Crystallographic procedures in the study of experimental rocks: x-ray single-crystal structure refinement of C2/c clinopyroxene from lunar 74275 high-pressure experimental basalt (original) (raw)

Summary

The possibility of applying X-ray single crystal techniques to minerals pertaining to rocks experimentally crystallized in laboratory has been investigated as a new approach to precisely relate crystal-chemistry to pressure, temperature, fO2, and composition of the “magma” from which the crystal formed.

A clinopyroxene from the experimental lunar basalt (74275 composition) of_Green_ et al. (1975) crystallized at 12 Kb and 1320 °C, was successfully refined to an R factor of 0.041. Its structure configuration turned out to be exceptional, expecially in relation to the record low Ca(M2) occupancy (0.57 atoms per formula unit henceforth a.f.u.), but comparable with the other lunar C2/c clinopyroxene previously studied with X-ray single crystal methodology: augite 12052 (Takeda 1972 a). Comparative analysis of the two lunar clinopyroxene structures revealed their common high temperature origin and the high-pressure imprint of the experimental specimen relative to the natural one.

Both lunar samples differ radically from the studied terrestrial clinopyroxenes of C2/c diopside-like structure: the main difference of lunar clinopyroxenes with respect to terrestrial analogues is the high Ally (T) (AlIV > 0.20 a.f.u.) in spite of low Ca(M2). Relative to the ideal diopside structure, Si4+ → AlIV and Ca → Mg + Fe2+ substitutions in T and M2 sites, respectively, turned out to be not compatible with terrestrial clinopyroxenes.

Zusammenfassung

Die Anwendung von Einkristallverfahren auf Minerale aus experimentell hergestellten Gesteinen wurde untersucht; sie wird als ein neuer Weg zur quantitativen Korrelation der Kristallchemie mit Druck, Temperatur, fO2 und der Zusammensetzung des „Magmas”, aus dem der Kristall gebildet wurde, gesehen.

Die Struktur eines Klinopyroxens aus dem experimentellen Mond-Basalt (Zusammensetzung 74275) von_Green_ et al. (1975), der bei 1320 °C kristallisierte, wurde mit Erfolg bis auf einen R-Faktor von 0.041 verfeinert. Es zeigte sich, daß seine Struktur-Konfiguration außergewöhnlich war, besonders im Hinblick auf die besonders niedrige Ca(M2) Besetzungsdichte (0.57 Atome pro Formeleinheit). Sie ist jedoch vergleichbar mit dem anderen lunaren C2/c Klinopyroxen der bisher mit Röntgen-Einkristallverfahren untersucht wurde, nämlich Augit 12052) (Takeda, 1972a). Vergleichsanalysen der beiden lunaren Klinopyroxen-Strukturen lassen ihre gemeinsame Hochtemperatur-Entstehung erkennen, sowie den — im Gegensatz zu der natürlichen Probe — deutlichen Einfluß hoher Drucke auf die experimentell hergestellte Probe.

Beide lunare Proben unterscheiden sich deutlich von den untersuchten terrestrischen Klinopyroxenen mit Diopsid-artiger C2/c Struktur: der Hauptunterschied zwischen lunaren Klinopyroxenen und ihren terrestrischen Analogen ist der hohe AlIV (T) (AlIV > 0.20 a.f.u.) trotz niedrigem Ca(M2). Im Vergleich mit der idealen Diopsidstruktur zeigte sich daß, Si4+ → AlIV und Ca → Mg + Fe2+ Substitutionen an T resp. M2 Plätzen nicht kompatibel mit terrestrischen Klinopyroxenen sind.

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1. C.N.R. “Centro di studio per i problemi dell'Orogeno delle Alpi Orientali”, c/o Dipartimento di Mineralogia e Petrologia, Corso Garibaldi 37, I-351000, Padova, Italy
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Manoli, S., Molin, G.M. Crystallographic procedures in the study of experimental rocks: x-ray single-crystal structure refinement of C2/c clinopyroxene from lunar 74275 high-pressure experimental basalt.Mineralogy and Petrology 39, 187–200 (1988). https://doi.org/10.1007/BF01163034

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• Received: 09 May 1988
• Accepted: 16 August 1988
• Issue Date: December 1988
• DOI: https://doi.org/10.1007/BF01163034

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