Wayne A. Dollase

The crystal chemistry of silica-rich, alkali-deficient nepheline

Coarse crystals of an extremely silica-rich, potassium-deficient nepheline have been synthesized hydrothermally with albite. Electron microprobe analysis yielded the formula: □1.76Ko.24Na6.00Al6.24-Si9.76O32. A crystal structure analysis of this nepheline has revealed (1) a disordered Si-Al distribution, (2) full occupancy of the smaller cavity site by sodium atoms, and (3) the larger cavity is vacant except for the minor potassium content, with the size of this cavity being the same as when largely occupied with potassium atoms. In addition, the crystal structure of another nepheline prepared by alkali exchange in molten NaCl showed that when Na atoms replace the K atoms of the large cavity: (1) this cavity does not collapse around the smaller atoms, and therefore, (2) the sodium atoms occupy an offcenter position displaced by about 0.3 Å from the cavity center in order to form a rather one-sided bonding configuration with some of the cavity wall oxygen atoms.These structures further support the strong site preference indicated earlier by Buerger and coworkers and restated by Barth: that for the large cavity K > □ ≫ Na and for the small cavity site Na>Ca(?)≫K, □. Perfect compliance with this site preference scheme would tend to restrict nephelines to the Barth compositional join: □2Na6Al6Si10O32-K2Na6Al8Si8O32, neglecting the minor Ca component usually present. Thirteen new electron microprobe analyses of nephelines from a variety of occurrences and sixteen additional microprobe analyses from the literature which comply with nepheline-structure allowed stoichiometry, are plotted showing that (1) natural nephelines closely conform to this site preference scheme with less than 10% of the large cavity sites containing sodium atoms, that is, the analyses cluster near the Barth join, and (2) examples of natural nephelines can be found at nearly all intermediate compositions along this join.

Si-Al ordering in nephelines

Different degrees of Si-Al ordering are demonstrated to exist in nepheline on the basis of five crystal structure refinements on specimens with different geologic origins and laboratory treatments. Disorder increases in the sequence; gneissic-igneous plutonic-volcanic nepheline and appears to increase as a result of laboratory heat treatment. The four independent tetrahedral sites can be divided into two pairs with one pair of sites always less ordered than a second pair in a simple regular relationship.

Generalized ellipsometry for biaxial absorbing materials: determination of crystal orientation and optical constants of Sb 2 S 3

We demonstrate generalized ellipsometry for precise measurement of the principal indices of refraction, the extinction coefficients, and the orientations of the crystal a, b, and c axes of orthorhombic absorbing materials. Stibnite (Sb(2)S(3)) single crystals cut approximately parallel to (100), (010), (001), and (313) are studied at a representative wavelength of 589 nm. The (313) surface is sufficient for retrieval of all optical constants. The expected effects of surface over-layer formation are removed numerically. We propose generalized ellipsometry as a powerful tool for measurement of anisotropic optical function spectra of biaxial materials.

Optimum distance model of relaxation around substitutional defects

The relaxation patterns around isovalent substituent atoms in the NaCl, CsCl, ZnS, CaF2, Li2O, and ReO3 structure types have been calculated. Such models (1) emphasize geometric structural criteria and are thus conceptually simple, (2) are computationally straightforward though perhaps computer-time consuming for large, low symmetry cases, and (3) agree reasonably well with more complex calculations, such as energy minimization methods. On the basis of such optimum interatomic distance models, atomic displacements resulting from an isovalent substitution: (1) depend strongly on the structural connectivity (structural type) involved, (2) are decidedly greater for atoms with radius vectors parallel to the substituents bonds and least for atoms with radius vectors oriented between such bonds, (3) decrease in magnitude approximately inversely proportional to the square of the distance to the substituent, regardless of direction, and (4) are mostly, but not strictly, radial. The simplest relationship to structural type is the dependence on the coordination of the substituents ligands — the greater the coordination number of the atoms bonded to the substituting atom, the less compliant the structure.

