Geology topics
J. R. Clark
Publications and source records attributed to J. R. Clark.
Bonding in eight ordered clinopyroxenes isostructural with diopside
Bond distances and angles in isostructural, ordered clinopyroxenes are compared for eight compositions, based on five new and three published crystal-structure refinements from X-ray diffraction data. Unit-cell parameters and configuration of the silicate chains are directly correlated with cation composition and distribution in the M2 and M1 sites.
Eclogitic pyroxenes, ordered with P2 symmetry
X-ray diffraction crystal-structure analysis of omphacite from eclogite, Tiburon Peninsula, Marin County, California, shows that this clinopyroxene has P 2 symmetry with a nearly ordered distribution of the multiple cation content defined by its approximate formula: (Na 0.5 Ca 0.5 ) (Mg 0.4 Fe 2 + 0.1 Al 0.4 Fe 3 + 0.1 )Si 2 O 6 . Na+ and Ca 2+ tend to assume alternate locations in the structure, and (Mg,Fe 2+ ) octahedra alternate with Al 3+ or (Al,F 3+ ) octahedra in chains along c .
Crystal chemistry and structure refinement of five hydrated calcium borates
The crystal structures of the five known members of the series Ca 2 B 6 O 11 ·xH 2 O (x = 1, 5, 5, 7, 9, and 13) have been refined by full-matrix least-squares techniques, yielding bond distances and angles with standard errors of less than 0·01 Å and 0·5°, respectively. The results illustrate the crystal chemical principles that govern the structures of hydrated borate compounds. The importance of hydrogen bonding in the ferroelectric transition of colemanite is confirmed by more accurate proton assignments.
Boron-oxygen polyanion in the crystal structure of tunellite
The crystal structure of tunellite, SrO·3B 2 O 3 ·4H 2 O, with infinite sheets of composition n[B 6 O 9 (OH) 2 ] 2- , has cations and water molecules in the spaces within the sheets. Adjacent sheets are held together by hydrogen bonding through the water molecules. The boron-oxygen polyanions provide the first example in hydrated borate crystals of one oxygen linked to three borons.
Crystal structure refinement of reedmergnerite, the boron analog of albite
Ordering of boron in a feldspar crystallographic site T 1 (0) has been found in reedmergnerite, which has silicon-oxygen and sodium-oxygen distances comparable to those in isostructural low albite. If a simple ionic model is assumed, calculated bond strengths yield a considerable charge imbalance in reedmergnerite, an indication of the inadequacy of the model with respect to these complex structures and of the speculative nature of conclusions based on such a model.