Mineralogical Magazine, February 1998, VoL 62(1), pp. 41-54

Cation distribution in natural Zn-aluminate spinels
S. LUCCHESI
Dipartimento di Scienze della Terra, Uuiversit~ di Roma "La Sapienza", P. le A. Moro 5, 00186 Rorna, Italy
A. DELLAGIUSTA
Dipartimento di Mineralogia e Petrologia, Universitfi di Padova, Corso Garibaldi 37, 35122 Padova, Italy
AND
U. Russo
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universitfi di Padova, Via Loredan 4, 35131
Padova, Italy

ABSTRACTThe intracrystalline cation distributions in fourteen natural Zn-aluminate spinels were determined bymeans of X-ray single-crystal structural refinement, supported for some samples by M6ssbauer
spectroscopy.
Zinc substitutes for Mg and subordinately Fe2+ and its relevant changes in content, from 0.10 to 0.96
atoms per formula unit (apfu), are not related to valiations of cell parameter. The latter is determined
mainly by the substitution Fe 3+ ~ A1. In agreement with data from synthetic samples, a small but
definite amount of Zn (up to 0.06 april) is located in the octahedral M site. Fe2+, when present, shows apreference for tetrahedral coordination.
An improved value of the tetrahedral Zn(T)-O distance (1.960 A) was obtained, integrating the setof interatomic distances used for the determination of cation distribution in spinels.

KEYWORDS: spinel, zinc, gatmite, crystal chemistry, intracrystalline disorder.

Introduction

SPINELS are a class of ubiquitous and importantrock-forming minerals, and their widespreadoccurrence has long suggested their use as
petrogenetic indicators (e.g. Spry and Scott,1986). A large number of investigations on
intracrystalline disorder (Delia Giusta et aL,
1996; Lucchesi and Della Giusta, 1997) on bothnatural and synthetic compounds has indicatedtheir possible use for geothermometric purposes.
In fact, in natural spinel cl~ystals, the two cationsites available in the AB204 structure (Fd3m
symmetry), one tetrahedral (T) site - - 43m point
symmetry - - and one octahedral (M) - - 3m point
sylmnetry - - both with fixed coordinates (Hafner,
1960; Hill et aL, 1979), can be occupied by

9 1998 The Mineralogical Society

several cations, mainly divalent (A) and trivalent(B), Some of these cations, e.g. Mg, A1, Fe2+ andFe 3+, under thermal treatment can exchange
between the T and M sites in amounts as large
as approximately 0.3 atoms (Wu and Mason,
1981; Wood et al., 1986; O'Neill, 1992;
Roelofsen et aL, 1992). The disordered configuration relations between the tetrahedral andoctahedral sites of the A and B cations may thus
be described by an inversion parameter i:IV(A1--iBi)Vl(B2-iAi)O4
Normal spinels are those with i = 0, inverse
those with i ~ 1, as magnetite. While a large
quantity of data is presently available for simple
binary spinels; both concerning their cation
distribution and its temperature dependence

S. LUCCHESI ETAL.

(e.g. O'Neill, 1992; Wu and Mason, 1981;
O'Neill et al., 1992), far tess work has been
done on multicomponent spinel solid-solutions(Nell et al., 1989; Larsson, 1995) and theprecision of these investigations is, at times,
poorly known.
Complete knowledge of cation distribution isthus fundamental for a full understanding ofspinel properties. This knowledge may beobtained by the precise characterization ofoctahedral and tetrahedral site preferences of theconstituent cations, in both normal and inverse
configurations.
Topochemical relations in natural spinels may
be determined by combining the information fromsingle-crystal X-ray diffraction and very accurate
microprobe analyses. In this method, each cationis characterized by its site bond-distance andvalency status; thus, the availability of an accurate
and self-consistent set of distances is essential.
Improvements to the ionic radii proposed by
Shannon (1976) have been developed for spinels
by several authors (e.g. O'Neill and Navrotsky,
1983; Marshall and Dollase, 1984). In previous
papers (Della Giusta et al., 1996; Lucchesi et al.,1997), an improved and self-consistent set ofbond-distances was obtained, for octahedral andtetrahedral coordination, for the commonest
cations in natural spinels (Mg, A1, Fez+, Fe 3+,
Mn, Cr). These data were used to calculate thecation intracrystalline distribution in a pleonaste
crystal from volcanic ejecta which turned out tobe highly disordered (Lucchesi and Della Giusta,
1997).
This paper presents the results of microprobe
analyses and single-crystal X-ray structuralrefinements of 14 natural aluminate spinels withthe g e n e r a l f o r m u l a Z n x ( M g , F e 2 + ) l _ x(A1,Fe3+)204, where 0.08 ~< x ~< 0.96. Other
cations occur only in minor amounts. The aim
was to investigate the effects of the presence of acation like Zn, with a relatively high preference
for tetrahedral coordination ( O ' N e i l l andNavrotsky, 1983; Navrotsky, 1986), on theintracrystalline distribution of the remaining
cations. The samples were chosen so as tocontain the lowest possible quantity of Mn2+: infact, with the adopted method, the presence of thiselement, which in large quantities may disorder
into the M site (Lucchesi et al., 1997), would hide
the expected low amounts of octahedral Zn.
Further studies will be devoted to the crystal
chemistry of Zn in ferrites (franklinite) and in thehaeterolite-hausmannite binary.

