE516.C7C47 1975 553'.464 75-33383
ISBN 0-08-019954-2
PRINTED IN NORTHERN IRELAND AT THE UNIVERSITIES PRESS, BELFAST
Geochimica et Cosmochimica Acta, 1975, Vol. 39, pp. 779 to 780. Pergamon Press. Printed in Northern Ireland
Chromium: its physicochemical behavior
and petrologic significance
Papers from a Carnegie Institution of Washington Conference
Geophysical Laboratory
Edited by
T. N. IRVINE
FOREWORD
This issue of Geochimica et Cosmochimica Acta comprises twenty-four papers
arising from an interdisciplinary conference on the physicochemical behavior and
petrologic significance of chromium held at the Geophysical Laboratory on
January 7-8, 1974. The conference, which was attended by 43 participants from
Canada, England, Germany, Japan and the United States, was organized by
T. N. Irvine (Geophysical Laboratory), J. S. Dickey (Massachusetts Institute of
Technology), and G. C. Ulmer (Temple University) and sponsored by the Carnegie
Institution of Washington.
Chromium is of special interest to industry and science because of its exceptional
tendency to form or concentrate in chemically resistant and refractory phases
and because of its variable valence. Although the element is most familiar as a
protective, decorative metallic plating, by far its main uses are in hardening and
toughening steel and increasing its resistance to corrosion, in the preparation of
high-temperature furnace linings, and in the chemicals industry. In the geological
sciences, chromium is estimated to be the tenth most abundant element in the
Earth and figures prominently in the constitution of the mantle. It and the min-
erals it forms are known to be important indicators of the physical and chemical
conditions accompanying formation of mafic and ultramafic rocks on the Earth
and Moon and in meteorites, and it appears that they may have much more
influence on the compositions of magmas generated in the upper mantle than one
might suspect from the low abundance of Cr in these magmas. The mineral
chromite forms some of the world's most remarkable magmatic ore deposits.
But the properties of chromium that make it of interest have also made it
difficult to investigate, and only recently has it become possible to attack in a
meaningful way many of the problems relating to the element. The advances,
however, have been rapid. Detailed phase equilibria, crystallographic and thermo-
chemical studies of chromium-bearing systems are now being conducted in several
laboratories; and modern analytical methods, such as those utilizing the electron
microprobe and Mössbauer and absorption spectroscopy, are yielding large
quantities of high-quality data on Cr-bearing phases. In view of this progress,
and given the added consideration that North American resources of chrome ore
amount to only a 4- to 5-year supply, it was an opportune time for specialists to
meet to integrate the knowledge at hand and broaden perspectives.
The subjects dealt with at the conference included phase equilibria studies of
chromium-bearing systems; thermodynamic and magnetic properties of chromium
779
780 T. N. IRVINE
phases; crystal-field stabilization of chromium and its behavior at high pressures;
processing of chrome ores in refractories technology and metallurgy; and distri-
bution and significance of Cr and Cr minerals in various kinds of ultramafic rocks,
kimberlites, lunar rocks and meteorites. The papers in this volume give a rep-
resentative overview of the problems, achievements, and discoveries reported
and they show in particular that chromium-bearing systems can be used to tackle
a variety of problems of much broader interest. Perhaps they will also serve to
stimulate similar investigations of other elements.
On behalf of the organizing committee, I wish to acknowledge the assistance
of Dr. H. S. Yoder, Jr., Director of the Geophysical Laboratory, in
arranging support for the conference and this publication. We are grateful also to
Dr. D. M. Shaw, Editor of Geochimica et Gosmochimica Ada, for authorizing the
publication. My personal thanks go to the authors, referees, and the staff of
Pergamon Press for their efforts and cooperation and to Mrs. M. B. Mattingly
for much assistance with correspondence.
T. N. IRVINE
Geophysical Laboratory
Geochimica et Co3mochimica Acta, 1975, Vol. 39, pp. 791 to 802. Pergamon Press. Printed in Northern Ireland
Phase relations in chromium oxide-containing systems
at elevated temperatures
ARNULF MTTAN
Department of Geosciences, The Pennsylvania State University,
University Park, Pennsylvania 16802, U.S.A.
(Received 22 March 1974; accepted in revised form 16 July 1974)
Abstract—The role of chromium in oxide and silicate systems is examined by reviewing typical
features of phase diagrams involving this component. Liquidus, solidus and sub-solidus phase
relations are discussed for a number of binary, ternary, quaternary and multicomponent systems
having a bearing on the behavior of chromium in minerals and rocks. The most dominant fea-
tures of the liquidus surface of Cr 0 -containing silicate systems are the large liquid miscibility
2 3
gaps and the dominance of the spinel (chromite) stability field. Subsolidus relations of especial
interest are those involving spinel phases (chromites). The oxygen pressure of the gas phase
is an important parameter in chromium oxide-containing systems, partly because iron oxide is
a common constituent of such systems, partly because chromium itself may occur in different
states of oxidation. At the lowest oxygen pressures considered, a significant proportion of the
chromium may be present as Cr2+, and at the highest oxygen pressures as Cr 6*.
