Crystal-chemical study of R3· c natural oxides along theeskolaite^karelianite^hematite (Cr2O3^V2O3^Fe2O3) join

L. SECCO1,*, F. NESTOLA1,2, A. DAL NEGRO

1AND L. Z. REZNITSKY

3

1 Dipartimento di Geoscienze, Universita di Padova, Via Giotto 1, I-35137, Padova, Italy2 Istituto di Geoscienze e Georisorse, CNR Sezione di Padova, Via Giotto 1, I-35137 Padova, Italy3 The Siberian Division of Russian Academy of Sciences, Institute of the Earth’s Crust, Irkutsk, 664033 Russia

[Received 14 March 2008; Accepted 11 August 2008]

ABSTRACT

Six natural crystals from the Sludyanka crystalline complex belonging to the eskolaite (Cr2O3)–karelianite (V2O3)�hematite (Fe2O3) solid solution were studied by means of X-ray diffraction andelectron microprobe. The Fe3+-poor samples show a general increase in a and c cell parameters withincreasing mean cationic radius (MCR), consistent with that shown by the synthetic crystals along theeskolaite–karelianite join. The Fe3+-richer sample deviates significantly from the behaviour shown bythe Fe3+-poor ones, similar to synthetic and natural hematites; with increasing MCR, the a and c cellparameters increase linearly along the eskolaite–karelianite join. However, for the samples rich in Fe3+,from karelianite to hematite, a shows a slightly steeper slope whereas the c parameter decreasesstrongly. The octahedral distortion increases slightly as a function of MCR along the eskolaite–karelianite join, whereas it increases markedly for Fe3+-rich samples. The evolution of the octahedraledges and of the octahedral distortions as a function of MCR are responsible for the behaviour of theunit-cell parameters along the eskolaite–karelianite–hematite join.

KEYWORDS: X-ray diffraction, single-crystal, solid-solution, eskolaite–karelianite–hematite.

Introduction

ESKOLAITE (Cr2O3), karelianite (V2O3) and hema-

tite (Fe2O3) are trigonal oxides (space group R3c)

present in several geological environments. Their

crystal structure has been described in several

works (e.g. Pauling and Hendricks, 1925; Kouvo

and Vuorelainen, 1958; Newnham and De Haan,

1962; Blake et al., 1966; Reid et al., 1972;

Robinson, 1975; Finger and Hazen, 1980;

Sawada, 1994; Oyama et al., 1999; Belonkoneva

and Shcherbakova, 2003; Kelm and Mader, 2005)

and they were also studied due to their common

use in industrial processes (e.g. glass ceramic,

Krishna et al., 2008; metal-insulator transitions,

Hague et al., 2008). Moreover, no crystal-

structure information on the solid solutions of

the natural crystals has been reported as far as we

are aware. Natural eskolaite and karelianite were

discovered as new minerals in the 1950s and

1960s at the famous Outokumpu deposit in

Finland (Long et al., 1963). There has been a

new find of eskolaite in some types of magmatic

and metamorphic rocks, meteorites and, recently,

kimberlites (Logvinova et al., 2008). Karelianite

is a relatively rare mineral. The isomorphic series

of (Cr,V)2O3 from eskolaite to eskolaite–karelia-

nite (50:50) was initially recognized in meta-

morphic rocks of the Sludyanka and Ol’khon

complexes at the shore of Lake Baikal, Russia

(Reznitsky et al., 1998; Koneva, 2002). At

present, almost complete Cr2O3-V2O3 binary

join and Fe-bearing oxides of the hematite-

eskolaite-karelianite series are known for the

Sludyanka complex (Koneva et al., 2001;

Reznitsky et al., 2005). However, there have* E-mail: luciano.secco@unipd.itDOI: 10.1180/minmag.2008.072.3.785

Mineralogical Magazine, June 2008, Vol. 72(3), pp. 785–792

# 2008 The Mineralogical Society

been no crystallographic studies focused on their

intermediate natural compositions, which are

needed in order to define how the Cr3+/V3+/Fe3+

substitutions affect the crystal structure of mixed

compositions.

