A. Della Giusta

Comparative Crystal Chemistry of Spinels from Some Suites of Ultramafic Rocks

SummaryMicroprobe analyses and X-ray crystal structure refinement data of Mg-Fe-Al-Cr spinels from different environments are compared. The investigated crystals represent both restitic and recrystallized spinels from an Alpine peridotitic massif, and restitic spinels from four suites of ultramafic xenoliths. Within each suite the crystals represent different steps of an increasing melting process, which causes a strong increase in Cr3+ and a moderate increase in Fe2+, with depletion of Al3+ and Mg2+. Within each suite, in spite of relevant bulk chemistry changes, the ratio of the octahedral to tetrahedral coordination distances, and consequently the oxygen positional parameterx, are kept constant. Conversely,x may differ in suites with similar bulk chemistry, mainly due to different Mg-Al ordering between octahedral and tetrahedral sites. This suggests that thex parameter is strongly affected by physical environment, and that consequently, within the range of the investigated compositions, it could be used as a marker.ZusammenfassungEMS-Analyse und röntgenograhsch verfeinerte Kristallstrukturen von Mg-Fe-Al-Cr Spinellen aus verschiedenen petrologischen Situationen wurden verglichen. Unter den untersuchten Kristallen befinden sich sowohl restitische und rekristallisierte Spinelle aus alpinen Peridotiten, als auch restitische Spinelle aus vier Suiten von ultramafischen Xenolithen. Die Kristalle in jeder Suite gehören zu verschiedenen Stufen eines Schmelzprozesses, der ein starkes Anwachsen der Cr3+ - und ein mäßiges Anwachsen der Fe2+- Konzentration bei gleichzeitiger Verminderung des Al3+ - und Mg2+-Gehaltes, verursacht. Jede Suite zeigt, trotz bedeutender chemischer Veränderungen, ein feststehendes Verhältnis von oktaedrischen zu tetraedrischen Polyederdimensionen, weshalb der Sauerstoffparameter x unveränderlich bleibt. Jedoch sehen wir, daß sich diex von den Spinellen zweier verschiedener Suiten mit ähnlichem Chernismus unterscheiden, was auf die unterschiedliche Mg-Al- Anordnung in oktaedrischen und tetraedrischen Positionen zurückzuführen ist. Dies läßt vermuten, daß der Parameterx sehr stark von der petrogenetischen Situation abhängig ist, was bedeutet, daß er als Anhaltspunkt zum Verständnis der physikalischen Entstehungsbedingungen verwendet werden kann.

Temperature, composition, and fO2 effects on intersite distribution of Mg and Fe2+ in olivines

Single-crystal X-ray structure refinement of natural olivines equilibrated at high temperature under controlled oxygen fugacity (fO2) conditions, coupled with a structure-energy model were used to establish the influence of T, fO2 and bulk chemistry on intracrystalline disorder.The results are: 1) The kD (kD = [(Fe M1·Mg M2)/(Fe M2·Mg M1)]) factor describing site population on M1, M2 polyhedra increases from values lower than 1 at T below 400–600° C (depending on composition) to values higher than 1 at higher temperature. 2) The increase of kD with T is quite regular. 3) At constant temperature and pressure, kD increases with increasing fayalite content in the mixture; 4) Contrary to previous observations (Will and Nover 1979; Nover and Will 1981) varying fO2, within the stability range of the substance, has a negligible influence on intracrystalline disorder. 5) As ancillary results, the model confirms the defect scheme of Nakamura and Schmalzried (1983) for the investigated solids. Moreover, it shows that cationic vacancies are always created on M 1 site at the expense of Mg ions, while trivalent iron is always stabilized on M2 site. This explains the marked anisotropies observed in Fe-Mg interdiffusion (Buening and Busek 1973; Misener 1974; Schock et al. 1989).

Cation distribution in natural Zn-aluminate spinels

Abstract The intracrystalline cation distributions in fourteen natural Zn-aluminate spinels were determined by means of X-ray single-crystal structural refinement, supported for some samples by Mössbauer 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 Fe3+ ⇌ Al. In agreement with data from synthetic samples, a small but definite amount of Zn (up to 0.06 apfu) is located in the octahedral M site. Fe2+, when present, shows a preference for tetrahedral coordination. An improved value of the tetrahedral Zn(T)-O distance (1.960 Å) was obtained, integrating the set of interatomic distances used for the determination of cation distribution in spinels.

