The strengths of MgO and NaCl polycrystals under confining pressures to 250 kbar have been determined in a diamond-anvil high-pressure cell. The stresses in the samples are inferred from the strains determined by x-ray diffraction. The strength of MgO polycrystals is found to rise from a 1-bar confining pressure value of about 4 kbar to a maximum of 30±10 kbar at a pressure of 50±20 kbar and to remain constant at that value to a pressure of 250 kbar. The strength of NaCl polycrystals is found to rise from a 1-bar confining pressure value of about 0.3 kbar to a maximum of 4.0±1.5 kbar at a confining pressure of 250 kbar. The interpretation of the form of strength versus pressure for these two compounds is discussed in terms of a brittle-ductile transition. Preliminary transmission electron microscope data are presented.

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Analyses of geomorphic, soil, and topographic data from the northern Yucatan Peninsula, Mexico, confirm that the buried Chicxulub impact crater has a distinct surface expression and that carbonate sedimentation throughout the Cenozoic has been influenced by the crater. Late Tertiary sedimentation was mostly restricted to the region within the buried crater, and a semicircular moat existed until at least Pliocene time. The topographic expression of the crater is a series of features concentric with the crater. The most prominent is an approximately 83-km-radius trough or moat containing sinkholes (the Cenote ring). Early Tertiary surfaces rise abruptly outside the moat and form a stepped topography with an outer trough and ridge crest at radii of approximately 103 and approximately 129 km, respectively. Two discontinuous troughs lie within the moat at radii of approximately 41 and approximately 62 km. The low ridge between the inner troughs corresponds to the buried peak ring. The moat corresponds to the outer edge of the crater floor demarcated by a major ring fault. The outer trough and the approximately 62-km-radius inner trough also mark buried ring faults. The ridge crest corresponds to the topographic rim of the crater as modified by postimpact processes. These interpretations support previous findings that the principal impact basin has a diameter of approximately 180 km, but concentric, low-relief slumping extends well beyond this diameter and the eroded crater rim may extend to a diameter of approximately 260 km.

A diamond anvil cell has been adapted for determination of the strain ellipsoid in a polycrystalline sample. An x-ray beam (Mo Kα) is directed through the sample perpendicular to the loading axis. Since the diamond anvils are transparent to Mo Kα radiation, a complete cone of diffraction is available from each set of crystallographic planes (hkl). These cones produce rings on a flat film placed in front reflection geometry. From the ellipticity of these rings it is possible to calculate directly the strain ellipsoid. Material strengths as a function of pressure can then be calculated.

The effect of pressure on the lattice parameter of magnetite has been determined at room temperature up to 320 kbar by means of X ray diffraction employing a diamond anvil high‐pressure cell. By using the Birch‐Murnaghan equation with a (∂KT/∂P)T∣P=0 value of 4 ± 0.4 the isothermal bulk modulus at zero pressure was calculated to be 1.83 ± 0.10 Mbar. The X ray diffraction study also revealed that at pressures greater than 250 kbar, magnetite transforms to a high‐pressure phase, which reverts to magnetite at pressures below 50 kbar. It is possible to index this high‐pressure phase as being monoclinic. If it is assumed that the unit cell contains two molecules, the density is in agreement with the density predicted for a phase of Fe3O4 having all of the iron atoms in sixfold coordination.