Antonio Della Giusta

Cation ordering and structural variations with temperature in MgAl2O4 spinel: An X-ray single-crystal study

Abstract The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl2O4 spinel was investigated between 600 and 1100 °C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 °C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O’Neill and Navrotsky (1983) gave α = 23 ± 2 KJ/mol and β = 13 ± 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and MO bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of -0.014 Å for T-O and +0.007 Å for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 × 10-6 °C-1 and 8.9 × 10-6 °C-1 were calculated for T and M sites, respectively.

Cation Distribution in Some Natural Spinels from X-ray Diffraction and Mössbauer Spectroscopy

Abstract Three natural spinels of different places of occurrence and compositions were investigated by means of microprobe chemical analysis, single crystal X-ray diffraction and Mössbauer spectroscopy. All cation distributions between T and M sites were calculated from microprobe and XRD experimental data, by means of a mathematical model with appropriate assumptions. Fe2+ and Fe3+ assignments calculated in this way were compared with those observed in Mössbauer spectra. The satisfactory agreement found proves, at least in the samples studied, the reliability of the model and the assumptions used. In the spinels examined, such results show Fe2+ and Fe3+ virtually ordered in T and M sites respectively. Mössbauer data also revealed Fe2+ in different tetrahedral sites due to the next-nearest neighbour effect, probably as a consequence of spinel genetic conditions.

Compositional dependence of cation distribution in some synthetic (Mg,Zn)(Al,Fe3+)2O4 spinels

The compositional dependence of the intracrystalline cation distribution was investigated in four synthetic spinels belonging to binary solid solutions. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe 3+ ) 2 O 4 system, and cation distribution was determined by means of single-crystal X-ray structural refinement, electron-microprobe analysis and Mossbauer spectroscopy. Experimental data were processed and a detailed topochemical model was obtained for each sample: IV (Mg 0.76 Al 0.24 ) VI (Al 1.76 Mg 0.24 )O 4 ; IV (Zn 0.65 Mg 0.22 Al 0.13 ) VI (Al 1.87 Mg 0.13 )O 4 ; IV (Mg 0.73 Al 0.18 Fe 3+ 0.09 ) VI (Al 1.62 Mg 0.27 Fe 3+ 0.11 )O 4 and IV (Mg 0.62 Zn 0.15 Al 0.15 Fe 3+ 0.08 ) VI (Al 1.61 Mg 0.23 Fe 3+ 0.16 )O 4 . Zinc was found to occupy only the tetrahedrally coordinated site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg ⟷ Al and Mg ⟷ Fe 3+ turned out to be limited. Zinc in octahedral coordination, which is rare in natural spinels, was shown to be favoured by high Fe 3+ contents and high equilibration temperatures. The cation distributions determined experimentally were found to be in close agreement with those calculated using the general thermodynamic model for spinel solid solutions by O9Neill & Navrotsky (1984).

CATION AND VACANCY DISTRIBUTION IN AN ARTIFICIALLY OXIDIZED NATURAL SPINEL

Abstract During research on the influence of temperature on cation partitioning in natural Mg-Al-Fe2+-Fe3+ spinels, some crystals were accidentally oxidized during heat treatment. The oxidation product, studied by means of single-crystal X-ray diffraction, turned out to be a phase retaining the Fd3m parent spinel structure, but with cell edge a and oxygen coordinate u considerably smaller than the parent ones (a ~ 8.087 as compared with ~8.111 Å; u ~0.2609 vs. 0.2617-0.2636) and with vacant sites due to oxidation. Assuming that the oxidation process must occur due to the addition of oxygen to the crystal boundary as cations are being preserved and rising in total valence, the site population was determined and compared with that of untreated and heated samples. It was found that, on oxidation, a charge enrichment in the tetrahedral site T had occurred, this phenomenon following that observed during heating at increasing temperatures also in other spinel series. This continuity was always in the direction of an increase in random charge distribution. Cation vacancies produced during oxidation were restricted to the octahedral site M. Examination of bulk sections by reflected light microscopy showed a few hematite lamellae as inclusions in the oxidized samples, not detectable by microprobe analysis or single-crystal structural refinement. However, hematite played a marginal part in oxidation. Vacancy-oxygen distances in oxidized spinels were determined from experimental data in the literature.

