Guy L. Hovis

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

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 δi 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 δi 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.

Solution calorimetric investigation of fluor-chlorapatite crystalline solutions

Abstract Solution calorimetric measurements have been made on 17 synthetic fluorapatite-chlorapatite crystalline solutions at 50°C in 20.0 wt% HCl under isoperibolic conditions. Analysis of the calorimetric data indicates that heats of mixing across the series may reach values as high as 8.3 kJ/mol. Normally such a high degree of thermodynamic nonideality would be associated with immiscibility, yet no such miscibility gap is indicated by either synthetic or natural fluor-chlorapatite specimens. Based on full chemical analyses, all Cl-rich samples (XCl > 0.65) of this study have halogen deficiencies that imply the presence of 4-11 mol% vacancies in the anion sites, which are interpreted to be associated with oxyapatite substitution. Separate analysis of data for the vacancy-free samples produces a linear fit for enthalpy of solution vs. composition, which yields an alternative interpretation of thermodynamic ideality. Together these models define the limits of enthalpy behavior for the fluor-chlorapatite system

Enthalpies of mixing for disordered alkali feldspars at high temperature: A test of regular solution thermodynamic models and a comparison of hydrofluoric acid and lead borate solution calorimetric techniques

The dehydration behavior of an analcime formed by ion exchange from leucite (X-type analcime, or secondary analcime) is contrasted with the behavior of an analcime formed by hydrothermal recrystallization from structurally dissimilar materials (hydrothermal analcime, or H-type analcime). The kinetics of dehydration of X-type analcime is consistent with a high specific surface area, showing surface equilibration with H20 vapor and fast initial H20 loss. The kinetic data approximately fit the Austin-Rickett equation, yielding an empirical activation energy for the dehydration of X-type analcime of 33 kl/mol, compared with 92 kl/mol for H-type analcime. Washed and ultrasonically cleaned samples of the same grain size have very different BET (N adsorption) surface areas, being 20 m2/g for X-type analcime and 2 m2/g for H-type analcime, consistent with the high porosity of the surface of X-type analcime, as observed by SEM. Further analysis of the N adsorption data shows the pore size distribution for the X-type analcime to have a maximum at 100 A. The rate ofNa drift under electron microprobe is more than 15 times higher for X-type analcime than for H-type analcime and could be a convenient means of distinguishing the two parageneses.

Thermal expansion of nepheline^kalsilite crystalline solutions

Abstract Eleven nepheline-kalsilite crystalline solutions with various proportions of K:Na have been studied from room temperature to 1050/1150°C by X-ray powder diffraction. Nepheline expansion is relatively high and little affected by composition, whereas kalsilite expansion is lower but affected to a significant degree by K:Na ratio. The generally higher rate of expansion in nepheline is apparently related to the collapse of the tetrahedral framework around the smaller of its two alkali sites. Occupancy of these sites by the relatively small Na ion further extends the potential for thermal vibration before the structure is stretched to the critical degree required for phase transformation. Once the structure changes to that of kalsilite, with its single alkali site, an increase in content of the larger K ion limits the degree to which kalsilite can expand. Crucial to the overall expansion behaviour of these minerals are the specific tetrahedral configurations of nepheline vs. kalsilite, the number and geometry of their alkali sites, the occupancies of those sites, and the flexibility inherent in each structure that allows for adjustment with increasing temperature.

A new era in hydrofluoric acid solution calorimetry; reduction of required sample size below ten milligrams

Abstract Significant advances have been made in hydrofluoric acid solution calorimetry at Lafayette College in the past 15 years. To determine the degree to which these developments enable the reduction of sample size, calorimetric experiments were performed on hexagonal germanium oxide as a function of sample weight. The resulting calorimetric data indicate that the highest degrees of reproducibility (±0.1%) are maintained down to sample sizes of 50 mg, and that precisions of ±1%, acceptable for many applications, are observed to sample sizes of 10 mg. Because silicate systems produce weight-based heats of solution that are about twice that of germanium oxide, the required sample size for these will be even less. The new minimum required sample size of 5 to 25 mg (depending on application) is about two orders of magnitude less than that used 20 or 30 years ago. This makes possible many new kinds of projects for HF solution calorimetric investigation, including those on high-pressure materials.