Zinc crystal chemistry in Fd3m aluminates
Zn-aluminates are generally assumed to be
essentially normal spinels (Navrotsky, 1986)
because of the excess energy stabilization of thezinc ion in the tetrahedral coordination, due to its
tendency to form sp 3 covalent bonds (O'Neill andNavrotsky, 1983). However, energy calculations
have led most authors to predict the existence of avery minor inversion in Zn(A1, Fe)204 (Spry and
Scott, 1986) and even in Zn(A1, Cr)204(Bruckmann-Benke et al., 1988), in which thehigh octahedral site-preference of Cr further
hinders the inversion process, as experimentally
noted by O'Neill and Dollase (1994) on synthetic
ZnCr204, a totally normal spinel. Neutron
diffraction experiments (Fischer, 1967) did notallow unambiguous direct observation of zinc
intracrystalline disorder, since a synthetic Znaluminate with a = 8.086(1) A and oxygen
coordinate u = 0.2636(1) had i = 0.03(3), avalue within the limits of experimental uncertainty. However, the presence of a very minor
amount of inversion was experimentally verified
in a synthetic Zn-aluminate by Cooley and Reed(1972) after it was annealed at high temperatures(i = 0.04 at 905~ and i = 0.06 at 1197~ andO'Neill and Dollase (1994), who observed small
departures from completely normal cation distribution (from i = 0.011(7) at 900~ to i = 0.053(6)at 1300~
Both papers highlighted the usual
tendency of ordering with progressive decrease ofannealing temperature.
In the presence of iron, the Zn behaviour seems
to depend on the different iron oxidation states. In
synthetic Zn(A1, Fe)204 spinels, the tendency ofzinc towards intracrystalline disorder, recognizedin synthetic Zn-ferrites (O'Neill, 1992), has been
observed (Waerenborgh et al., 1994b). In the latter
paper, zinc was unambiguously assigned tooctahedral sites (i = 0.02 for Zn(Ala.gFe0.1)O4 andi = 0.05 for Zn(All.4Fe0.6)O4, both at 727~
theFe3+ fraction in tetrahedral sites being larger thanthe AI(T) one. An increase in annealing temperature to 1077~ resulted in the usual increase ininversion (i = 0.04 and 0.08 respectively).
However, the same authors (Waerenborgh et al.,1994a) could not positively show the existence ofoctahedral zinc in (Zn, Fe)A1204 synthetic crystals,
which were thus considered as totally normal, with
only one case of 0.009 Zn(M) apfu. In thesecrystals, minor amounts of Fe3+ due to partial ironoxidation were noted, and their disordering intetrahedral coordination was considered to be
42

CATION DISTRIBUTION IN ZN.ALUMINATE SPINELS

responsible for possible, although highly improbable, zinc inversion.
Studies on natural samples are comparatively
less abundant and more contradictory. An X-ray
investigation (Saalfeld, 1964) on a gahnite crystal
from Falun, Sweden, showed a totally normal
character with a =~]k and T - O = 1.88
and M - O = 1.96 A, notwithstanding the presenceof large amounts of Fe and Mg in both T and M
sites. M/Sssbauer investigations (Waerenborgh etal., 1990) on three natural gahnites and kreittonites suggested totally ordered iron distribution
with Fe e+ in tetrahedral sites and Fe 3+ inoctahedral ones.

Experimental
The examined samples (Table 1) constitute a setof 14 crystals in the gahnite-spinel (s.s.)hercynite system, kindly made available by theMuseum of Mineralogy, University of Rome "La
Sapienza", Italy.
Small equidimensional fragments of theselected crystals were mounted, for X-ray datacollection, on a Siemens P4 automated fourcircle, singie-crystal diffractometer. Details ofdata collection are shown in Table 2, and further
information may be found in Lucchesi et al.
(1997).
No significant deviations from Fd3m symmetry
were noted: the appearance of forbidden spacegroup reflections such as 200 was attributed, on
the basis of Psi-scan checks, to double reflection(Tokonami and Horiuchi, 1980). The oxygen
coordinate, M and T occupancies, secondary
extinction coefficient, scale factor and thermal
factors were the variable parameters. The starting
oxygen coordinate was that proposed by
Princivalle et al. (1989), setting the origin at
3m. Isotropic secondary extinction was correctedaccording to Larson's (1970) algorithm. No
chemical constraints were used during refinement.
Fully ionized scattering curves for all elements
except oxygen (20% ionized) were used, since
they proved to furnish the best values of
conventional agreement factors over all sin0/L
intervals and the best coherence between
observed and calculated F(222), the latterstructure factor being particularly sensitive to M
and O site occupancies (Della Giusta et al., 1986).
Off-diagonal thermal factors were extremely
small and of the same magnitude as their ~,when not forced by symmetry to zero values, andthus only Uaa values are shown in Table 3. Three
isotropic, full-matrix, refinement cycles were
followed by anisotropic cycles until convergence
was attained. The disagreement coefficient (R)values were very satisfactory (Table 3).
The same crystals used for X-ray datacollection were mounted on a glass slide andpolished for electron microprobe analysis (WDS
method) on a Cameca-Camebax instrument with
the PAP data reduction program. Synthetic oxide
standards (MgO, FeO, MnO, ZnO, NiO, CuO,
A1203, Cr203, V203, TiO2, SiO2) were used (seeLucchesi et al. (1997) for details of instrtmaent

TABLE 1. Description and provenance of natural Zn-aluminate spinels
SampleVariety

SPN 112SPN 115a
SPN 115b
SPN 116
SPN 117
SPN 118
SPN 127
SPN 190
SPN 193
SPN 228
SPN 230
SPN 232
SPN 244
SPN 249

GahniteDysluite
Dysluite
Dysluite
Dysluite
Dysluite
GalmiteZn-Spinel
Zn-Spinel
Kreittonite
Dysluite
Kreittonite
GahniteZn-Spinel

ProvenanceFalun, Sweden
New York, USA
New York, USA
Sterling, N.J., USA
Franklin, N.J., USA
Franklin, N.J., USA
Falun, Sweden
Tiriolo, Calabria, Italy
Tiriolo, Calabria, Italy
Bodenmais, Bavaria, Germany
Sterling, N.J., USA
Bodelmaais, Bavaria, Germany
Charlemont, Mass., USA
Tiriolo, Calabria, Italy

