LOW ORTHOPYROXENE FROM A LUNAR DEEP CRUSTAL ROCK: A NEW PYROXENE POLYMORPH OF SPACE GROUP P21ca

Bronzite crystals (En86 FsllWo3) from a slowly-cooled lunar troctolitic granulite, have space group P21ca, a postulated, but previously unreported space group. Diffractions violating the b-glide extinction conditions have been observed in long-exposure X-ray precession photographs from three of these crystals and on an automated X-ray diffractometer. P21ca is a subgroup of the common orthopyroxene space group Pbca and cell dimensions; a=18.235 +/- 0.004 angstrom b=8.831 +/- 0.002 angstrom c=5.189 +/- 0.001 angstrom are similar to those of terrestrial bronzites. It is postulated that the lower symmetry space group has developed as a result of very slow cooling at pressures of one to two kilobars deep in the lunar crust.