Temperature dependence of the cation distribution in CuAl2O4 spinel

The temperature dependence of the cation distribution in CuAl 2 O 4 spinel has been determined from 600 to 1100°C on quenched specimens by powder X-ray diffraction, and from room temperature to 1000°C by in-situ high-temperature powder neutron diffraction. The results, in agreement with earlier work, show that CuAl 2 O 4 is a largely normal spinel, which is already highly disordered at 600°C, the lowest temperature at which the cation distribution is inferred to be in equilibrium, with an inversion parameter, x, at this temperature of 0.35 ± 0.005 from the neutron diffraction experiments. Increasing temperature to 1000°C only causes a modest increase in x, to 0.40 ± 0.005. This somewhat unusual behaviour may be ascribed to local distortions from the Jahn-Teller effect in both octahedrally and tetrahedrally coordinated Cu 2+ . Thermodynamically, the ordering may be described either with a large entropy of disordering, or alternatively, with a large and positive quadratic term in the enthalpy of disordering. The Jahn-Teller effect produces no long-range distortion of the structure, which has cubic symmetry (space group Fd 3 m ) under all investigated conditions.

Temperature dependence of the cation distribution in CuAl2O4.

The temperature dependence of the cation distribution in CuAl 2 O 4 spinel has been determined from 600 to 1100°C on quenched specimens by powder X-ray diffraction, and from room temperature to 1000°C by in-situ high-temperature powder neutron diffraction. The results, in agreement with earlier work, show that CuAl 2 O 4 is a largely normal spinel, which is already highly disordered at 600°C, the lowest temperature at which the cation distribution is inferred to be in equilibrium, with an inversion parameter, x, at this temperature of 0.35 ± 0.005 from the neutron diffraction experiments. Increasing temperature to 1000°C only causes a modest increase in x, to 0.40 ± 0.005. This somewhat unusual behaviour may be ascribed to local distortions from the Jahn-Teller effect in both octahedrally and tetrahedrally coordinated Cu 2+ . Thermodynamically, the ordering may be described either with a large entropy of disordering, or alternatively, with a large and positive quadratic term in the enthalpy of disordering. The Jahn-Teller effect produces no long-range distortion of the structure, which has cubic symmetry (space group Fd 3 m ) under all investigated conditions.

THE EFFECT OF MANGANESE ON OLIVINE-QUARTZ-ORTHOPYROXENE STABILITY

• • . opx-oliv microprobe analyses of run products indicate a small preference of Mn for pyroxene over olivine with K~ Mn-Fe = 1.2-1.5, similar to values observed for natural pairs. M6ssbauer spectra are consistent with a random distribution of Mn between the M1 and M2 sites in the orthopyroxene. These experimental data allow downward revision of pressure estimates based on the orthopyroxene barometer in areas where Mn is a significant component in orthopyroxene.

Modified lattice parameter/Curie temperature diagrams for Titanomagnetite/titanomaghemite within the quadrilateral Fe3O4‐ Fe2TiO4‐Fe2O3‐Fe2TiO5

Magnetic and mineralogical data were obtained on a sequence of titanomagnetite - titanomaghemite solid solutions from four basalt samples (0 to 70 Ma) dredged from four sites along a traverse normal to the mid-Atlantic Ridge. Data which characterized magnetic iron oxides with the spinel structure include Mossbauer, XRD (including Rietveld structure refinement), TEM/AEM, and magnetic data. The data indicate a sequence ranging from nearly unoxidized titanomagnetite to near end-member titanomaghemite. Compositions of samples with intermediate values of oxidation parameter, Z, varied significantly from those estimated using previously determined relations between lattice parameters and compositions. The data of this study, in conjunction with those of selected references, lead to more accurate contoured diagrams for Curie temperature and lattice parameter (as consistent with Vegards raw) for the quadrilateral Fe 3 O 4 -Fe 2 TiO 4 -Fe 2 O 3 -Fe 2 TiO 5 .

Crystalline dimethylaminomethane-diphosphonic acid: the existence of anhydrous and monohydrate forms

Abstract There has been some discrepancy in the literature as to whether solid dimethylarninornethanediphosphonic acid existed as the anhydrous form or the monohydrate. We report the synthesis and characterization of both forms. 31P solid-state NMR data are given.