43

Reference #Colour2988/1122991 / 115
2991/115
2992/116
2993/1172994/118
5171/127
13747/190
13958/193
18820/228
19970/230
20822/232
22759/244
23838/249

Dark green
Blue
Light green
Honey yellow
Dark green
Honey yellow
Dark green
Blue-green
Blue-green
Black
Black-green
Blue-green
Black-green
Blue-green

S. LUCCHESI ETAL.

TABLE 2. Parameters for X-ray data collection
Unit cell parameter determination:
Radiation (A)Reflections used
Range (20)Temperature (K)Mo-K~I (0.70930)
12 (Friedel pairs on both +20 and - 2 0 )
83~
~
296

Diffraction !ntensity collection:
Radiation (A)Monochromator
Range (20)Reciprocal space range
Scan method
Scan range (20)
Scan speed (20/min)
Temperature (K)

Omega
2.4 ~
Variable 2.93~
296

Data reduction:
Refinement
Corrections
Absorption correction
Set of observed reflections
Set of unique reflections

SHELXTL
Lorentz, Polarization
Semi-empirical, 13 psi scans (0~707-717 (I > 2or)147-149

Mo-Kc~ (0.71073)
High crystallinity graphite crystal
30_95 ~

O<~h,k,l<~17calibration). Precision for major elements (Mg,
A1, Fe) was usually within 1% of the actual
amount present, while that of minor elements waswithin about 5%. Contents of Fe 3+ were
calculated on the basis of stoichiometry: 3
cations for 4 oxygen atoms. Standard deviationsof cations were calculated according to Wood andVirgo (1989). The elements Ni, Cu, V and Si were
not observed. Results are shown in Table 4. Verygreat care was devoted to the microprobe analyses
no less than 15 point analyses were performedon each sample - - since only two samples,
SPN116 and SPN 118, were available in sufficientamounts for M/Sssbauer verification o f the
FeZ+/Fe 3+ ratio.
The samples for M6ssbauer measurements
were prepared by finely grinding 16 mg of eachspinel in an agate mortar under acetone. The
powder was then suspended in vaseline andwrapped in thin plastic foil. Spectra were obtainedin a conventional constant acceleration spectrometer using a rhodium matrix 57Co source(nominal strength 50 mCi) kept at r o o mtemperature; the spectrometer was calibrated atroom temperature with s-iron foil. Mirror
symmetric spectra were accumulated in a 512
channel analyser, folded, and fitted to pure
Lorentzian line-shapes with the aid of a leastsquares fitting program.
-

~

~ 20)

Results

Cation distribution
The cation distribution of the investigated Znspinels is rather complex, involving five cations(Al, Zn, Mg, Fe 2§ Fe 3+) that may be distributedon both T and M sites.
Intracrystalline distribution was obtained by aminimization program which takes into accountboth structural and chemical data (Carbonin et al.,1996). The minimized function is:-f(Xi) = ~.~=l[ Oj --C~j(xi) )]where Oj is the observed quantity, (~j its standard
deviation and Cj the same quantity calculated bymeans of X,. parameters. The n Cj quantities were:a, u, the site electrons e-(T) and e (M), thenumber of atoms per formula unit (3) and thenumber in T and M sites (1 and 2 respectively),
the number of charges for balance and the total
atomic proportions from microprobe analyses.
The Xg parameters were the cation fractions inboth sites.
The a and u values depend on the T-O andM - O site bond distances (Hafner, 1960; Hill etal., 1979) and are thus linked to the cation site
44

CATION DISTRIBUTION IN ZN-ALUMINATE SPINELS

o

'=o A

~

~x

o[~

"O
c~

"O

~m

N

N

N

~

~o~
n~

d d d d d d d d d d d d d d~ d M ~ d d ~ d d d d ~ d

X X X X X X X X X X X X X X

X X X X X •

~.o

45

S./LICCHESI ETAL.
TABLE 4. Electron microprobe analyses of Zn-aluminate natural spinels
Sample

MgO

ZnO

MnO

FeO

A1203

Fe203*

Cr203

Total

SPN 112SPN 115a
SPN l15b
SPN 116
SPN 117
SPN 118
SPN 127
SPN 190
SPN 193
SPN 228
SPN 230
SPN 232
SPN 244
SPN 249

3.91
1.80
0.49
0.45
0.48
0.37
4.35
23.48
21.43
2.43
0.84
1.63
1.47
23.14

35.09
33.71
41.89
41.56
42.00
40.76
34.28
4.33
9.09
28.73
40.44
33.09
38.15
5.96

0.12
0.68
0.72
0.51
0.60
0.80
0.13
0.37
0.30
0.41
0.89
0.41
0.08
0.45

3.87
6.47
0.14
0.39
2.99
2.31
1.30
9.97
0.57
7.01
3.22
1.51

56.76
55.85
49.35
45.13
50.00
46.74
57.43
67.17
66.47
54.30
49.47
54.03
54.98
67.29

1.76
8.27
13.26
7.52
10.22
1.33
2.38
2.15
4.03
8.47
3.10
1.95
2.25

tr.
0.13
tr.
tr.
0.17
0.06
tr.
-

99.83
100.35100.93101.57100.8999.60
100.60100.11100.7799.94
100.8599.34
99.93
100.62

