Orthopyroxene 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 augite, 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 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 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 calculated 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(obs), were 2.11 and 1.78% for two crystals with different augite contents. This refinement procedure confirms Pbca as the correct space group and provides more accurate structural parameters for the Serra de Mage orthopyroxene.