In this study we investigated six natural

samples belonging to the eskolaite–karelianite–

hematite solid solution by single-crystal X-ray

diffraction (XRD) and electron microprobe

(EMP) analysis. The samples come from the

same locality studied by Reznitsky et al. (1998).

Our samples were compared with synthetic

eskolaite and hematite as published by Finger

and Hazen (1980), the synthetic eskolaite 70%–

karelianite 30%, eskolaite 50%�karelianite 50%,

eskolaite 15%�karelianite 85% of Oyama et al.

(1999), the synthetic karelianite of Robinson

(1975) and the natural hematite of Blake et al.

(1966). The aim of our study is to determine the

crystallographic features related to the Cr3+/V3+/

Fe3+ substitutions in natural crystals for which the

compositions are different from those of synthetic

ones; our work will also provide new information

not only for geological purpose but also for those

research fields dedicated to technological indus-

trial processes for which the mechanical and

physical properties of such oxides are crucial.

Experimental

Sample characterizationThe Sludyanka complex belongs to one of the

metamorphic terranes of the Central Asian fold

belt. The terrane is situated to the south of Lake

Baikal near the boundary with the Siberian craton.

The Sludyanka complex includes a folded

supracrustal series of intercalated mafic schist,

gneiss, marble and carbonate-silicate rocks

metamorphosed at granulite facies in the early

Palaeozoic.

The Cr-V mineral assemblage is related to

certain lithological types of metamorphites,

known as quartz-diopside rock suites, derived

from siliceous-dolomite sediments. According to

the ratio between the main rock-forming minerals

(quartz, diopside, calcite, and rarely, dolomite), a

series of petrographical types from diopsidites to

diopside-bearing quartzites and calciphires were

recognized.

The suite of the quartz-diopside rocks includes

thin layers and lenses of Cr-V-bearing varieties,

which contain a large series of Cr-V-bearing

minerals. The main minerals present are clinopyr-

oxenes, amphiboles, garnets, dioctahedral and

trioctahedral micas, spinels, tourmalines,

sulphides and Ti-Cr-V oxides (Reznitsky and

Sklyarov, 1996).

Accessory eskolaite-karelianite is present

within all varieties of the quartz-bearing rocks.

Eskolaite-karelianite inclusions were recognized

in various Cr-V minerals, but euhedral and

subhedral crystals up to 0.1�0.3 mm in size

were found in quartz and rarely in calcite. The

general range of compositions of the Sludyanka

oxides is from eskolaite (97.8 wt.% Cr2O3) to

karelianite (93.4 wt.% V2O3). In general, V-

eskolaite and eskolaite-karelianite with a Cr:V

ratio up to 1:1 are predominant (Reznitsky et al.,

1998).

Ambient quartz-diopside rocks contain practi-

cally no Fe and so the Cr-V minerals also have

small Fe contents. Fe-bearing eskolaite-karelia-

nites were recognized in metasomatically altered

(skarned) 20�40 cm wide rock zones at contacts

with granitoids. Compared to initial quartz-

diopside rocks, the skarned rock types are

enriched in Fe, Al and oxides of alkaline metals

and characterized by the appearance of feldspars,

scapolites and epidotes. As a result the Cr-V

minerals become more aluminous and ferric. In

particular, uvarovite-goldmanites, spinelides and

eskolaite-karelianites attain andradite, magnetite

and hematite components, respectively (Reznitsky

et al., 2005). The Fe2O3 content in various grains

of the eskolaite-karelianite ranges from 2 to

3 wt.% up to a maximum value of 31.9 wt.%.

EMPand XRD analysis

The natural samples studied in this work were

analysed at the ‘Istituto di Geoscienze e

Georisorse’, CNR, Padova (Italy) using a

CAMECA-CAMEBAX EMP operating with a

fine-focused beam (~1 mm) at an acceleration

voltage of 15 kV and a beam current of 15 nA in

wavelength dispersive mode (WDS), with 20 s

counting times for both peak and total back-

ground. X-ray counts were converted to

oxide wt.% using the PAP correction program

supplied by CAMECA. Standards, spectral lines

and analytical crystals used were: Al2O3 (Al-Ka,TAP), MnTiO3 (Mn-Ka, LiF; Ti-Ka, PET), Cr2O3

(Cr-Ka, LiF), Pb5Cl(VO4)3 (V-Ka, LiF), Fe2O3

(Fe-Ka, LiF), olivine for Mg (Ka, TAP). The

estimated errors are ~2.5% and 10% for major and

minor elements, respectively.