Cations (4 oxygens)
Sample
Mg

Zn

Mn

Fe 2+

AI

Fe 3§

Cr

Total

0.762(7)0.742(8)0.959(8)0.965(10)0.960(13)0.954(11)0.737(10)0.079(3)0.168(4)0.633(5)0.923(11)0.740(8)0.850(9)0.109(23)0.003(1)0.017(2)0.019(2)0.027(2)0.016(3)0.021(2)0.003(1)0.008(1)0.006(1)0.011(1)0.023(2)0.011(1)0.002(1)0.009(4)0.063(3)0.161(5)
0.000
0.000
0.002(18)0.008(5)0.071(3)0.048(1)0.026(3)0.248(5)0.015(16)0.176(6)0.081(9)0.030(3)1.968(7)1.961(10)1.803(9)1.673(10)1.823(17)1.747(11)1.970(10)1.956(4)1.959(5)1.909(7)1.802(14)1.928(9)1.956(11)1.958(30)0.032(6)0.039(8)0.193(6)0.314(9)0.177(16)0.246(9)0.030(9)0.044(3)0.041(4)0.091(6)0.197(14)0.072(8)0.044(10)0.043(27)0.003(1)0.004(1)0.002(1)
-

3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000

SPN 112SPN 115a
SPN l15b
SPN 116
SPN 117
SPN 118"*
SPN 127
SPN 190
SPN 193
SPN 228
SPN 230
SPN 232
SPN 244
SPN 249

0.172(2)0.080(3)0.023(2)0.021(1)0.022(1)0.018(2)0.189(6)0.865(3)0.799(3)0.108(3)0.039(1)0.073(2)0.066(1)0.852(29)* Calculated.
** SPN118 also contains Ti 0.002(1) apfu.

e-(M) = ]~iXM,iZi, that is, as a linear contributionof the atomic number (Zi) of each cation and itssite molar fraction (Xi).
The following assumptions were made at the
beginning of the minimization: (a) Zn, Mg, A1, Fe 2§
and Fe 3§ are equally distributed in both T and M
sites; (b) on the basis of their general site preference,the small amounts of Cr and Ti are fbxed in the Msite (Marshall and Dollase, 1984; O'Neill andDollase, 1994; Della Giusta et aL, 1996) and
Mn 2+ in the T site (Lucchesi et al., 1997); (c) thesum (in T + M sites) of the cation fractions need not
be equal to the microprobe value; (d) no constraints
are fixed for maxima and minima.

fractions (Xi) according to:
T - O = ~iXT,i(W-O)iand
M - O = ]~iXM,i(M--O)i
where XT,i and XM,i are the cation site-fractions intetrahedral (T) and octahedral (M) coordinationand ( T - O ) i and ( M - O ) i the corresponding 'ideal'bond distances. The ( T - O ) i and ( M - O ) i set usedin the minimization is that proposed by Della
Giusta et al. (1996), integrated with theMn2+(T)-O distance (Lucchesi et al., 1997).
Site electronic densities e-(T) and e-(M) were
calculated according to: e-(T) = ~ i X T , i Z i and46

CATION DISTRIBUTION IN ZN-ALUMINATE SPINELSAs already noted, the site occupancies of theseevaluation of the standard deviation of thespinels are strongly dependent on the ionization
atomic fractions in each site is not simple, butlevel of the adopted scattering curves. However,
we may assume a value about twice thatwhile the total number of electrons e-(tot.) may
calculated in Table 4 as a sufficient estimate.
change by up to 1 e-, the e-(T)/e-(M) ratio
remains unchanged. This is most probably a
Crystal chemistryconsequence of the isotropic extinction factor in
the adopted refinement program, since measureThe aluminate spinels investigated here cover aments of the intensities of the equivalentrelatively wide compositional range (Table 4) andreflections often showed anisotropic extinction.
contain, besides Mg, A1 and Zn, relevant amounts
The experience of our laboratories is thatof Fe z+ (up to c. 0.25 apfu in SPN228 anddifferences between the total number of electrons
SPN232, both samples being of the kreittonite
from the accurately calibrated microprobe andvariety) and Fe 3+, up to 0.31 apfu. The elements
those from site occupancies are always lower thanA1 and Fe 3+ are the only important trivalent
0.5%. Consequently, the e-(T) and e-(M) values
cations. Minor amounts of Mn2+ and Cr are also
were rescaled to the microprobe e-(tot.), in orderpresent. Of the divalent cations, the main
to avoid overestimation of this factor in thesubstitution is Zn for Mg (Fig. 1) and Fe2+, theminimization.
latter mainly in the kreittonites. Progressive
Results showed a systematic increase inenrichment in zinc does not determine variations
minimization residuals with Zn content. Thisin a. Considering samples from the same locality,
suggested that the Zn2+(T)-O bond distance
it is apparent that a may decrease with increasing
should be changed, repeating the minimization
Zn content (Tiriolo, Silberberg, Falun), remainof all samples. The Zn2+(T)-O bond distance wasunchanged (Franklin, Sterling), or even increasechanged by 0.0005 A at a time, and the best result(New York) (Table 3). Instead, cell parameter
was obtained at a value of 1.960 ,~, i.e. slight!yvariations are essentially determined by thelower than that of Mg2+ (Mg(T)-O = 1.996 A,
substitution Fe3+ ~-- A13+ (Fig. 2). The small
Della Giusta et al., 1996).
quantities of Mn2+ (except in SPN228) generally
We used this method since data from synthetic
increase with Fe 3+.
Zn spinels show great differences according tocomposition. For instance, Zn(T)-O turns out tobe 1.953 A in ZnA1204 and 1.967/k in ZnCr204
1.00(O'Neill and Dollase, 1994) and 1.984 A in
ZnFe204 (O'Neill, 1992). Similar differences - up to 0.030 A, more than ten times the
0.80
experimental sigmas - - are also shown by othercations. A consequence of this is that it is
impossible to obtain a single set of bond distances
0.60
which satisfactorily fits the geometric parametersof synthetic spinels. Instead, this is possible fornatural spinels, as well as for other minerals suchas olivine, garnet, clino- and orthopyroxene.
0.40
Ample literature about the use of bond distances
to determine the cation distribution of theseminerals for petrological purposes exists, and
0.20+ ~+ + ~ +testifies that bond distances are approximately
constant - - with an uncertainty of some digits tothe third decimal point - - at least within each
0,00
t
t
i
class of compounds.
0.0
0.20 0.40 0.60 0.80 1.00
Intracrystalline distributions are reported inZn afuTable 5. Table 6 shows the minimization data
Fro.
1.
Natural
Zn-aluminates:
Mg vs Zn content
compared with the observed data. The atomic
fractions in Table 5 are shown to the fourth variations. Scatter of points to the left of the line
decimal point, only to allow comparison with the representing the Mg ~ Zn substitution is due to (Mn2" +Fe2--) ~ Zn substitution.
calculated data collected in Table 6. Correct47