Crystal chemistry of a highly disordered Mg-Al natural spinel

SummaryA Mg-Al spinel from a xenolith found in the Sacrofano volcanic area (Latium, Italy) was investigated by X-ray single crystal diffraction and electron microprobe. Its main structural parameters were: a0 = 8.0922(3) Å and the oxygen coordinate u = 0.26213(4), which gave to tetrahedral and octahedral bond distances of 1.9220(6) Å and 1.9299(3) Å respectively. Cation distribution, obtained from structural and chemical data, showed a very high Mg-Al disorder, with an Al(T) content of 0.205 and an inversion parameter, i, of about 0.23. Comparison with the cation distribution of a spinel with similar chemical composition but occurring in chlorite schists (Shishimsk Mountains, Urals, Russia) with a0 = 8.1003(3) Å and u=0.26330(3), and with data from the literature concerning spinels from metamorphosed limestones (Kalkbro, Sweden) and websteritic dikes (Balmuccia, Italy), all characterized by much lower Mg-Al disorder (i about 0.13–0.12), suggests a completely different thermal history, with rapid cooling rates and consequent high closure temperature of intracrystalline exchange, consistent with the Sacrofano geological environment.ZusammenfassungEin Mg-Al-Spinell aus einem Xenolith des Vulkangebiets von Sacrifano (Latium, Italien) wurde mit Röngten-Einkristallmethoden und der Elektronenmikrosonde untersucht. Die wichtigsten Strukturparameter sind ao = 8,0922(3) Å und die Sauerstoffkoordinate u = 0,26213(4), was zu den tetraedrischen und oktaedrischen Bindungsabständen von 1,9220(6) Å bzw. 1,9299(3) Å führt. Die aus den strukturellen und chemischen Daten erhaltene Kationenverteilung zeigt eine sehr starke Unordnung Mg-Al mit einem Al (T)-Gehalt von 0,205 und einem Inversionsparameteri von ca. 0,23. Der Vergleich mit der Kationenverteilung eines Spinells ähnlicher chemischer Zusammensetzung, aber aus Chloritschiefern (Schischimsk-Berge, Ural, Rußland) mit a0 = 8,1003(3) Å und u=0,26330(3), sowie mit Literaturdaten über Spinelle aus metamorphen Kalken (Kalkbro, Schweden) und websteritischen Gängen (Balmuccia, Italien), die alle durch eine viel geringere Mg-Al-Unordnung charakterisiert sind (i etwa 0,13–0,12), weist auf eine völlig andere thermische Geschichte hin, und zwar mit rascher Abkühlungsgeschwindigkeit und hoher Schließungstemperatur des intrakristallinen Austausches, was mit den geologischen Verhältnissen von Sacrofino in Übereinstimmung steht.

Crystal structure and cation distribution in some natural magnetites

SummaryThe crystal structures of two natural magnetites were refined. Both turned out to have equipoints 8a and 16d (Fd3m) fully occupied and hence different from type I and type II defect structures investigated by Fleet (1981, 1982). It was possible to improve the cation radii for Fe2+ ad Fe3+ in octahedral and tetrahedral coordinations for pure and almost pure magnetites obtaining very good agreement between observed and calculated values of the two independent geometric structure parameters, i.e. cell edge and oxygen coordinate. The present results lead to an estimate of inversion parameter i in (Fe1−i2+ Fei3+)(Fei2+ Fe2t-i3+)O4, of about 0.90, equal for all the four pure magnetites, independently of type of structure and also of cooling history.ZusammenfassungDie Kristallstrukturen von zwei natürlichen Magnetiten wurden verfeinert. Es stellte sich heraus, daß ihre Gitterplätze 8 a und 16d (Fd 3 m) voll besetzt sind, und sich somit von den von Fleet (1981, 1982) bestimmten Defektstrukturen unterscheiden. Die Kationradien für Fe2+ und Fe3+ in Oktaeder- und Tetraeder-Koordinationen wurden für reine und fast reine Magnetite verbessert. Dabei wurde eine gute Korrelation zwischen beobachteten und berechneten Werten der beiden unabhängigen Parameter in der Struktur, d.h. Gitterkonstante und Sauerstoff-Ortsparameter gefunden. Die vorliegenden Ergebnisse bringen einen Umkehrparameter i in (Fe1−i2+ Fei3+)(Fei2+ Fe2−i3+)O4 von ungefähr 0.90, der in allen reinen Magnetiten gleich und unabhängig von dem Strukturtyp sowie dem Abkühlverhalten ist.

