27Al, 29Si,and 23Na MAS NMR study of an Al,Si ordered alkali feldspar solid solution series     

Brian L. Phillips  R. James Kirkpatrick  Guy L. Hovis 《Physics and Chemistry of Minerals》1988,16(3):262-275

Solid-state ²⁷Al, ²⁹Si and ²³Na MAS NMR spectra have been obtained for an Al,Si ordered low albite to low microcline ion exchange series for which unit-cell parameters and ²⁹Si NMR data have previously been reported. ²⁷Al δ_i vary continuously with composition from 63.4 (±0.5) ppm for albite to 58.9 (±0.5) ppm for microcline, and parallel the ²⁹Si chemical shifts assigned to the T2m-site. The ²⁷Al and ²⁹Si chemical shifts for this series correlate well with composition-dependent lattice parameters, most notably cell volume and the angle [201]¹b. The linewidths of the ²⁹Si and ²⁷Al resonances indicate a significant amount of structural disorder in the intermediate compositions due to Na, K substitution. The 1 σ width of the distribution of average Si-O-T angles for each T-site is estimated to be about 1° for the Or33 sample. The average ²³Na δ_i varies monotonically from -8.5 (±1) ppm for albite to -24.3 (±1)ppm for Or83. Similarly, the average ²³Na nuclear quadrupole coupling constant decreases from 2.60 to 1.15 (±0.05) MHz and the asymmetry parameter of the electric field gradient increases from 0.25 to 0.6 with increasing K-content from albite to Or83. The observed variations in the quadrupole coupling parameters are consistent with simple electrostatic calculations. Higher resolution ²³Na spectra of the intermediate compositions obtained at 11.7 T indicate the presence of an inhomogeneous linebroadening which is related to the distribution of Na-environments. A model based on a random distribution of local compositions does not simulate the spectra, suggesting that the distribution of Na is skewed toward Na-rich clusters. Observation of the ²³Na NMR lineshape of Or49 after short periods of heat treatment indicate that ²³Na NMR is very sensitive to the changes in the Na, K distribution accompanying the early stages of exsolution. Reversible changes occur after heating at 530° C for 3 h, whereas heating at 600° C produces no changes, possibly bracketing the position of the coherent spinodal for Al, Si ordered alkali feldspars at this composition.  相似文献   

Energetics of water dissolution in trachyte glasses and liquids     

Pascal Richet  Guy Hovis  Jacques Roux 《Geochimica et cosmochimica acta》2004,68(24):5151-5158

Enthalpies of dissolution in HF solutions have been measured at 323 K for a series of hydrous trachyte glasses. Enthalpies of mixing between water and molten trachyte have then been calculated from heat capacity data for the same set of samples and available enthalpy for pure water. The moderately negative enthalpies of mixing suggested at 1 bar by the measurements made on glasses almost disappear when trachyte liquids and water are referred to the same temperature, and particularly so when enthalpies of mixing are calculated for a few kbars pressure. As found for albite and phonolite liquids, trachyte melts thus appear to mix nearly ideally as far as enthalpy is concerned. These results imply that the enthalpy of exsolution of water from magmas is very small or negligible under the P-T-X conditions relevant to trachytic volcanism, even for complete degassing of up to 5 wt% H₂O. Furthermore, the viscosity increase associated with exsolution-driven cooling is negligible compared to the decrease caused directly by water exsolution.  相似文献   

A hydrofluoric acid solution calorimetric investigation of glasses in the systems NaAlSi₃O₈-KAlSi₃O₈ and NaAlSi₃O₈-Si₄O₈     

Guy L. Hovis 《Geochimica et cosmochimica acta》1984,48(3):523-525

Glasses in the systems NaAlSi₃O₈-KAlSi₃O₈ and NaAlSi₃O₈-Si₄O₈ have been studied by means of hydrofluoric acid solution calorimetry at 50°C. Results indicate small negative enthalpies of mixing in the former system and small positive departures from ideality in the latter.  相似文献   

A new X-ray investigation of maximum microcline crystalline solutions     

Guy L. Hovis  Eric Peckins 《Contributions to Mineralogy and Petrology》1978,66(4):345-349

Refined unit-cell dimensions and molar volumes of a mircocline-low albite ion-exchange series previously studied by Waldbaum and Robie (1971) have been redetermined. Members in the sodic half of the series were found to have significantly larger b dimensions than those previously reported, resolving the major difference between the above study and that of Orville (1967). W _v, the Margules parameter related to the volume of mixing, is significantly lower (0.078 cal/bar mol) than those determined in either of the investigations above. However, it is close to values of W _v determined for several alkali feldspar series with topochemically monoclinic Al-Si distributions. Thus, the dependence of K-Na volume of mixing properties on Al-Si distribution in alkali feldspars is doubtful.  相似文献