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Up to 17 GPa, the crystalline phases of N2 are characterized by pronounced orientational disorder, whereas the higher-pressure phases of molecular N2 are ordered. This raises the question about long-term relaxation of orientational disorder within the low- to intermediate-pressure regime. Here, this question is addressed by comparing synthetic with natural, chemically pure, solid N2 that resides as inclusions in diamonds at 300 K for about 108 years at pressures up to 11 GPa. It is shown that disorder prevails at 8.7 GPa, 300 K, where both synthetic and natural N2 assume the same structure. However, at 10.8 GPa, natural solid N2 exhibits monoclinic distortion and partial orientational ordering of the molecules, both of which are not observed in synthetic material. This difference is interpreted as the result of long-term structural relaxation. The ordering mechanism is examined and placed into the context of the δ- to ε-N2 transition. We present explanations for the absence of complete ordering of δ-N2. [ABSTRACT FROM AUTHOR]

Fluid-bearing microinclusions in diamonds ( Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely. Keywords: Crystal structure, high-density fluid, high-silica mica, carbonate, halide, secondary minerals, multi-phase assemblage

Nielson and Wilshire (1993) promised a review of geochemically based models for magma transport and metasomatism in the mantle. However, they focused on discrediting the chromatographic model of Navon and Stolper (1987). Nielson and Wilshire concluded that the process operating during melt percolation in the mantle 'resembles ion-exchange chromatography for [H.sub.2]O purification, rather than the model of chromatographic species separation proposed by Navon and Stolper (1987).' Our objective here is to show that Nielson and Wilshire (1993) did not offer any new theoretical or modeling advances. We demonstrate that the 'water-purification model' (based on the finite-plate model of Helfferich 1962) is controlled by the same physics and mathematics as the frontal chromatography model discussed by Navon and Stolper (1987). Thus, the distinction made by Nielson and Wilshire between the two models has no basis. Moreover, we show that with appropriate boundary conditions, the model of Navon and Stolper (1987) produces an excellent fit to the data that Nielson et al. (1993) fitted using the 'water-purification model.' We emphasize that the model of Navon and Stolper can be used to model both small- and large-scale processes and that it allows the recovery of temporal information, which cannot be retrieved using the finite-plate model. The usefulness of these models for understanding the geochemistry of mantle rocks is a subject of active research and beyond the scope of this discussion.

The discovery of solid CO2 in natural diamond gives rise to speculations about the origin of the trapping of the CO2 molecule in the diamond crystal. The likelihood of the reaction of CO2, present in the form of free stable phase at pressures exceeding 5 GPa, with the upper-mantle rocks at these pressures is analyzed. CO2's presence in submicroscopic inclusions is indicated by the resemblance between the spectra of colored zones of diamonds and the spectrum of solid CO2, which is illustrative of the occurrence of a distinct group of CO2 molecules. The prevalence of fluids at depths of 220-270 km and the subsequent potential of diamond in bearing these fluids in their crystal lattice structure, which facilitates the transfer of these fluids to the surface of the earth, is demonstrated.

The article discusses the precipitation of diamonds from high-density carbon-hydrogen-oxygen (CHO) fluids. It mentions the emergence of a precise scenario on the chemical and mineralogical composition of diamond substrates. Also outlined are the appoaches in constraining the diamond-forming processes including the use of model calculations to yield the carbon content and speciation of the CHO fluids and the study on the presence of some fluid inclusions in diamonds like the hydrous saline.

Presents a letter to the editor and the authors' response concerning the article "Geology and Geochronology of the Amram Massif, Southern Negev Desert, Israel," which appeared in a 1999 issue of the "Israel Journal of Earth Sciences."