Intracrystalline relationships in olivine, orthopyroxene, clinopyroxene and spinel from a suite of spinel lherzolite xenoliths from Mt. Noorat, Victoria, Australia.

A detailed crystal chemical study of coexisting olivine, orthopyroxene, clinopyroxene and spinel from selected Victorian (Australia) lherzolite suites was carried out by means of single crystal x-ray diffraction and electron probe microanalysis to obtain actual site occupancies. The aim of this study was primarily to characterise the intracrystalline configurations and related cation ordering on sites in major mantle constituents. The results demonstrate that cation ordering on sites is subject to distinctive crystallographic controls which depend on the petrological evolution of the suite. Mg-Fe2+ ordering in M1–M2 pyroxene sites depends on variations of the smaller cations, mainly Alvi, Ti4+, Fe3+, and related configurations of M 1. Pressuresensitive Alvi is crucial to Fe2+, the more ordered clinopyroxene showing high Alvi configurations which tend to exclude the larger bivalent cations and yield small polyhedral volumes for M 1, M 2, T sites and the unit cell. Conversely, the coexisting orthopyroxene, characterised by lower Alvi configuration and higher M 1 and unit cell volumes, is relatively more disordered. Olivine is consistent with the coexisting clinopyroxene, the more disordered crystals coexisting with more disordered clinopyroxene, while Al-Mg order in the coexisting spinel shows the reverse relationship. Estimated temperatures of apparent equilibration based on current geothermometers are not considered realistic. Assumptions of ideal cation mixing on sites in pyroxene and spinel are not supported.

A Structure Energy Model for C2/c Pyroxenes in the System Na-Mg-Ca-Mn-Fe-Al-Cr-Ti-Si-O

Pyroxenes, like feldspars, occupy a position that is “chemically central to the composition realm of rocks” (Robinson, 1980) and occur ubiquitously in most part of igneous and metamorphic terrains. Understanding their crystal-chemical and thermodynamic properties is thus of primary importance in the earth sciences. Due to their importance in earth sciences, pyroxenes have been the object of various petrologic and thermodynamic investigations. Most published experimental data concern the pyroxene quadrilateral and have been restricted to the binary joins. The work of modeling multicomponent pyroxene mixtures using the binary solution data has begun only recently, and we wish to contribute to its development by presenting a series of structure-energy calculation procedures for the various phases of interest. This first work concerns the structural class C2/c that is the most representative of pyroxenes in nature and for which most crystal-chemical and thermodynamic data are available.

The magnetite ore districts of the southern Aosta Valley (Western Alps, Italy): a mineralogical study of metasomatized chromite ore

Abstract In the southern Aosta Valley (Italian Western Alps), several massive magnetite bodies occur within serpentinized ultramafic rocks belonging to the Mesozoic meta-ophiolite nappe. The ultramafic rocks consist of lherzolite with minor dunite bodies and show a high pressure metamorphic overprint. The results of a multi-methodological study, based on optical microscopy, electron microprobe analysis and single-crystal X-ray diffraction, are reported here in order to give new insights into (1) the mineralogy and crystal chemistry of spinels and silicates and (2) the genesis of the massive magnetite bodies. Chromium-rich relict cores inside the magnetite grains suggest a derivation from primary chromite concentrations. The major-element behaviour shows the presence of two chromite types: a Cr2O3-rich (Al2O3-poor) type and a Cr2O3-poor (Al2O3-rich) type. Magnetite ore deposits probably represent the product of transformation from a chromite proto-ore which formed in ultramafic rocks pertaining to an ophiolite suite. The transformation of chromite to magnetite occurred during multiple stages: the premetamorphic setting of the ultramafics and the petrographic evidence suggest that metasomatism started before the onset of the alpine metamorphism and was active during the early alpine, eclogite-facies metamorphic overprint related to a subduction process under high fluid activity.

Influence of the Jahn-Teller effect on the linewidth of the Mössbauer spectra of natural spinels

The temperature dependence of the Mossbauer doublet linewidth of tetrahedrally coordinated Fe2+ ions, distorted by the Jahn-Teller effect, was analysed by means of a new mathematical approach. The idea was that the linewidth depends on a distribution of quadrupole splittings that are in turn dependent on temperature. The variation of the quadrupole splitting was supposed to be connected with the temperature dependent population of the ground state Eg orbitals, split by the local Jahn-Teller effect. The model was applied to three sets of experimental data. The barrier splitting between the two Eg electronic states as well as their widths, , were obtained.