Physics
Scientific paper
Dec 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990pcm....17..438s&link_type=abstract
Physics and Chemistry of Minerals, Volume 17, Issue 5, pp.438-443
Physics
2
Scientific paper
The space group of an orthopyroxene (En86) from a deep crustal lunar rock (sample 76535) that was previously reported as having space group P21 ca has been re-examined on an automated X-ray diffractometer. In addition to diffractions violating the b-glide of the conventional space group, Pbca (0kl,k-odd) reported in the earlier study, diffractions violating the a-glide of Pbca are also present. Careful examination of both the a-glide- and b-glide-violations shows them to be sharp, with no evidence of diffuse streaks parallel to a *, and with consistent intensities at several rotations about ψ. Diffractions violating the b-glide are in registry with the host, however, those violating the a-glide appear to be out of registry and result from a cell with a slightly longer a of about 18.4 Å, consistent with previous electron diffraction studies. The most reasonable explanation for the observed space group violations is that both the a- and b-glide violations result from ordering of Ca into (100) Guinier-Preston (G-P) zones that possess orthopyroxene topology, but have space group P21/c and a cell of a=18.4 Å, b=8.83 Å, c=5.18 Å, and β=90.0°; whereas the Cadepleted host has space group Pbca and a cell of a= 18.230(6) Å, b = 8.828(2) Å, and c=5.1946(9) Å. In addition to the G-P zones which may compose 12% or more of the sample, the crystal contains (100) lamellae of pigeonite, and other samples from the same rock contain lamellae of augite.
Smyth Joseph R.
Swope Jeffrey R.
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