The crystallographic polarity of the noncentrosymmetric material zincite (ZnO) has been determined by a rapid x-ray absorption edge method. The polarity results have been correlated to marked differences in etching behavior and crystal morphology in opposite polar directions of ZnO crystals. These differences are shown to be consistent with the proposed surface bonding model for AII-Bv! compounds. Crystal morphology is shown to be a useful criterion to indicate crystallographic polarity in well-formed wurtzite-type crystals.

A new polymorph of carbon, hexagonal diamond, has been discovered in the Canyon Diablo and Goalpara meteorites. This phase had been synthesized recently under specific high-pressure conditions in the laboratory. Our results: provide strong evidence that diamonds found in these meteorites were produced by intense shock pressures acting on crystalline graphite inclusions present within the meteorite before impact, rather than by disintegration of larger, statically grown diamonds, as some theories propose.

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The key features of the complete, stable nickel—carbon phase diagram have been established at pressures up to 54 kbar, and these features have been directly related to the nucleation and growth characteristics of diamonds in this system. At pressures in excess of 52.5 kbar, a liquid+graphite+diamond eutectic is formed at approximately 0.3 at. % nickel which results in an effective displacement of the diamond—graphite equilibrium line. This displacement is shown to be of considerable importance in the nucleation and growth of diamonds in this system. Under low and moderate driving forces, the rate-controlling mechanism in the growth on a speed diamond is the diffusion of carbon across the molten metal film surrounding the diamond. The observed diffusion coefficients at 54 kbar and 1660°<T<1800°K lie in the range: 2×10−5<D<4×10−5 cm2/sec.

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