ORTHO-PYROXENE FROM THE SERRA DE MAGE METEORITE - A STRUCTURE-REFINEMENT PROCEDURE FOR A PBCA PHASE COEXISTING WITH A C2 C EXSOLVED PHASE/

An X-ray structure-refinement procedure was developed to characterize orthopyroxene from the Serra de Mage meteorite. In the studied sample, the orthorhombic phase coexists with exsolution lamellae of C2/c augi te, parallel to (100), with a(Opx) = a(Aug)*, [101](Opt)* = c(Aug)*, b(Opx) = b(Aug)*. Diffraction maxima of the monoclinic phase overlap those of the orthorhombic phase with h + l = 2n, yielding violations o f the extinction conditions for space group Pbca and simulating the lo wer symmetry space group P2(1)ca. Structural parameters of the Pbca ph ase together with the W-Cpx parameter, which expresses the fraction of the C2/c phase present, were refined first. F-c values were calculate d using the equation F-c = root(1-W-Cpx)[F-c](2)(Opx) + W-Cpx[F-c](2)( Cpx), with [F-c](Cpx) values taken from the structure refinement of an augite. Observed F-o values were then corrected by subtracting the ca lculated contribution of the monoclinic phase with use of the equation F-o(corr) = root[F-o](2) - [F-c](2)(Cpx) W-Cpx and used for a further refinement of the orthorhombic phase. The final residual indices, R(o bs), were 2.11 and 1.78% for two crystals with different augite conten ts. This refinement procedure confirms Pbca as the correct space group and provides more accurate structural parameters for the Serra de Mag e orthopyroxene.