The oxide wt.% obtained by averaging ~10

EMP analyses for each single crystal are reported

786

L. SECCO ET AL.

in Table 1, the following compositions were

obtained:

Crystal 79: (Cr3+1.79V3+0.15Fe

3+0.01Al0.03Ti

4+0.01Mn2+0.01)O3

Crystal 100/1:(Cr3+1.47V

3+0.44Fe

3+0.04Al0.03Ti

4+0.01Mg0.01)O3

Crystal 82/1: (Cr3+0.70V3+1.26Fe

3+0.03Al0.01)O3

Crystal 92: (Cr3+0.51V3+1.48Fe

3+0.01)O3

Crystal 82/2: (Cr3+0.36V3+1.57Fe

3+0.06Al0.01)O3

Crystal 100/2:(Cr3+0.33V

3+1.08Fe

3+0.50Al0.01Ti

4+0.04Mg0.01Fe

2+0.03)O3

The errors evaluated using the chemical

formulae and the crystal-structure refinements

are 0.01 per formula unit (p.f.u.) (see below for

details).

The same single crystals were used for the

XRD study. Intensity data were collected using a

STOE single-crystal diffractometer with graphite

monochromated Mo-Ka radiation using a CCD

detector (Oxford Diffraction) in a 5 4 2y 4 85º

range using a 0.2º o-scan with exposure time of

10 s under ambient conditions. The sample–

detector distance was 60 mm. The Crysalis Red

program (Oxford Diffraction) was used to

integrate the intensity data applying the Lorentz-

polarization correction. The X-red (Stoe and Cie,

2001) and X-shape (Stoe and Cie, 1999) programs

were used to correct for absorption. Weighted

structural anisotropic refinements were done using

the SHELX-97 package (Sheldrick, 1997) starting

from the atomic coordinates of Newnham and De

Haan (1962). We used the Cr and V atomic

scattering curves for the Cr-rich and V-rich

samples, respectively, and the partly-ionized

curve for oxygen. The atomic scattering curves

were taken from the International Tables for

X-ray Crystallography (Ibers and Hamilton,

1974). Final occupancies obtained by the

crystal-structure refinements were in excellent

agreement with the chemical compositions for all

samples, with a maximum difference between the

number of electrons obtained by the chemical

analysis and crystal structure refinement of 0.1 for

the samples 79 and 100/1. In Table 2 the unit-cell

parameters, together with all the refinement and

structural data of the samples studied here, are

reported.

The cif files, together with the Fo-Fc, sft and

ADP tables for all the samples studied here are

deposited with the Principal Editor and can be

found at www.minersoc.org/pages/ e_journals/

dep_mat_mm.html.

Results

Unit-cell parametersIn order to account for the superposition of

different cations on the M site in the XRD

experiments, a mean value for the MCR is

calculated using the effective ionic radii from

Shannon (1976) weighted by the individual

occupancies of the cations on the octahedral

site. In Fig. 1, the a and c parameters are plotted

vs. MCR. The c parameter is the most affected by

the composition. Using this figure and comparing

all the data it is possible to note that c increases

linearly from eskolaite (Finger and Hazen, 1980)

to karelianite (Robinson, 1975) by ~2.7% up to

MCR = 0.640 A, where it shows a significant

decrease from karelianite to hematite (Finger and

Hazen, 1980; Blake et al., 1966) by ~1.3% from

MCR = 0.640 to 0.645 A. We observe a similar

behaviour in our natural samples: in fact the Fe3+-

rich crystal (100/2 sample) lies out of the

TABLE 1. Electron microprobe chemical analyses for the samples studied in this work. The data are reported asoxides in wt.%.