S. LUCCt-tfiSIs
TABLE 5. Cation distribution o f natural Zn-aluminate spinels, as obtained from the minimization procedure
Sample

Site

Mn

Fe 2+

Zn

Mg

A1

Fe 3+

Cr

Total

SPN 112

T
M
T
M
T
M
T
M
T
M
T
M*
T
M
T
M
T
M
T
M
T
M
T
M
T
M
T
M

0.00290.01730.01870.02690.01560.02140.00310.00770.00630.01060.02330.01050.00210.0093-0.06310.00010.14650.01480.00000.00000.00000.00000.00000.00210.00000.00830.07150.00000.04790.00010.02640.00000.22850.02060.00000.01500.16490.01120.06510.01610.03010.00000.75290.00810.72040.02120.91180.04570.90170.05410.93120.02890.89670.05880.72980,00630,07610.00280.16730.0002
0.63490.00010.88870.03290.73690.0026
0.84940.00000.10840.00000.10380.06750.0236
0.05610.00000.02270.00000.02090.00000.02290.00000.01800.09430.09410.75050.11030.68530.10930.04340.06490.00000.03930.00500.06840.00000.06640.73350.11330.06061.90850.05301.90790.00491.79980.0028
1,67950.00001.82430.00001.74680.07791.89240.08941.87010.08771.8747
0.03671.87020.00011.8046
0.0561
1.87210.03801.9186
0.0962
1.86520.01650.01550.03910.00000.06430.1286
0.06790.24720.05100.12440.08110.16360.02330.00710.02770.01590.02620.01480.04640.04470.08670.10900.02660.04560.04410.00000.02160.02060.00000.00000.00310.00000.00000.00400.00000.00000.00000.00000.00010.00000.00000.00000.9997
1.99971.00002.00000.99981.99990.99932.00170.99792.00260.99922.00130.99981.99980.9993
1.99920.9992
1.99911.00052.00050.99872.00100.9999
1.99990.99862.00100.9992
1.9990SPN l15a
SPN l15b
SPN 116
SPN 117
SPN 118
SPN 127
SPN 190
SPN193
SPN228SPN230
SPN232SPN244
SPN249

Atomic frequencies are given to the fourth decimal point for calculation purposes only. For an estimate of theuncertainty, refer to standard deviations of Table 4.
* Contains Ti 0.0018.

The T - O bond distance ranges from 1.944 ,& to1.960 A, due to the substitution (Zn + Mg) forFe 2+ (Table 5). These T - O values, as a consequence o f the fact that the values o f Zn and Mg
tetrahedral bond-distances are similar, are notrelated to the variations in Zn content with
progressive Mg substitution. Instead, ferrous
iron, which is mainly ordered in the tetrahedral
site (Fe2+(T)-O = 1.996 A), when present insignificant amounts, causes an increase in the
T - O values (Fig. 3). In contrast, given their shortsite bond-distances ( A I ( T ) - O = 1.767 A andFe3+(T)-O = 1.891 A), even small amounts o f
AI(T) + Fe3+(T) result in a contraction o f the
T - O values. The electronic density o f the T site
is highly variable, ranging from 14.7 e - (SPN249)
to about 29.7 e - (SPN116), essentially because o f

the amply variable Zn(T) site contents. Despite
the r e c o g n i z e d t e n d e n c y o f Mn 2+ towards
intracrystalline disorder in galaxite and jacobsite(Lucchesi et al., 1997), this cation was fixed in
the T site due to its low content (less than 0.03
apfu), since significant amounts o f Mn 2+ are
known to occupy octahedral sites significantly
only in crystals with total Mn content higher than
0.2 apfu (Lucchesi et al., 1997).
In all samples the octahedral bond-distance isshorter than the tetrahedral one, as a consequenceo f the oxygen fractional coordinate, which ishigher than 0.2625 (for this value T - O = M - O ;Hill et al., 1976). Such a short M - O bonddistance, from 1.917 A to 1.932 A, is a directconsequence o f the large amounts o f A1 and o f its
general preference for the M site (Fig. 4).
48

CATION DISTRIBUTION IN ZN-ALUMINATE SPINELS

~D

_=
~q

c~
O
~D

"6
C~

I

"5
~3
o

r

9

m
.<

~5
..=
o

o
r./~ r.,~O r.,~ r./3 r./3 r~o r./~ r.,/3 r.~ r./3 cfJ r./3 r./3 rJ~

49

S. LUCCHESIETAL.
8.20

0.40

8.18

0.30
8.16

:3

A
~~ 8.14

8.12+

I-- 0.20
r
L,I-

u

.-Jr

[3

+
+

'~
0.10
+

8,10

++

+
++

8.08

I

0.00

i

0.10 0,20

i

I

I

I

0.30

0.40

0.50

0.60

0.00
1.940

0.70

t

I

t

t

1.945

1.950

1,955

1.960

Fe 3+ afu

FIG. 2. Natural Zn-aluminates: a vs Fe3+ content
variations. Crosses: data from the present work; Fullsquares: natural samples (Waerenborgh et aL, 1990);
Full circle: natural sample (Saalfeld, 1964); Opensquares: synthetic Zn(A1, Fe)204 (Carvalho and Sclar,1988); Open circles: synthetic Zn(A1, Fe)204 (Waerenborgh et aL, 1994a). Line shows the calculated values
for Zn(A1, Fe)204 cubic spinels as obtained from thebond distances used in this paper. All natural data lie
above this line because of the substitution of Zn by otherbivalent cations. Synthetic data from literature, relative
to nominal compositions Fe3+ 0.2 and 0.6, show aremarkable scatter presumably due to poor chemical
characterization. Literature data concerning synthetic
Z n A I 2 0 4 end-member are all very close to 8.088 A (see
tex0.