Energy and long-range disorder in simple spinels

Interionic potential calculations conducted on simple spinels in the framework of the Born model indicate that, within experimental uncertainties, observed control of P, T intensive variables on long-range disorder may be completely due to static effects. Parameterization of repulsive potentials allows us to rationalize the existing experimental data and quantify the variation of static energy, configuration entropy and static Gibbs free energy with temperature. The static energy of the investigated phases cannot be expressed in simple quadratic terms of the inversion parameter, as commonly assumed, but requires higher order terms. Knowledge of the longrange disorder at various temperatures allows us to express configurational entropy as a Maier-Kelley type polynomial expansion on temperature. As configurational entropy simply depends on the static part of the internal energy, the integration of the obtained functions in d T allows rapid evaluation of the effect of long-range disorder on the static part of energy.

Structural relationships in (Mn1−xZnx)Mn2O4 (0 ≤ x ≤ 0.26): The “dragging effect” of the tetrahedron on the octahedron

Abstract Ten hausmannite crystals (from Ilfeld and Friedrichrode, Harz, Germany), belonging to the (Mn1-xZnx)Mn2O4 (0 ≤ x ≤ 0.26) system (I41/amd hausmannite structure type), were characterized by chemical (electron microprobe) and structural (single-crystal X-ray diffractometer) analysis. The prevailing trivalent cation is Mn3+, with very minor Al (not higher than 0.005 apfu). Among divalent cations, the main substitution involves Zn → Mn2+. Cation distribution was obtained by comparing chemical and structural data, and results confirm normal distribution, with Mn3+ ordered on the octahedral site. A specific bond distance of 2.030 Å was refined for VIMn3+-O. Unit-cell parameters a and c range from 5.752 to 5.763 Å and from 9.408 to 9.461 Å, respectively. The smallest values are characteristic of the sample with the highest hetaerolite content. T-O bond distance (2.027-2.041 Å) shows a strong positive correlation with unit-cell constants, while the O-T-O angle (103.3-103.7°) is related only to the oxygen coordinate, z. The two octahedral bond distances show limited variations: the shorter one, M-OS, ranges from 1.927 to 1.930 Å, and is not significantly correlated with unit-cell parameters. The longer one, M-OL, shows a larger variation, from 2.281 to 2.290 Å, and is positively correlated with c. Regularization of the octahedron with increasing hetaerolite content coincides with an increase in the oxygen coordinate y and a decrease in c and c/a. Of particular interest is the positive linear relation between octahedral elongation and VT. As the octahedral content of all samples is almost constant, given the closeness of Mn3+ to stoichiometry, all structural distortions are linked to IVZn → IVMn2+ that reduces the T-O bond distance and causes movement of the structure toward cubic symmetry. This interaction is due to the “dragging effect” of the tetrahedron on the octahedron. In hausmannite-type structures, besides the main structural distortion produced by the Jahn-Teller effect, a secondary one, without symmetry modification, is determined by the geometrical effects of the tetrahedron on the octahedron.

Cation distribution and cooling rates of Cr-substituted Mg-Al spinel from the Olkhon metamorphic complex, Russia

A crystal chemical study was carried out on seven Mg-Al rich and Cr-bearing spinels from a calciphyre of the Olkhon metamorphic complex (western Lake Baikal, Russia) in order to: a) verify the structural effects related to the progressive substitution of Cr 3+ , the main substituent cation in this suite pertaining to the MgAl 2 O 4 - MgCr 2 O 4 binary join; b) estimate the closure temperature of the exchange reaction involving Mg and Al in tetrahedral (T) and octahedral (M) sites. This will be a first step towards evaluation of the cooling rate of the host rock. Chemical composition is Mg (Al 2-p , Cr p ) O 4 with 0.03 ≤ p ≤ 0.23 afu. With increasing p, the cell edge a increases from 8.090 to 8.117 A, whereas the oxygen positional parameter u is almost constant. As u is related to the (M-O)/(T-O) ratio, Cr substituting for Al in the M site causes an increase not only of the M-O bond distance, but also that of the tetrahedral T-O. The observed T-O increase is due to Mg ordering into the T site, with increasing Cr content. Cation distributions where used to estimate the closure temperature of the exchange reaction applying two available models. Both estimates are rather consistent for very low Cr content, but substantial disagreement is observed for Cr-rich samples. The inversion trend of the studied spinel suite extrapolated to Cr= 0, enables us to calculate the closure temperature of the MgAl 2 O 4 end-member with the same cooling rate. This suggests that kinetic results from cooling experiments on MgAl 2 O 4 may be applied to cation distributions measured in samples along the MgAl 2 O 4 - MgCr 2 O 4 binary join.