Samples 79 100/1 82/1 92 82/2 100/2

Cr2O3 90.7 (2.3) 74.0 (1.8) 35.3 (0.9) 25.6 (0.6) 18.2 (0.4) 16.6 (0.4)V2O3 7.6 (0.2) 22.1 (0.6) 62.7 (1.6) 73.7 (1.8) 78.0 (1.9) 53.3 (1.3)Fe2O3 0.44 (0.04) 2.31 (0.20) 1.64 (0.21) 0.31 (0.03) 3.25 (0.35) 26.4 (0.6)Al2O3 1.17 (0.11) 0.99 (0.10) 0.20 (0.03) 0.10 (0.01) 0.22 (0.04) 0.41 (0.03)TiO2 0.28 (0.03) 0.30 (0.04) 0.18 (0.02) 0.00 (0.01) 0.09 (0.01) 2.08 (0.21)MgO 0.10 (0.01) 0.13 (0.01) 0.09 (0.01) 0.00 (0.01) 0.04 (0.01) 0.27 (0.03)FeO 0.00 (0.01) 0.04 (0.01) 0.00 (0.01) 0.00 (0.01) 0.01 (0.01) 1.30 (0.14)MnO 0.07 (0.01) 0.00 (0.02) 0.00 (0.01) 0.00 (0.01) 0.00 (0.01) 0.10 (0.02)

Total 100.42 99.82 100.05 99.70 99.76 100.46

CRYSTAL CHEMISTRY OF OXIDES OF CR, V AND FE

787

eskolaite–karelianite linear trend as its c para-

meter is significantly smaller. The a parameter

shows an almost linear increase with increasing

MCR; however, a slight change in slope is

observed corresponding to the increased Fe3+

content (sample 100/2 and hematite crystals).

Octahedral site

The volume of the M octahedral site, VM,

increases along the eskolaite–karelianite join,

whereas synthetic and natural hematites show an

increase in MCR from 0.640 A (karelianite) to

0.645 A (hematites) at constant VM. Concerning

TABLE 2. Unit-cell parameters, structure-refinement data, atomic coordinates, polyhedral volumes, geometricalparameters and bond lengths for the samples investigated in this work.

79 100/1 82/1 92 82/2 100/2

a (A) 4.965 (1) 4.972 (1) 4.994 (1) 4.994 (1) 5.001 (1) 5.016 (1)c (A) 13.620 (2) 13.648 (2) 13.808 (2) 13.831 (5) 13.873 (3) 13.835 (2)V (A3) 290.8 (1) 292.2 (1) 298.2 (1) 298.7 (1) 300.5 (1) 301.5 (1)Rall (%) 3.3 3.3 1.8 2.2 2.2 1.8Un. Refl. 97 79 60 46 67 70Rint (%) 6.9 3.7 4.3 3.5 2.2 2.1Rs (%) 2.6 2.0 2.4 1.3 1.2 1.6Ext 0.0458 0.0090 0.0668 0.1096 0.0040 0.0075WR2 (%) 8.2 8.3 6.3 6.0 7.1 4.1GooF 1.19 1.15 0.96 1.36 0.97 0.99

M x 0 0 0 0 0 0y 0 0 0 0 0 0z 0.3478 (1) 0.3482 (1) 0.3487 (1) 0.3487 (1) 0.3490 (1) 0.3506 (1)

Ueq (A2) 0.0027 (1) 0.0027 (1) 0.0047 (1) 0.0070 (1) 0.0074 (1) 0.0065 (1)

O x 0.3056 (6) 0.3064 (5) 0.3067 (3) 0.3063 (4) 0.3082 (5) 0.3066 (3)y 0 0 0 0 0 0z 0.25 0.25 0.25 0.25 0.25 0.25

Ueq (A2) 0.0041 (1) 0.0040 (1) 0.0053 (1) 0.0089 (1) 0.0095 (1) 0.0081 (1)

