Omphacite

Article By:

Holland, Timothy J. B. Department of Mineralogy and Petrology, University of Cambridge, Cambridge, United Kingdom.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.468800

The pale to bright green monoclinic pyroxene found in eclogites and related rocks. Omphacites are essentially members of the solid solution series between jadeite (NaAlSi₂O₆) and diopside (CaMgSi₂O₆), but contain smaller amounts of the components acmite (NaFe³⁺Si₂O₆), hedenbergite (CaFe²⁺Si₂O₆) and Tschermak's pyroxene (CaAlAlSiO₆). The density ranges from 3.16 to 3.43 g/cm³ (1.8–2.0 oz/in.³), hardness is 5–6, cleavage is well developed on {110} and simple or lamellar twinning on {100} is common. With four formula units to the unit cell, omphacite crystallizes in space groups Cs/c, as well as in the lower symmetry group P2/n. The possibility of space groups P2 and P2/c has also been suggested for some omphacites. C2/c structure is characteristic of omphacites crystallized above a P2/n-C2/c inversion temperature (just over 1300°F or 700°C), although there is some evidence for metastable growth at lower temperatures. In this structure (Ca,Na) and (Al,Mg) are more or less randomly distributed over M2 and M1 sites. See also: Diopside; Eclogite; Jadeite; Solid solution

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