Summary
The 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.
Zusammenfassung
Die 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.
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Carbonin, S., Salviulol, G., Munno, R. et al. Crystal-chemical examination of natural diopsides: Some geometrical indications of Si-Ti tetrahedral substitution. Mineralogy and Petrology 41, 1–10 (1989). https://doi.org/10.1007/BF01164806
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DOI: https://doi.org/10.1007/BF01164806