Chromite to magnetite transformation: compositional variations and cation distributions (southern Aosta Valley, Western Alps, Italy)

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 1-17 - DOI: 10.2451/2011PM0001 Special Issue in memory of Sergio Lucchesi Chromite to magnetite transformation: compositional variations and cation distributions (southern Aosta Valley, Western Alps, Italy) Antonio Della Giusta 1 , Susanna Carbonin 1,* and Umberto Russo 2 1 Dipartimento di Geoscienze, Universita di Padova, Italy 2 Dipartimento di Scienze Chimiche, Universita di Padova, Italy *Corresponding author: susanna.carbonin@unipd.it Abstract Massive magnetite bodies in the southern Aosta Valley, NW Italy, are most probably the product of transformation from a chromite proto-ore forming in the u ltramafic rocks of an ophiolite suite. Chromium was detected in all the magnetite bodies, except in the Cogne deposit. Chromite-magnetite transformation was examined in spinel samples from the Ussel ore deposit by EMPA, XRD and Mossbauer spectroscopy. Chemical compositions reveal strong chemical inhomogeneity, mainly for Cr and Fe 3+ . Spot analyses, from core to rim, in the composite chromite-magnetite grains show significant Al decrease accompanied by Mg and Zn depletion, while the sharp Cr decrease is assisted by Fe 3+ increase. Cr shows positive linear correlations with Mg, Mn and Zn, and negative with Fe 3+ and Fe 2+ . Cr/(Cr+Al) (~ 0.75) and Mg/(Mg+Fe 2+ ) (~ 0.15) reveal substantial modifications of the original chromite. At the chromite-magnetite boundary (Cr ~ 0.8 a.f.u.) variations in both major and minor elements are sharp, defining a compositional gap between the normal structure of chromite and the inverse structure of magnetite. At this boundary, Ti4+ shows a prominent peak (~ 0.02 a.f.u.) with respect to core and rim, 0.003 and 0.006 a.f.u., respectively. R 2+ 2TiO 4 component (R = Fe 2+ , Mg, Mn 2+ ) is relevant for charge balance requirement and in the transition from chromite to magnetite, due to the inversion of structure. Single crystal X-ray investigation of seven Cr-magnetites was undertaken to detail intracrystalline cation distributions and to compare them with that of the Cr-free magnetite (Cogne). The non-homogeneity of the chemical composition greatly affects the cell parameters, which do not show any definite trend, in spite of considerable Cr variation. M.a.n. (T) and m.a.n. ( M) are almost constant; the only parameter which substantially changes, decreasing Cr, is the oxygen coordinate, from 0.2574 to 0.2553. The bulk sample from which the seven crystals were selected was also examined by Mossbauer spectroscopy. Its spectrum is typical of magnetite with iron partially replaced by other divalent and trivalent cations, and consists of two partially overlapping sextets. Cr is replaced by both Fe 3+ and Fe 2+ in the M site, always with a definite disorder of Fe 2+ in T site. This feature was also observed in the Cogne Cr-free magnetite. All samples retain an average formal iron valence near 2.5 + on the M site, suggesting a structure-stabilising role due to charge hopping. Data suggest that the Cogne magnetite may represent the last stage of the chromite to magnetite transformation, with the sole survival of the inverse spinel structure. Samples from all the other deposits, retaining appreciable amounts of relict Cr-enriched areas, represent a transitional situation, where both normal and inverse structures still coexist. The inhomogeneous chemical composition revealed at the micrometer scale seems to be the consequence of a complex transformation responsible for the survival of microsites the compositions of which recall, but do not fit, those of a chromite proto-ore. Key words : chromite, magnetite, mineral chemistry, X-ray structure refinement, cation distribution.

Mössbauer Spectral Studies of Natural Substituted Spinels

Oxide minerals are widespread in a large variety of terrestrial and lunar rocks, and in meteorites.1 They generally occur as minor components, that is, only as a few percent of the rock weight, but they may also be concentrated to form large ore deposits in the form of hematite, chromite, magnetite, and corundum. They are usually grouped according to structural symmetry as reported in Table 1. Modifications of coexisting oxides following changes in temperature, pressure, and oxygen fugacity, fO2, during rock evolution have a direct influence on the magnetic properties of the earth. A fundamental parameter controlling their distribution, abundance, and composition is the level of initial fO2 and its variations upon cooling. For instance, at magmatic temperatures, for high fO2 values, iron oxides crystallize in preference to iron-rich silicates, whereas, with decreasing fO2, iron is partitioned between oxide and silicate phases. A full comprehension of these phenomena is still unavailable, because of the difficulties arising from the large number of phases, solid, liquid, and gaseous, usually occurring in natural systems at various stages of their evolution.