Joan R. Clark

A crystal-chemical classification of borate structures with emphasis on hydrated borates

The rules governing formation of hydrated borate polyanions that were proposed by C.L. Christ in 1960 are critically reviewed and new rules added on the basis of recent crystal structure determinations. Principles and classifications previously published by others are also critically reviewed briefly. The fundamental building blocks from which borate polyanions can be constructed are defined on the basis of the number n of boron atoms, and the fully hydrated polyanions are illustrated. Known structures are grouped accordingly, and a shorthand notation using n and symbols Δ = triangle, T = tetrahedron is introduced so that the polyanions can be easily characterized. For example, 3:Δ+2T describes [B3O3(OH)5]2−. Correct structural formulas are assigned borates with known structures whereas borates of unknown structure are grouped separately.

Crystal chemistry and structure refinement of five hydrated calcium borates

Abstract 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 A 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.

Atomic displacements in ferroelectric trigonal and orthorhombic boracite structures

Abstract New crystal-structure refinements of Pca21 boracite, Mg3ClB7O13, and Rϵc ericaite, Fe2.4Mg0.6ClB7O13, show that some boron and oxygen atoms are involved in the ‘ferro’ transitions as well as the metal and halogen atoms. The atomic displacements associated with the polarity changes are as large as 0.6A.

Lunar clinopyroxenes: Chemical composition, structural state, and texture

Single-crystal x-ray diffraction, microprobe, optical and electron optical examinations of clinopyroxenes from Apollo 11 lunar samples 10003, 10047, 10050, and 10084 show that generally the crystals are composed of (001) augitepigeonite intergrowths in varying ratios. Transmission electron micrographs reveal abundant exsolution lamellae, many only 60 A thick. In addition to the phase inhomogeneities, primary chemical inhomogeneities are clearly demonstrated. There are reciprocal relationships between calcium and iron and between Ti4+ + 2Ai and 2Si. Our evidence suggests that a chemically inhomogeneous subcalcic C2/c augite was the only primary pyroxene from which pigeonite later exsolved.

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 P2 symmetry with a nearly ordered distribution of the multiple cation content defined by its approximate formula: (Nao.5 Cao.5) (Mgo.4Fe2+ 0.1 Al0.4 Fe3 +0.1) Si206. Na+ and Ca2+ tend to assume alternate locations in the structure, and ( Mg,Fe2+) octahedra alternate with Al3+. or (Al,F3+) octahedra in chains along c.

Pentaborate Polyanion in the Crystal Structure of Ulexite, NaCaB5O6(OH)6·5H2O

Triclinic ulexite crystals contain isolated borate polyanions [B5O6(OH)6]3- related to the well known pentaborate polyanion [B5O6(OH)4]- by addition of two hydroxyl groups to two opposite B-O triangles. The isolated ulexite polyanions form the [B5O7(OH)4]n3n- chains previously found in crystals of the related mineral probertite, NaCaB5O7(OH)4�3H2O.

Crystallography of Some Lunar Plagioclases

Crystals of calcic bytownite from type B rocks have space group I1 with c ≈ 14 angstroms. Bytownite crystals from type A rocks are more sodic and have space group C1, c ≈ 7 angstroms. Cell parameters of eight bulk feldspar separates from crystalline rocks indicate that the range of angle gamma is about 23 times the standard error of measurement, and its value might be useful for estimation of composition. Cell parameters of seven ilmenites are close to those of pure FeTiO3.

Crystal structure refinement of reedmergnerite, the boron analog of albite

Ordering of boron in a feldspar crystallographic site T1(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.