Three natural orthopyroxene single crystals, measured in the laboratory over the temperature range 850 ø-1200øC, are more than } order of magnitude more electrically conducting than previously measured crystals. Small concentrations (1-2%) of AI:O8 and Cr:O8 present in these crystals may be responsible for their relatively high conductivity. Such pyroxenes, which contain trivalent elements, are more representative of pyroxenes expected to be present in the lunar mantle than those which have been measured by other investigators. The new conductivity values for pyroxene are responsible for a relatively large bulk conductivity calculated for (polymineralic) lunar mantle assemblages. The results permit a somewhat cooler lunar temperature profile than previously proposed. Such lower profiles, several hundred degrees CelsiUs below the solidus, are quite consistent with low seismic attenuation and deep moonquakes observed in the lunar mantle. Egorov, 1977] in the earth and moon. Conclusions based on these measurements are uncertain at best, as there are at least five sources or classes of possible error, some of which have not received adequate attention. These include (1) selection of a suitable mineral composition for measurement, (2) the actual electrical conductivity measurement of a niques. Unit cell dimensions are included in Table 1. Each crystal is of a single phase, free of exsolution lamellae, and chemically homogeneous (Figure 1). George Rossman of the California Institute of Technology kindly determined by optical absorption spectroscopy that the chromium in crystal MF-I is trivalent. Electron microscopy reveals that the crystals are mineral, performed in the laboratory, (3) selection of the min-remarkably devoid of defects and contain no Gunier-Preston eral assemblage which represents the planetary interior of zones (G. L. Nord, personal communication, 1978). interest, (4) the effects of fabric, grain boundaries, intra-Conductivity was independent of frequency(between 50 Hz
1975; Vanyan and
As a model of an internal displacement reaction involving a ternary oxide "line" compound, the following reaction was studied at 1273 K as a function of time, t:Both polycrystalline and single-crystal materials were used as the starting NiTiO 3 oxide. During the reaction, the Ni in the oxide compound is displaced by Fe and it precipitates as a ␥-(Ni-Fe) alloy. The reaction preserves the starting ilmenite structure. The product oxide has a constant Ti concentration across the reaction zone, with variation in the concentration of Fe and Ni, consistent with ilmenite composition. In the case of single-crystal NiTiO 3 as the starting oxide, the ␥ alloy has a "layered" structure and the layer separation is suggestive of Liesegang-type precipitation. In the case of polycrystalline NiTiO 3 as the starting oxide, the alloy precipitates mainly along grain boundaries, with some particles inside the grains. A concentration gradient exists in the alloy across the reaction zone and the composition is Ͼ95 at. pct Ni at the reaction front. The parabolic rate constant for the reaction is k p ϭ 1.3 ϫ 10 Ϫ12 m 2 s Ϫ1 and is nearly the same for both single-crystal and polycrystalline oxides. Fe ϩ NiTiO 3 ϭ "Ni" ϩ "FeTiO 3 "
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