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An X-ray structure-refinement procedure was developed to characterize orthopyroxene from the Serra de Magé meteorite. In the studied sample, the orthorhombic phase coexists with exsolution lamellae of C2/c augite, parallel to (100), with a* Opx  = a* Aug , [101]* Opx  = c* Aug , b* Opx  = b* Aug . Diffraction maxima of the monoclinic phase overlap those of the ortho­rhombic phase with h + l = 2n, yielding violations of the extinction conditions for space group Pbca and simulating the lower symmetry space group P2 1 ca. Structural parameters of the Pbca phase together with the W cpx  parameter, which expresses the fraction of the C2/c phase present, were refined first. F c  values were calculated using the equationwith [F c ]C px  values taken from the structure refinement of an augite. Observed F 0  values were then corrected by subtracting the calculated contri­bution of the monoclinic phase with use of the equationand used for a further refinement of the orthorhombic phase. The final residual indices, R obs , were 2.11 and 1.78% for two crystals with different augite contents. This refinement pro­cedure confirms Pbca aS the corect space group and provides more accurate structural parameters for the Serra de Magé orthopyroxene.

Orthopyroxene from the Serra de Mage meteorite: A structure-refinement procedure for a Pbca phase coexisting with a C2/ c exsolved phase