INTRODUCTION
THE OBJECTIVE of this introductory paper is to establish part of the framework
needed for an understanding of the role of chromium in oxide phase assemblages.
No attempt will be made to review all available phase-equilibrium data on chromium
oxide-containing systems, or to discuss details of phase relations in specific systems.
Many such descriptions have appeared in previous literature, and some will be pre-
sented by other authors in subsequent chapters of this volume. The approach
adopted in this chapter is to present a limited number of phase diagrams involving
chromium oxide as a component and to point out features of these diagrams that
demonstrate the characterisics of this component in oxide and silicate phases. The
emphasis will be on systems judged to be of particular interest in relation to the
natural occurrences of chromium-bearing minerals. The presentation will proceed
from the simple systems toward the more complex, starting with binary systems and
ending up with multicomponent systems. While the emphasis will be on geometrical
aspects of such relations, it should be recognized that the phase diagrams are geo-
metrical manifestations of the thermodynamic properties of the phases involved in
the equilibria. No quantitative thermodynamic derivations will be presented in this
chapter, but qualitative inferences will be made between typical features of the
diagrams and some of the characteristic thermodynamic properties of Cr 0 and
2 3
its compounds. The latter relations will be discussed in some detail in the succeeding
paper of this volume (NAVROTSKY, 1975).
Many of the phase-equilibrium data on chromium oxide-containing systems to
be presented in the following have been obtained by ceramists or metallurgists
rather than geoscientists, emphasizing the close interrelations among these fields.
It is particularly gratifying to be able to draw on data from related technological
fields for presentation at a meeting held at a geophysical laboratory founded by
Andrew Carnegie.
781
782 ABNULF MU AN
SOME GENERAL PROPERTIES OF CHROMIUM OXIDES
The most stable oxide of chromium under the conditions of primary interest in
high-temperature silicate chemistry is Cr 0 . This oxide is moderately stable with
2 3
respect to its constituent elements, the change in standard free energy (Δ6?°) for the
reaction
|Cr + 0 = |Cr 0
2 2 3
being approximately —120 kcal at 1200° C (ELLIOTT and GLEISER, 1960). For com-
parison, we note that Cr 0 is considerably more stable than TeO' (2Fe + 0 =
2 3 2
2FeO, AO° Q* -80 kcal at 1200°C), but less stable than MnO (2Mn + 0 = 2MnO,
2
AG0^ -132 kcal at 1200°C) and Si0 (Si + 0 = Si0 , AG° ^ -147 kcal at
2 2 2
1200°C) (all data from ELLIOTT and GLEISER, 1960). The structure of Cr203 ('eskol-
aite') is that of corundum, and it is a very 'refractory' oxide, having a melting point
in air of approximately 2265°C (BUNTING, 1930; KEITH, 1954). Although the most
common oxidation state of chromium under geologically important conditions is
Cr3+, Cr2+ may be present in significant amounts under strongly reducing conditions,
such as prevail, for instance, in lunar rocks. Furthermore, chromium may occur in
oxidation states higher than Cr 3+, notably as Cr6+, under strongly oxidizing condi-
tions, and in the presence of high concentrations of cations of low field strengths
[i.e. very 'basic oxides' (Lux, 1939), e.g. CaO, BaO]. We also note that Cr 0 is a
2 3
relatively 'neutral' oxide in terms of acid-base characteristics (Lux, 1939), and hence
has a high degree of compatibility with both 'acid' (e.g. Si0 ) and 'basic' oxides
2
(e.g. MgO). The success of Cr 0 as a refractory material is related to this property
2 3
as well as to its high melting point.
In addition to its main occurrence in oxide phase, chromium may also be present
as an important constituent of sulfides, for instance in chalcospinels. In order to
keep this presentation within reasonable limits of length, we will omit the latter
subject and refer the interested reader to descriptions of such phases in recent
literature (see, for instance, BOBBINS et al., 1970), and to a later paper in this volume
(BOBBINS, 1975).
