Wolfgang Morgenroth

Structural change in molten basalt at deep mantle conditions

Silicate liquids play a key part at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to present-day volcanic activity. Quantitative models of these processes require knowledge of the structural changes and compression mechanisms that take place in liquid silicates at the high pressures and temperatures in the Earth’s interior. However, obtaining such knowledge has long been impeded by the challenging nature of the experiments. In recent years, structural and density information for silica glass was obtained at record pressures of up to 100 GPa (ref. 1), a major step towards obtaining data on the molten state. Here we report the structure of molten basalt up to 60 GPa by means of in situ X-ray diffraction. The coordination of silicon increases from four under ambient conditions to six at 35 GPa, similar to what has been reported in silica glass. The compressibility of the melt after the completion of the coordination change is lower than at lower pressure, implying that only a high-order equation of state can accurately describe the density evolution of silicate melts over the pressure range of the whole mantle. The transition pressure coincides with a marked change in the pressure-evolution of nickel partitioning between molten iron and molten silicates, indicating that melt compressibility controls siderophile-element partitioning.

Electron density studies on hydrogen bonding in two chromone derivatives

The experimental electron densities of two chromone derivatives have been determined from X-ray synchrotron diffraction data at low temperature (100 K). Topological analysis of the electron density has been used to analyze the formation of resonance-assisted hydrogen bonds (RAHBs). Geometrical and topological parameters confirm π-electron delocalization within the hydrogen-bonded ring. In addition, weak C-H···O interactions were identified in both structures. Hydrogen-bond energies allowed medium and weak hydrogen bonds to be distinguished.

Crystal structure of the high-pressure phase of the oxonitridosilicate chloride Ce4[Si4O3 + xN7 − x]Cl1 − xOx, x≃ 0.2

The structural compression mechanism of Ce4[Si4O3 + xN7 − x]Cl1 − xOx, x≃ 0.2, was investigated by in situ single-crystal synchrotron X-ray diffraction at pressures of 3.0, 8.5 and 8.6 GPa using the diamond–anvil cell technique. On increasing pressure the low-pressure cubic structure first undergoes only minor structural changes. Between 8.5 and 8.6 GPa a first-order phase transition occurs, accompanied by a change of the single-crystal colour from light orange to dark red. The main structural mechanisms, leading to a volume reduction of about 5% at the phase transition, are an increase in and a rearrangement of the Ce coordination, the loss of the Ce2, Ce3 split position, and a bending of some of the inter-polyhedral Si—N—Si angles in the arrangement of the corner-sharing Si tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared with the cell parameter of the cubic low-pressure structure.