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The effect of pressure on chemical interactions in molecular mixtures is important for problems spanning fundamental chemistry, planetary science, and materials science. Diamond-anvil cell studies reveal pressure-induced chemistry in the CH4-H2 system. The system, which has no known compounds at ambient conditions, formed four molecular compounds, CH4(H2)2, (CH4)2H2, CH4(H2)4, and CH4H2, at pressures up to 10 gigapascals. These have been characterized by synchrotron single-crystal x-ray diffraction, polycrystalline x-ray diffraction, Raman spectroscopy, and visual observation. Although CH4(H2)2 crystallizes in the MgZn2-type, hexagonal Laves phase structure, (CH4)H2 has a body-centered tetragonal structure that is similar to that of Al2Cu. The 1:1 and 1:2 compounds are stable to at least 30 gigapascals.

Metallic filaments with submicrometer diametere have been fabricated. Standard diffraction techniques with conventional x-ray sources were unsuccessful in identifying the structure of these materials. However, with the use of synchrotron radiation produced on a wiggler beam line, diffraction data were obtained in measurement periods as short as 10 milliseconds. Two cylindrical single crystals of bismuth were studied, each with a diameter of 0.22 +/- 0.02 micrometer. The volume of sample illuminated for these measurements was 0.38 cubic micrometer, less than 0.5 femtoliter. The crystals are grown in glass capillaries, and, because bismuth expands on solidification, they are under a residual hoop stress. The crystallographic data indicate the presence of a linear compressive strain of about 2 percent, which is assumed to be the result of a residual stress of about 2 gigapascals.

A new mineral (approximately Fe(0.5)Mg(0.5)Ti(2)O(5)) related to the pseudobrookite series has been discovered in section 10071,28. Electron-probe analyses for this mineral, a coexisting ilmenite, and a chromian ulvöspinel-ilmenite assemblage in section 10020,40 indicate crystallization under highly reducing conditions. Analytical and optical absorption studies of the olivine in 10020 show it to contain unusually high Cr (1400 parts per million) probably as Cr(2+).

Transformations involving a change from tetrahedrally coordinated to octahedrally coordinated silicon ((IV)Si --(VI)Si) are observed to occur at high pressure when the mean (IV)Si-O bond compresses to approximately 1.59 angstroms based on known room-pressure crystal structures, Si-O bond compressibilities, and pressures of (IV)Si --(VI)Si transformations. The lower two-thirds of the mantle transition zone of high-density gradient (500 to 900 kilometers) corresponds to the predicted range of (IV)Si --(VI)Si transformations. The 10 percent density increase of this zone at zero pressure is attributed primarily to the density increase associated with the change in silicon coordination. Below 900 kilometers all silicon is predicted to be in octahedral or greater coordination. The concept of cation polyhedral stability fields is defined.

Petrogenetic hypotheses involving fractional crystallization, assimilation, or mixing of magmas may be expressed and tested as problems in leastsquares approximation. The calculation uses all of the data and yields a unique solution for each model, thus avoiding the ambiguity inherent in graphical or trial-and-error procedures. The compositional change in the 1960 lavas of Kilauea Volcano, Hawaii, is used to illustrate the method of calculation.