PHASE BELATIONS IN SELECTED CHROMIUM OXIDE-CONTAINING SYSTEMS
Binary systems, and systems that can be treated similarly
Fundamental to an understanding of the role of chromium in oxide phases is
the binary system Cr—O. Unfortunately, our knowledge of equilibrium relations in
this system is very limited. A sketch illustrating approximate relations is shown in
Fig. 1. It appears that no intermediate crystalline phase between Cr and Cr 0 is
2 3
stable in the binary system, and that the alleged 'Cr 0 ' phase of structure similar
3 4
to that of spinel owes its occurrence to the presence of iron oxide in the materials used
by some of the previous authors (HILTY et al., 1955; LAYDEN, 1965). It is to be
noted that liquidus temperatures decrease sharply from the reported melting point
of Cr 0 in air, ~2265°C (right side of Fig. 1), to ~1650°C at the Cr-Cr 0 eutectic
2 3 2 3
point. The experimental data available are insufficient to permit superposition of
isobars describing quantitatively the change in oxygen pressure along the liquidus
curve in the Cr203 field (compare similar relations in the system Fe-0 ; DARKEN and
Phase relations in chromium oxide-containing systems at elevated temperatures 783
— OL SHANSKII and SHLEPOV
2400 A Cr203 + LIQUID 1
Cr + LIQUID PRESENT
230o| Cr + Cr2 03 INVESTIGATION
-EUTECTIC
2200 />
2100
I
^2000 ■•I
cc LIQUID /
z> TWO LIQUIDS
< 1900
ce /
LU J8IOe Λ
/ Cr203
g 1800 , /
Cr + LIQUID
\ / LIQUID
1700 4
1600
Ι50θ|
Cr + Cr203
1400
Cr 30 40 50 60
WEIGHT %
Fig. 1. Sketch showing approximate phase relations in part of the system Cr—O,
based mainly on data of OL'SHANSKII and SHLEPOV (1953) and of JOHNSON and
MUAN (1968).
GURRY, 1945, 1946; MUAN, 1958), but the oxygen pressure at the eutectic point is
inferred to be of the order of 10~12 atm, based on the available data for the free energy
of formation of Cr 0 from its elements (compare earlier section). Further work on
2 3
equilibria in the system Cr-0 is in progress in our laboratories at the present time,
and the data reported above (Fig. 1) should be considered tentative only.
Studies of the system Cr-0 at oxygen pressures above 1 atm have been reported
by numerous authors. Although data obtained under such conditions are not of
obvious direct importance in petrology, they nevertheless serve to expand our knowl-
edge of the role of chromium in oxide structures. The reader is referred to a later
paper in the present volume (WHITE and ROY, 1975) for further discussion of this
subject.
Phase relations have been determined for a number of binary systems where
Cr 0 is one of the components present. The system Cr 0 -Si0 as portrayed in Fig.
2 3 2 3 2
2 shows very limited mutual solubility between these two components in the solid
as well as in the liquid state, and the lowest liquidus (and solidus) temperature
(~1710°C) is only slightly lower than the melting point of Si0 (1723°C). Clearly,
2
a large miscibility gap will be an essential feature of the liquidus surface of ternary
and multicomponent silicate systems where Cr 0 is a component. The diagram in
2 3
Fig. 2 was determined in air, under which conditions essentially all the chromium
is in the trivalent state and the system therefore for all practical purposes may be
considered binary. Under strongly reducing conditions, the system is far from
binary (HEALY and SCHOTTMILLER, 1964). A liquid phase has been reported to form
at as low a temperature as <^1450°C, and chromium orthosilicate (Cr Si0 )
2 4
784 AUNULF MUAN
—ι 1 1 i r r 1 1 I
f -^. LIQUID
f
I
TWO LIQUIDS
1 1-
2300 1-
Cr.,0, ♦ LIQUID
"2250 LIQ'.JO-- J
■■ Ti
2 200 - \-
1
1
2100
1-
1
2000 - Cr203 * LIQUID i1 -
!1
1900 -
1
1800 -
CRISTOBALITE U
-I720
1700 - LIQUID ^ " " \J -j
1600 - Cr203-CRISTOBALITE
» I II
50
WEIGHT %
Fig. 2. The system Cr 0 -Si0 in air, based on data of BUNTING (1930) as discussed
2 3 2
by KEITH (1954). (Reproduced from MUAN and OSBORN, 1965.)
presumably containing most of its chromium in the divalent state, has been observed
in the quenched samples. However, the stability of the latter phase has not been
established. Hence, drawing a phase equilibrium diagram for the system under these
conditions is premature. An interesting observation in the above work was the blue
color of the orthosilicate phase. Similar colors have subsequently been reported to
occur in other, more complex chromium oxide-containing systems (DICKEY et al.9
1971).
Combination of Cr 0 with another sesquioxide of similar structure gives a dia-
2 3
gram of the type shown in Fig. 3 for the system Al 0 -Cr 0 . A complete solid-
2 3 2 3
solution series is formed between the two isomorphic end members at high tempera-
tures, resulting in continuous slopes of the liquidus and solidus curves. According
to a recent report (BARKS and ROY, 1972), a miscibility gap exists at temperatures
below <^900°C, as shown in the lower part of Fig. 3.
