Sheet conductivity was found in oxygen reduced . The sheets are aligned along the twin boundaries of the unreduced starting material . The bulk transforms during the oxygen loss to a phase with tetragonal crystal structure. The space group is with lattice parameters a = 0.739 nm and c = 0.388 nm. The perovskite-like structure contains distorted octahedra with W - O distances between 0.17 nm and 0.218 nm.

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Abstract-The iron-magnesium exchange between MI and M2 sites in orthopyroxene is a reversible reaction that records the latest event in the thermal evolution of the host rock. A kinetic analysis of this process has been applied to 16 orthopyroxene single crystals from 7 different diogenites to constrain the cooling history of their parent body. The Fe2+-Mg ordering degrees have been determined by single-crystal x-ray difiction. The Fe2+-Mg ordering closure temperatures were very homogeneous within each sample and ranged, for all diogenites studied, between 31 1 2 29 "C and 408 2 10 "C. Cooling rates at these closure temperatures were calculated using a numerical method developed by Ganguly (1982). These ranged between -5 "C/104 year in Johnstown and -0.8 "C/year in Roda.A comparison with other achondrites studied with the same method showed that increasing closure temperatures correspond to increasing cooling rates and that meteorites from a same parent body exhibit similar closure temperature and cooling rate values.The cooling rates obtained for these diogenites, at their low closure temperatures, should probably be ascribed to a complex thermal history of their parent body, thus confirming Miyamoto and Takeda's theory (1 994a) of excavation of deep crustal material due to impact events. The differences on cooling rate values for different diogenites could be due to different burial depths in the fragment ejected from the parent body.

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