M�O4 (A) 1.966 (1) 1.965 (1) 1.973 (1) 1.974 (2) 1.971 (1) 1.968 (1)M�O1 (A) 2.019 (2) 2.029 (2) 2.050 (1) 2.050 (2) 2.064 (2) 2.074 (1)Mean (A) 1.993 (2) 1.997 (2) 2.012 (1) 2.012 (2) 2.018 (2) 2.021 (1)VM (A3) 10.34 (2) 10.40 (2) 10.62 (2) 10.63 (2) 10.73 (2) 10.74 (2)loct

# 1.0136 1.0139 1.0146 1.0147 1.0147 1.0176soct* 46.99 47.90 50.00 50.51 49.82 59.02M site refinedoccupancies

Cr: 0.990 (30) Cr: 0.981 (21) Cr: 0.970 (21) V: 1.011 (27) V: 1.010 (24) V: 1.036 (12)

Electrons refinedat M site

23.8 23.5 23.3 23.2 23.2 23.8

O1�O2 (A) 2.628 (2) 2.639 (3) 2.653 (2) 2.650 (3) 2.670 (3) 2.664 (2)O2�O4 (A) 2.853 (1) 2.857 (2) 2.882 (1) 2.886 (2) 2.890 (2) 2.890 (1)O4�O5 (A) 2.993 (1) 2.993 (2) 3.005 (2) 3.007 (2) 3.002 (2) 3.019 (1)

M1�M2 (A) 2.664 (2) 2.681 (2) 2.726 (2) 2.730 (4) 2.747(2) 2.784 (2)

# Octahedral quadratic elongation and *octahedral angular distortion of M site are calculated as in Robinson et al.(1971). We measured the data with �6 < h < 6, �6 < k < 6, �12 < l < 12 for all samples, collecting a total of 840reflections for each sample (Z = 6). The polyhedral volumes and relative errors were calculated using the IVTONsoftware (Balic-Zunic and Vickovic, 1996). The errors on the average bond lengths were calculated as in Hazen andFinger (1982).

788

L. SECCO ET AL.

the M site distortions, in Fig. 2 s2oct (a) and

loct (b) (Robinson et al., 1971) are reported as a

function of MCR. These two parameters display

identical behaviour, showing only a slight

increase with increasing MCR for low-Fe3+

samples and a strong increase for the Fe3+-

richest natural sample studied here, according to

the octahedral distortion parameters of the M site

of hematite.

Discussion

The structure of R3c oxides is well known and in

general can be described in terms of octahedral

layers stacked normal to the c axis. A particular

feature of the octahedron is represented by the

presence of two triangular faces lying on the ab

plane, one shared (O(x 0 1/4); O(0 x 1/4); O(�x�x 1/4)

equivalent oxygens) and one unshared

FIG. 1. MCR at the M site vs. unit-cell parameters a and c. The data for the cell parameters are reported using the

same scale in order to show the real relative variations as a function of MCR. Filled circles: synthetic Cr2O3 and

Fe2O3 (Finger and Hazen, 1980); filled triangles: synthetic Cr1.4V0.6O3, Cr1.0V1.0O3, Cr0.3V1.7O3 (Oyama et al.,

1999); open triangle: synthetic Cr0.02V1.98O3 (Robinson, 1975); star: natural Fe2O3 (Blake et al., 1966); open

diamonds: samples studied in this work.

FIG. 2. MCR at the M site vs. (a) s2oct; and (b) loct calculated as by Robinson et al. (1971). Symbols as in Fig. 1.

CRYSTAL CHEMISTRY OF OXIDES OF CR, V AND FE

789

(O(1/3 2/3�x 5/12); O(x�2/3 x�1/3 5/12); O(1/3�x �1/3 5/12)

equivalent oxygens) with another octahedron

(Fig. 3). Hereafter, and in figures presented in

this paper, the following labels will be used:

O(x 0 1/4) = O1; O(0 x 1/4) = O2; O(�x �x 1/4) = O3;

O(1/3 2/3�x 5/12) = O4; O(x�2/3 x�1/3 5/12) = O5;

O(1/3�x �1/3 5/12) = O6.

The variation in the z coordinate of the M site

with increasing MCR causes the increase in

distance between the equivalent M cations

located at 0 0 z (hereafter referred to as M1) and

0 0 1/2�z (hereafter referred to as M2) (Fig. 3),

making the M cations closer to the unshared faces.