The liquidus-solidus relations in the system Fe 0 -Cr 0 would presumably be
2 3 2 3
similar to those for Cr 0 -Al 0 . However, high oxygen pressures would be required
2 3 2 3
to realize these equilibria experimentally in the iron oxide-containing system. It is
thought that the tendency for unmixing at lower temperatures is smaller in the iron
oxide-containing system than in the system Al 0 -Cr 0 . We will expect that ex-
2 3 2 3
tensive solid-solution formation among phases containing major amounts of the
three cations Cr3+, Al3+ and Fe3+ will be a characteristic feature of many of the dia-
grams for more complex systems to be presented in later parts of this paper.
Phase relations in chromium oxide-containing systems at elevated temperatures 785
~l I I n
2400 LIQUID
2200
^^^-— "RêOvU — — *
2000
1800
o
u 1600
cr
3 R203 (SESQUIOXIDE SOLID SOLUTION)
Ü
u 1400
Σ£L
liJ
1200
1000
800
600
J L_
400
AI;
WEIGHT %
Fig. 3. The system Al 0 -Cr 0 , mainly after BUNTING (1931), with relations at
2 3 2 3
low temperatures ( <900°C) drawn in accordance with more recent data of BABKS
and ROY (1972).
Combination of chromium oxide with an oxide of a medium-sized divalent cation
gives a diagram of the type sketched in Fig. 4 for the system MgO-Cr 0 . The
2 3
locations of boundary curves at the very high temperatures involved in this system
are not accurately known, but the general relations are thought to be approximately
as shown in Fig. 4. Of particular note in this diagram is the persistence to high
temperatures of the spinel phase (MgCr 0 , picrochromite). Clearly, one will expect
2 4
to see a dominant area of this phase on the liquidus surface of multicomponent sys-
tems where MgO and Cr 0 are two of the components present.
2 3
Relations similar to those illustrated in Fig. 4 prevail if Cr 0 is combined with
2 3
NiO, CoO or TeO'. Because of the ready oxidation of Fe 2+ to Fe3+, special pre-
cautions must be taken to control the oxygen potential in the case of the iron oxide-
containing systems. Phase relations for parts of the system TeO'-Cr 0 under
2 3
strongly reducing conditions have been reported by a number of authors (see for
instance RIBOUD and MUAN, 1964; HOFFMANN, 1965). However, many uncertain-
ties still remain. In particular, phase relations attending the melting of the spinel
phase, iron chromite (FeCr 0 ), are poorly known. In view of these uncertainties,
2 4
and inasmuch as extensive studies of this system are now in progress in our labora-
tories, a phase diagram for this system will not be presented here. As the oxygen
pressure of the gas phase in equilibrium with iron oxide-Cr 0 mixtures increases,
2 3
increasing proportions of the iron will be present as Fe 3+, and the compositions of the
phases are represented by points within the triangle FeO-Fe 0 -Cr 0 . However,
2 3 2 3
786 ARNULF MUAN
i Γ
2 8001 -2800
2700
Λ *X
2600 LIQUID
\ M>^ PICR0CHR0MITE.
2500
\
¥ 2400 I \ C/n *Of ^ v πυηυυπκυινΐι
X %0 *\ AlQUIDN
\
\xL-f2\ 35>0V^2 265|
V-
LIQUID
PICROCHROMITEi
PERICLASE
PICROCHROMITE
PICROCHROMITE
Cr203 .
30 40 50 60 MgOCr203 C'rr22°^;3
WEIGHT %
Fig. 4. Sketch of the system MgO-Cr 0 based mainly on data of AUPER et al.
2 3
(1964). The dashed curves indicate uncertainties in locations of boundary curves.
for practical purposes it is useful to present the relations in a simplified diagram with
appearance similar to that of a binary system, as illustrated in Fig. 5 for the system
iron oxide-Cr 0 in air. Here the compositions of the mixtures have been projected
2 3
along 'oxygen reaction lines' on to the join Fe 0 -Cr 0 . The diagram illustrates the
3 4 2 3
strong stabilizing effect of Cr 0 on phases in which the iron is present in the trivalent
2 3
state. Similar relations have been observed in the system manganese oxide-Cr 0
2 3
in air (SPEIDEL and MUAN, 1963).
~226£l
LIQUID
/SESQUIOXIDE
Χζοπ LIQUID
/ /
/SPINEL /
/ilQUlD
/ / /3ESQUÎOXIDE
•^ 1800 / /
UJ
en
K<I 700 ■ / /SPINEL
1/ /
^1600
UJ
1500
SESQUIOXIDE
1300 l·
I I & iû 4b 50 60 7Ό 80 9OQ2O3
FeOFe203 WEIGHT%
Fig. 5. The system iron oxide-Cr 0 in air, after MUAN and SÖMIYA (1960).
2 3
(Reproduced from MUAN and OSBORN, 1965.)