1.965

T-O (A)
FIG. 3. Natural Zn-aluminates: Fe2+(T) content variations vs T-O bond distance. Crosses: data from thepresent work; Open squares: synthetic Znl_xFexAl204(Waerenborgh et al., 1994b).

2.00
1.90
+

1.8o

~

+
+

1.70

Substitution of Fe3+(M) for AI(M) is in fact
responsible for the M - O bond distance increase,
A I ( M ) - O b e i n g d e f i n i t e l y shorter thanFe3+(M)-O (1.909 A and 2.020 A, respectively).
This relation holds, no matter which otheroctahedral cations ( Z n ( M ) - O = 2.104 A,
Mg(M)-O = 2.095 A or Fe2+(M)-O = 2.138 A)
substitute for AI(M). However, the latter substitutions are responsible for the variations in site
electronic density. In any case, M site occupancy,
which is dominated by A1, is less variable thanthat of the T site and is generally very close to 13
e , although in a few samples the value increases
up to 14.8 e- (SPN 116) as a consequence of
Fe3+(M) ~ AI(M) substitution and, in somesamples, of the particularly high Zn(M) content(up to 0.06 apfu in SPNll8).

1.60

1.50

1.40

1.30
1.905

'
1.915

I
1,925

1.935

1,945

1,955

M-O (A)
FIG. 4. Natural Zn-aluminates: AI(M) content variations
vs M - O bond distance. Crosses: data from the present
work; Open squares: synthetic Zn(A1, Fe)204 (Waerenborgh et al., 1994a). Line shows calculated M - O
variations with AI(M) content using bond-distance datafrom the present work.
50

CATION DISTRIBUTION IN ZN-ALUMINATE SPINELS

Mi~ssbauer spectral results

and narrower, while that of sample SPN118 has avalue which is sightly too large for the linewidth.
Moreover, as the diffraction and microprobe
results supported the presence of two distinct
iron sites, on the basis of this suggestion and thespectral findings, an attempt was made to fit both
spectra to two, strongly overlapping doublets.
This approach resulted in better ;(2 values andmore reliable parameters than the fitting to justone doublet, and was accepted as the mostrepresentative (Table 7). The samples are fully
oxidized and contain Fe 3+ in both M and T sites ina 3:2 (SPNll8) and 4:1 (SPNll6) quantitative
ratio. This attribution is supported by isomer shift
values characteristic of the two types of geometriccoordination. The quadrupote splittings for bothsites of sample SPNll8 are higher than thecorresponding values of sample SPNll6, prob-

The iron distribution data supplied by them i n i m i z a t i o n procedure were tested by
M6ssbauer spectroscopy on the two samples,
SPNll6 and SPNll8, that were available insufficient amounts and were highly purified. The
spectra show only one quadrupole split doublet
due to Fe 3+ cations. The absence of any
absorption at about 2.0 mm/s is a sure indicationthat no Fe2§ is present in these crystals. The
spectra can easily be fitted to just one doublet
with parameters that are reasonable for Fe 3+ in M
sites. Differently from synthetic ZnFe204, where
Fe3+(M) and Fe3+(T) doublets completely overlap(O'Neill, 1992), the spectrum of sample SPNt 16(Fig. 5) shows evident asymmetry between thetwo peaks, the one at higher velocity being deeper

-"N
E

.-e

oo
, o O

m

9

9

Q
O

I

-4

I

I

I

0

Velocity

4mm/s

Fra. 5. Natural Zn-ahtminates: M6ssbauer spectra of samples SPN116 and SPN118 at room temperature.

S. LUCCHESI ETAL.

TABLE 7. M6ssbauer effect spectral parameters for SPN116 and SPN118
spinels collected at room temperature
Sample

~
mm/s

AEQ
mm/s

F
mm/s

Area
%

Attribution

SPN116

0.368
0.236
0.385
0.214

0.604
0.664
0.717
0.748

0.36
0.22
0.35
0.32

80
20
60
40

M
T
M
T

SPNll8

8: Isomer shift; AEQ: Quadrupole splitting; F: Line full width at half height.

with the general features of that proposed by
Waerenborgh et aL (1994a) for synthetic Znspinels in the ZnxFe]_xA1204 system. The sameauthors (Waerenborgh et al., 1994b) postulated
the existence of intracrystalline disorder, although
not well quantifiable, between Zn and (AI+Fe3+)
in synthetic ZnFeyAle_yO4 spinels, with adefinitely lower contribution by Al. The latterhypothesis, of exchange between zinc andtrivalent cations is only partly supported by the
relation between Zn(M) and Fe3+(T) observed innatural samples (Fig. 6), since Fe3+(T) and AI(T)exhibit the opposite behaviour. In any case, we
confirm the tendency of Zn towards crystalline

ably due to the presence in the former of moredisordered second-coordination spheres. In each
spectrum, the quadrupole splitting value of the Tsite is larger than that of the M site, probably
because of greater disorder in the octahedral sites,
which are occupied by various metal cations.
These results are very consistent with themicroprobe and XRD results, no Fe2+ being
detected in either sample (Table 4). The small
residual Fe2+ of SPNll8 is close to its standard
deviation within the (7 of cation distribution. The
Fe 3§ distribution obtained by the two methods isthe same. The MSssbauer data give 20% Fe3+ in
the T site for SPNll6 and 40% for SPNll8.
These values compare well with those of 19% and32%, obtained from the minimization procedure.

