Abstract
Modified single‐crystal X‐ray diffraction techniques have been adopted in the study of the crystal structure of andradite garnet at several pressures to 19.0 GPa (190 kbar) and the compression of a mantle pyrope to 6.0 GPa. Bulk modulus, K, of end‐member natural andradite from Val Malenco, Italy, is 159 ± 2 GPa (assuming dK/dP = 4), whereas that of pyrope from Kakanui, New Zealand, is 179 ± 3 GPa. In andradite, the Ca-containing 8‐coordinated polyhedron, Fe3+‐containing octahedron, and Si‐containing tetrahedron all display significant compression. The larger Ca‐containing polyhedron, which is more compressible than Fe- or Si‐containing polyhedra, has the same bulk modulus as the bulk crystal. Below l0 GPa, bond compression is accompanied by a significant decrease in Si‐O‐Fe angle. This behavior is similar to that of framework silicates, in which a corner-linked network of smaller, more rigid polyhedra (Si and Fe) collapses about a larger, more compressible polyhedron (Ca). At higher pressure, however, polyhedral distortion may play a significant role in garnet compression.
We use an optimized procedure for collecting X‐ray diffraction data from single crystals for which peak‐to‐background ratios are small, and the ratio of accessible reflections to structural parameters is large. A subset of reflections-those most influenced by variable atom positional parameters‐is scanned at relatively slow rates. By counting a select subset of reflections for longer times, we greatly increased the percentage of observed reflections, improved precision of refined positional parameters and interatomic distances, and reduced the total diffractometer time for the data‐collection experiment.
