Susanna Carbonin

Intracrystalline Cation Distribution in Natural Clinopyroxenes of Tholeiitic, Transitional, and Alkaline Basaltic Rocks

Thermodynamic studies of Mueller (1962) and Thompson (1969) and the crystallographic work of Ghose (1965) gave new impetus to a rapidly developing branch of mineralogy. Orthopyroxenes were studied by Virgo and Hafner (1969) and Saxena and Ghose (1971) using Mossbauer technique. Site-occupancy data have been used in many mineralogical and petrological problems, e.g., to estimate the thermodynamic solution properties of pyroxenes and to understand the cooling history of rocks. A parallel development in the mineralogy of clinopyroxene did not take place, particularly because the Mossbauer technique proved unsatisfactory for calcic pyroxenes. Recognizing this, we undertook a detailed X-ray crystallographic study of the clinopyroxenes and succeeded to determine crystal-structural parameters for Fe-Mg site occupancies of the nonequivalent Ml and M2 sites. Until now there are no published site-occupancy data on a series of clinopyroxene of intermediate composition. This paper attempts to fill this important gap in the data on clinopyroxenes. Besides our concern for the crystal-chemical and thermodynamic aspects of such a study, we were also motivated by the possible use of pyroxene crystallographic parameters in the classification and characterization of magmatic rocks. With the advent of automation in X-ray crystallography, it is possible to study a number of crystals from one or more rock samples in a relatively short time and provide petrologically useful information.

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.

Crystal structures of Ca-rich clinopyroxenes on the CaMgSi2O6-Mg2Si2O6 join

The structural changes occurring in the clinopyroxenes with composition Di100, Di90En10 and Di80En20, due to the Ca-Mg substitution in the M2 site, have been studied. Evidence is given that with increasing Mg content a small percentage of the atoms converts from the M2 position to a new M2′ position which is solely occupied by Mg. The maximum conversion of M2 to M2′ found in this study is 7%. The closest parallel to the M2′ geometry is found in the ZnSiO3 pyroxene (C2/c). The presence of this new site causes significant changes in the tetrahedral configuration, because the M2′ atoms are not bonded to 03. The intermediate compositions, Di90En10 and Di80En20, may be thought of as the coexistence of two structural models: diopside and ZnSiO3 pyroxene (C2/c).

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.

Crystal chemistry and evolution of the clinopyroxene in a suite of high pressure ultramafic nodules from the Newer Volcanics of Victoria, Australia

A clinopyroxene suite from lherzolite inclusions associated with the Victorian (Australia) “Newer Volcanics” has been investigated with the aim of understanding the clinopyroxene crystal-chemical response to increasing temperature (e.g. a melting model and/or crystallization processes prevailing at high pressure).The M1 clinopyroxene polyhedron dominates the intracrystalline physical-chemical variations, essentially given by the triple substitution AlVIFeM12Ti4+⇌Cr3+ Fe3+MgM12+corresponding to an increase in the volume of M1 with increasing Mg/Mg+Fe2+ (mg) for the clinopyroxene. A relative Ca2+ increase in M2 ensures the necessary charge balance. However, Na+ occupancy of M2 persists to the highest mg values, i.e. maximum thermal stability, where the volume of M2 is the largest due to FeM22+depletion. The variations of M1 and M2 volumes are greater than, and opposite to, the variations in the volume of T (tetrahedron) by factors of ca. 3 and 1.5, respectively. Inclusions with relatively low clinopyroxene content (Mt. Porndon specimens) show distinct intracrystalline variations, essentially reflecting lower AlVI, i.e. higher volume of M1, and implying a lower pressure regime compared to clinopyroxene-rich analogues (Mt. Leura specimens). The intracrystalline relationships of the Mt. Porndon clinopyroxene suggest that the host peridotite inclusions survived larger degree of mantle melting at shallower depths relative to the Leura analogues.

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.

Crystal chemistry of Ca-rich pyroxenes from undersaturated to oversaturated trachytic rocks, and their relationships with pyroxenes from basalts

Abstract A study of 23 Ca-rich pyroxenes from silica-undersaturated and -oversaturated rocks, shows that all the pyroxenes from trachytes are characterized by a relatively large M1 site, related to its high Fe 2+ occupancy. The volume of the M1 site is inversely related to that of the tetrahedral site. The high Si content of the pyroxene from trachytes causes significant increase in the longest M2-O3C2,D2 distances. The most striking difference between the pyroxenes from trachytes and those from basalts is found in the volumes of the M1 and T sites, and in the degree of distortion of the M2 polyhedron (ΔM2). The relationship between the β angle and ΔM2 provides an estimate of: (1) the degree of alkalinity in trachytic melts, and (2) indications on the nature of their possible parental “basaltic” magmas.

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.

Cation and vacancy distribution in a synthetic defect spinel

Crystal structure refinement was carried out on a defect spinel with composition Mg0.388Al2.408O4 synthesized at 2050°C. Combining information from site occupancies and bond distances for tetrahedral and octahedral sites, the following cation and vacancy distribution were obtained: (Al0.653Mg0.347)(Al,.755Mg0.04iD0.204)O4. Structural analysis performed after heating the same crystal at 1150°C led to the same result, suggesting that the obtained distribution is stable over a wide range of temperatures.

Crystal-chemical examination of natural diopsides: Some geometrical indications of Si-Ti tetrahedral substitution

SummaryThe crystal structures of 20 natural diopsides have been refined. For some samples, microprobe analyses show a Si deficiency in the tetrahedron with no A1 present; this suggests that some Ti4+ may enter the tetrahedral site. Geometrical variations in these samples with respect to the others are discussed in the light of this Si → Ti4+ substitution. The volumes of their M1 sites are larger because of the absence of R3+ cations, the octahedron being very regular; in the same way, the volumes of M2 sites of these samples are greater not because of different site occupancies but because of significant charge modifications around the O3 and O1 oxygens, caused by the tetrahedral site. The low distortion of M1 site and the lengthening of (M2-O1) and 〈M2-O3〉 bond lengths are a clue to the presence of small amounts of Ti at the tetrahedral site.ZusammenfassungDie Kristallstrukturen von zwanzig natürlichen Diopsiden sind verfeinert worden. In einigen Proben zeigen Mikrosondenanalysen zu geringe Si-Werte im Tetraeder ohne daß A1 anwesend wäre. Dies weist darauf hin, daß Ti4+ Tetraederplätze einnehmen könnte. Geometrische Variationen in diesen Proben werden im Lichte der genannten Substitution diskutiert. Die Volumen von M1-Plätzen sind größer wegen der Abwesenheit von R3+ Kationen, wobei das Oktaeder sehr gleichmäßig ist. Die Volumina von M2-Plätzen sind noch größer nicht wegen verschiedener Besetzungen dieser Plätze sondern wegen signifikanter Ladungsmodifikationen im Bereich von O3 und O1 Sauerstoffen, die durch die tetraedrische Position bewirkt wird. Die geringe Distortion der M1-Plätze und die Verlängerung der (M2-O1) und (M2-O3) Abstände weisen auf die Anwesenheit von geringen Titangehalten auf Tetraederplätzen hin.