0.06 )

Conclusions

0

The main features of cation distribution in Zn-richaluminate spinels concern evidence of Zn
intracrystalline disorder. Zinc, as expected,
shows a strong preference for tetrahedral coordination, but definite amounts (up to about 0.06
apfu) are also found in octahedral coordination.
This is true, not only for the samples with thehighest Zn content (> 0.88 apfu, samples
SPNll5b, 116, 117, 118, 230), but also for
those with high Fe3+ content. Considering thequoted {; values, a satisfactory positive correlation exists between Zn(M) and Fe3+(T) (Fig. 6), inagreement with the observation of the little
inversion in natural franklinites (work in progress)
and, at high temperatures, in synthetic Zn-ferrites(O'Neill, 1992). A negative correlation between
AI(T) and Zn(M) can be seen in Fig. 6. As in MgA1 rich spinels, AI(T) and Mg(M) are highly
correlated (Fig. 7). This cation distribution agrees

+

0

+

0.04

+

E
N
+

0.0~

§

0

o

+

0

0
0.O0

0.00

II

0.02

!

~

II

0.04

0
I

i

0.06

0.08

c~

O

0.10

afu

F~o. 6. Natural Zn-aluminates: Zn(M) content variations
vs Fe3+(T): crosses, and AI(T): circles.
52

CATION DISTRIBUTION IN ZN-ALUMINATE SPINELS
0.10

agreement with the M6ssbauer investigations by
Waerenborgh et al. (1990) on natural iron-richgahnites of which two samples, from Falun andfrom Bodenmais, were compositionally similar toour samples SPN127 and SPN228. However ourdata, which show a general tendency towards theintracrystalline disorder of ferric iron on T and M
sites in particular in SPN228, do not agree withthe data of the latter authors, who showed acomplete ordering of Fe3+ in octahedral sites.

§

++
0.08

3-

+
+

0.06
+

--< 0.04i++

Acknowledgements
0.02

0.00
0.00

~i'+~

(I

I

I

I

0.02

0,04

006

0.08

0.10

The Director and Curator of the Museum ofMineralogy of the University of Rome "La
Sapienza", Italy, are thanked for kindly making
samples available. Mr. R. Carampin is thanked for
his assistance during microprobe analysis at the
CNR laboratory in Padova and Mrs Gabriel
Walton for revision of the English text.
Comments by the editor and an anonymousreferee substantially improved the manuscript.
This research was carried out within the scientificprograms and with the financial support of the
CNR (grant no. 95.04078.CTll) and a MURST
grant.

0.12

Mg(M) afu
FIG. 7. Natural Zn-aluminates: AI(T) content variations
vs Mg(M).

disorder only in the presence of appreciable
amounts of trivalent iron.
The Zn(T)-O distance was optimized at 1.960
A, shorter than the value accepted for Mg(Mg(T)-O = 1.964 A; Della Giusta et al.,1996). This is consistent both with the fact thata does not change relative to the main substitutionof these samples, Zn ~ Mg (Fig. 1), and with thesimilarity of the a parameter in synthetic
MgA1204 and ZnA1204, which are 8.090-8.086
A (Grimes et al., 1983; Finger et al., 1986) and8.088-8.086 A (Fischer, 1967; BruckmannBenke et al., 1988; Carvalho and Sclar, 1988;
O'Neill and Dollase, 1994; Waerenborgh et al.,1994b), respectively.
In the investigated spinels, AI shows its usual
preference for the M site but with some'inversion' with respect to Mg. The trend of theexchange AI(T) ~ Mg(M) common to all
samples, from high to low Zn content, can beseen in Fig. 7.
Concerning iron distribution, Fe2+ shows adefinite preference for the T site, as alreadyproposed by work on other natural aluminate
spinels containing iron (Della Giusta et al., 1996);
the reported amounts of octahedral ferrous ironare very probably of the same magnitude as thecorresponding error. The Fe2+(T) content islinearly related to T - O bond distance (Fig. 3).
The ordering of Fe2+ in tetrahedral sites is in good

References
Bmckmann-Benke, P., Chatterjee, N.D. mad Aksyuk, M.
(1988) Thermodynamic properties of Zn(A1,Cr)204
spinels at high temperatures and pressures. Contrib.
Mineral. Petrol., 98, 91-6.
Carbonin, S., Russo, U. and Della Giusta, A. (1996)
Cation distribution in some natural spinels from Xray diffraction and M6ssbauer spectroscopy.
Mineral. Mag., 60, 335-68.
Carvalho, III, A.V. and Sclar, C.B. (1988) Experimental
determination of the ZnFe204-ZnA1204 miscibility
gap with application to franklinite-gahnite exolution
intergrowths from the Sterling Hill mine deposit,
New Jersey. Econ. Geol., 83, 1447-52.
Cooley, R.F. and Reed, J.S. (1972) Equilibrium cation
distributions in NiA1204, CuA1204 and ZnAl204 . J.
Amer. Ceram. Soc., 55, 395-8.
Della Giusta, A., Princivalle, F. and Carbonin, S. (1986)
Crystal chemistry of a suite of natural Cr-bearing
spinels with 0.15<Cr<1.07. Neues Jahrb. Mineral.
Abh., 155, 319-30.
Della Giusta, A., Carbonin, S. and Ottonello, G. (1996)
Temperature-dependent disorder in a natural Mg-A1Fe2+-Fe3+-spinel.Mineral. Mag., 60, 603-16.
Finger, L.W., Hazen, R.M. and Hofmeister, A.M. (1986)
High-pressure crystal chemistry of spinel (MgA1204)
and magnetite (Fe304): comparisons with silicate
53

