X-ray Diffraction Procedures for Clay Mineral Identification

Thermochemistry of the high structural state plagioclases

The chemical bonding in silica polymorphs and silicates is being studied by both experimental and theoretical methods1. Spinning at the octahedral (magic) angle yields sharp peaks in the NMR patterns for 29Si (ref. 2) and 27Al (ref. 3). We report magic angle spinning (MAS) spectra for seven polymorphs of silica and interpret the chemical shifts empirically in terms of interatomic distances and angles determined by X-ray diffraction. The conclusions provide a partial basis for quantitative interpretation of spectra for framework aluminosilicates, especially zeolite molecular sieves; additional effects from aluminium substitution and non-framework atoms4 will need to be evaluated.

Nuclear magnetic resonance of silica polymorphs

Dependence of albite dissolution kinetics on ph and time at 25°c and 70°c

Comparison of new crystal-structure refinements for spodumene, LiFe3tSi,O" synthetic ureyite, acmite, diopside, augite, a CZ/c omphacite, and a PZ omphacite with published refinements for jadeite, johannsenite, NaIn'+Si,O" fassaite, and another PZ omphacite indicates that the structures are strikingly similar throughout the range of chemical compositions. The "end-member" clinopyroxenes are ordered within the limits of resolution. Bonding can be satisfactorily explained in terms of an essentially ionic model without invoking additional covalent effects. Except for PZ omphacites, the charge and size of the Mz cations determine the structure type: if Mz is Ca or Na, CZ/c is expected; if Mz is Li, CZ; and if Mz is Mg or Fe", PZ,jc or orthorhombic. Precise bond distances for ordered "end-member" clinopyroxenes can be used in determining T and M1 site contents of intermediate-composition clinopyroxenes, and ferrous iron is disordered in such clinopyroxenes. "Ideal" PZ omphacite, Cao.,Na,,.,Mgo.5Alo.,Si,O,, is expected to have Mg and AI fully ordered in the M1 octahedral chains, but in the Mz sites the maximum amount of order in alternating sites is jCa, j Na, and jNa, j Ca.

I. Y. Borg

Papers

Calculated X-ray Powder Patterns for Silicate Minerals

X-ray diffraction studies on CdTe at high pressure☆

Mechanical Twinning in Sphene at 8 Kbar, 25° to 500°C