In Fig. 4a the M1�M2 distance increases nearly

linearly with increasing MCR from eskolaite to

karelianite and shows a strong jump to very large

values (e.g. 2.90 A) for hematites. In the same

way, our Fe3+-rich sample (100/2) shows an

M1�M2 distance greater than the eskolaite–

karelianite trend. The same behaviour is shown

by the O4�O5 distances as a function of MCR

(Fig. 4b), whereas the O1�O2 edge increases

with increasing MCR showing no corresponding

change in slope. As a consequence of the increase

in O4�O5 (and at the same time as O1�O2) an

enlargement of both shared and unshared faces

lying on the ab plane is observed, which is

responsible for the increase in the a cell parameter

with increasing MCR. Combining Fig. 1 and

Fig. 4b it is possible to verify that the slight

increase in slope of the a parameter evolution

showed by Fe3+-richer 100/2 crystal and hema-

tites is related to the stronger enlargement of the

unshared face displayed by these crystals. The

different behaviours shown by the O1�O2 edge

of the shared faces and the O4�O5 edge of

unshared ones lying on the ab plane could be

partially responsible for the strong increase in the

octahedral distortion involving the crystals char-

acterized by large Fe3+ content (e.g. Fig. 2a, b).

The O2�O4 edge (connecting the shared and

unshared faces of the M octahedron), inclined

towards the c axis (<20º), shows a linear increase

with increasing MCR up to the synthetic

karelianite (Fig. 5), whereas sample 100/2 devi-

ates slightly from the linear eskolaite–karelianite

trend due to its large Fe3+ content, as confirmed

by the behaviour found from karelianite to

FIG. 3. Crystal structure of trigonal oxides studied in this

work along a clinographic view and with the a and c

axes indicated. M1 at 0 0 z; M2 at 0 0 1/2�z; O1 at x 0 1/

4; O2 at 0 x 1/4; O3 at �x�x 1/4; O4 at 1/3 2/3�x 5/12;

O5 at x�2/3 x�1/3 5/12; O6 at 1/3�x�1/3 5/12.

FIG. 4. MCR at the M site vs. (a) M1�M2 distance; and (b) O4�O5 octahedral edge. Symbols as in Fig. 1,

nomenclature as in Fig. 3.

790

L. SECCO ET AL.

synthetic and natural hematites (Fig. 5); such

behaviour could be related to the general decrease

in the c cell parameter observed in Fig. 1 for the

Fe3+-rich samples.

The strong increase in s2oct and loct distortion

parameters of the octahedral M site with

increasing MCR displayed by the 100/2 Fe3+-

rich crystal studied here (Fig. 2a and b) is mainly

related to the evolution of the O4�O5 edge of the

unshared face lying on the ab plane. This is

confirmed by synthetic and natural hematites.

Conclusions

(1) The R3c natural oxides investigated here are

characterized by a wide Cr3+/V3+ substitution

causing a significant increase in both the a and c

cell parameters with increasing MCR of the M

site.

(2) For the Fe3+-richest samples (100/2 and

hematites), a significant decrease in the c cell

parameter as a function of MCR is observed,

showing a significant deviation from the linear

increase with MCR found for the eskolaite–

karelianite join.

(3) The evolution as a function of MCR of the

O1�O2 and O4�O5 edges strongly affect the

behaviour of a unit-cell parameter over all the

compositional range investigated. From the other

side, the evolution of O2�O4 edge with MCR

clearly determines the behaviour of the c unit-cell

parameter.

(4) The differences in behaviour with MCR of

O1�O2 and O4�O5 edges mainly affects the

distortion parameters of the octahedral M site.

Acknowledgements

The authors thank R. Carampin for assistance

with the electron microprobe analyses, R. Fischer

and F. Bosi for their helpful review and M. Welch

and F. Camara for their useful comments. This

research was made possible with the financial

support of MURST and RFBR (project 06-05-

64300).

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FIG. 5. MCR at M site vs. O2�O4 octahedral edge.

Symbols as in Fig. 1, nomenclature as in Fig. 3.

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