S. LUCCHESI ETAL.

spinels. Phys. Chem. Minerals, 13, 215-20.
Fischer, P. (1967) Neutronenbeugungsuntersuchungender Strukturen von MgA1204- und ZnAlzO4Spinellen, in Abh5ngigkeit vonder Vorgeschichte.
Zeits. Krist., 124, 275-302.
Grimes N.W., Thompson, P. and Kay, H.F. (1983) New
symmetry and structure for spinel Proc. Royal Soc.
London, A386, 333-45.
Hafner, S. (1960) Metalloxyde mit Spinellstruktur.
Schweiz. Mineral. Petrogr. Mitt., 40, 208-40.
Hill, R.J., Craig, J.R. and Gibbs, G.V. (1979)
Systematics of the spinel structure type. Phys.
Chem. Minerals, 4, 317-39.
Larson, F.K. (1970) Crystallographic Computing. Ed.
F.R. Ahmed, Munksgaard, Copenhagen.
Larsson, L. (1995) Temperature dependent cation
distribution in a natural Mg0.4Feo.6Al204 spinel.
Neues Jahrb. MineraL, Mh., 173-84.
Lucchesi, S. and Della Giusta, A. (1997) Crystal
chemistry of an highly disordered Mg-A1 naturalspinel. Mineral. Petrol., in press.
Lucchesi, S., Russo, U. and Della Giusta, A. (1997)
Crystal chemistry and cation distribution in someMn-rich natural and synthetic spinels. Eur. J.
MineraL, 9, 31-42.
Marshall, C.P. and Dollase, W.A. (1984) Cation
arrangement in iron-zinc-chromium spinel oxides.
Amer. Mineral., 69, 928-36.
Navrotsky, A. (1986) Cation-distribution energetics andheats of mixing in MgFe204-MgAlzO4-ZnA1204
and NiAlzO4-ZnA1204 spinels: Study by hightemperature calorimetry. Amer. Mineral., 71,
1160-9.
Nell, J., Wood, B.J. and Mason, T.O. (1989) Hightemperature cation distributions in Fe304-MgAlzO4MgFe204-FeA1204 spinels from thermopower andconductivity measurements. Amer. Mineral., 74,
339-51.
O'Neill, H.St.C. (1992) Temperature dependence of thecation distribution in zinc ferrite (ZnFe204) frompowder XRD structural refinements. Eur. J.
Mineral., 4, 571-80.
O'Neill, H.St.C. and Dollase, W.A. (1994) Crystal
Structures and Cation Distributions in Simple
Spinels from Powder XRD Structural Refinements:
MgCr204, ZnCr204, Fe304 and the Temperature
Dependence of the Cation Distribution in ZnA1204.
Phys. Chem. Minerals, 20, 541-55.
O'Neill, H.St.C. and Navrotsky, A. (1983) Simple
spinel: crystallographic parameters, cation radii,
lattice energies and cation distribution. Amer.
Mineral., 68, 181-94.
O'Neill, H.St.C., Annersten, H. and Virgo, D. (1992)

The temperature dependence of the cation distribution in magnesioferrite (MgFe204) from powder
XRD structural refinements and Mrssbauer spectroscopy. Amer. Mineral., 77, 725-40.
Princivalle, F., Della Giusta, A, and Carbonin, S. (1989)
Comparative crystal chemistry of spinels from somesuites of ultramafic rocks, Mineral Petrol., 40,
117-26.
Roelofsen, J.N., Peterson, R.C. and Raudsepp, M.
(1992) Structural variation in a nickel aluminate
spinel (NiAI204). Amer. Mineral., 77, 522-8.
Saalfeld, H. (1964) Strukturdaten von Gahnit, ZnA1204.
Zeits. Krist., 120, 476-8.
Shannon, R.D. (1976) Revised effective ionic radii andsystematic studies of interatomic distances in halides
and chalcogenides. Aeta Crystall., A32, 751-67.
Spry, P.G. and Scott, S.D. (1986) The stability ofzincian
spinels in sulfide systems and their potential as
exploration guides for metamorphosed massive
sulfide deposits. Econ. Geol., 81, 1446-63.
Tokonami, M. and Horiuchi, H. (1980) On the spacegroup of spinel MgAI204. Acta Cryst., A36, 122-6.
Waerenborgh, J.C., Annersten, H., Ericsson, T.,
Figueiredo, M.O. and Cabral, J.M.P. (1990) A
M6ssbauer study of natural gahnite spinels showing
strongly temperature-dependent quadrupole splitting
distributions. Eur. J. Mineral., 2, 267-71.
Waerenborgh, J.C,, Figueiredo, M.O., Cabral, J.M.P.
and Pereira, L.C.J. (1994a) Powder XRD Structure
Refinements and 57Fe M~Sssbaner Effect Study ofSynthetic Znl_xFexAl204 (0<x< 1) Spinels Annealed
at Different Temperatures. Phys. Chem. Minerals,
21, 460-8.
Waerenborgh, J.C., Figueiredo, M.O., Cabral, J.M.P.
and Pereira, L.C.J. (1994b) Temperature andC o m p o s i t i o n D e p e n d e n c e of the Cation
Distribution in Synthetic ZnFeyAlz_yO4 (0 ~< y ~<1) Spinels. J. Solid State Chem., 111, 300-9.
Wood, B.J. and Virgo, D. (1989) Upper mantle
oxidation state: Ferric iron contents of lherzolite
spinels by 57Fe M~Sssbauer spectroscopy andresultant oxygen fugacity. Geoehim. Cosmochim.
Aeta, 53, 1277-91.
Wood, B.J., Kirkpatrick, R.J. and Montez, B. (1986)
Order-disorder phenomena in MgA1204 spinel.
Amer. MineraL, 71, 999-1006.
Wu, C.C. and Mason, T.O. (1981) Thermopower
measurement of cation distribution in magnetite. J.
Amer. Ceram. Soc., 64, 520-2.
[Manuscript received 3 June 1996:
revised 24 April 1997]

54