Volumes of K-Na mixing for low albite-microcline crystalline solutions at elevated temperature: A test of regular solution thermodynamic models

Abstract High-temperature volumes of K-Na mixing have been investigated for a seven-member low albite-microcline ion-exchange series by conducting X-ray powder diffraction measurements from room temperature to approximately 1000 °C using Guinier techniques. Volume expansion is a linear function of temperature for all series members and is due mainly to the lengthening of the a unit-cell axis, although some expansion of band c occurs for relatively sodic members. The maximum expansion possible for each feldspar is determined both by chemistry and temperature, thus ΔV/ΔT slopes are steepest for the sodic third of the series, then decrease abruptly for increasingly potassic feldspars. Volumes of K-Na mixing for this series are essentially constant with temperature, regardless of the mixing model utilized, and thus are adequately represented by regular solution thermodynamic models.

Thermodynamic and structural behavior of analcime-leucite analogue systems

Abstract Two synthetic solid-solution series, analcime to Rb-leucite and analcime to Cs-leucite (pollucite), have been investigated to understand more fully the thermodynamic and structural behavior of analcime- leucite and similar mineral systems. Unit-cell dimensions and volumes in these series expand with the substitution of analcime component in either Rb-leucite or pollucite, as H2O molecules structurally replace the smaller entities Rb+1 or Cs+1, respectively. Unit-cell volumes vary linearly as functions of composition, but with changing slopes over several segments of compositional space, akin to thermal expansion in K-, Rb-, and Cs-end-member materials studied by previous workers. When symmetry changes displacively from tetragonal to isometric, as in the Rb-bearing series, the slope of volume expansion changes. Once structures have reached full expansion, volume slopes flatten and are little affected by additional analcime component. Enthalpies of solution measured at 50 °C in 20.1 wt% hydrofluoric acid show single-slope linear relationships over the entire compositional ranges of both series. Thus, despite positive volumes of mixing, there are no enthalpies of mixing in either series, nor is there energetic evidence of displacive tetragonal/isometric inversion or the various stages of structural expansion. Overall, the data suggest that the analcime-leucite system also can be modeled as close to thermodynamically ideal. The limited solid solution between natural analcime and leucite must be attributed to energetically favored heterogeneous equilibria involving minerals such as feldspars and other feldspathoids, and not to immiscibility between the end-members.

Principles of thermal expansion in the feldspar system

Abstract Following the recent thermal expansion work of Hovis et al. (2008) on AlSi3 feldspars, we have investigated the thermal expansion of plagioclase, Ba-K, and Ca-K feldspar crystalline solutions. X-ray powder diffraction data were collected between room temperature and 925 °C on six natural plagioclase specimens ranging in composition from anorthite to oligoclase (and as well on their Kexchanged equivalents) and five synthetic Ba-K feldspars with compositions from 25 to 99 mol% BaAl2Si2O8. The resulting thermal expansion coefficients for volume (αV) have been combined with earlier results for end-member Na- and K-feldspars. Unlike AlSi3 feldspars, Al2Si2 feldspars, including anorthite and celsian from the present study, and Sr- and Pb-feldspar from other workers, show essentially constant, and relatively limited, thermal expansion. In the context of structures where the Lowenstein rule requires Al and Si to alternate among tetrahedra, the proximity of bridging Al-O-Si O atoms to divalent neighbors (ranging from 0 to 2) results in Ca-O (or Ba-O) bonds that are especially short. It is suggested that short bonds such as these have a partly covalent character resulting from the requirement for local charge balance. This, in turn, stiffens the structure. For feldspar series with coupled substitution, the change away from a purely divalent M-site occupant gives the substituting (less strongly bonded) monovalent cations increasingly greater influence on thermal expansion. Overall, thermal expansion in the feldspar system is well represented on a plot of αV against room-temperature volume (VRT), where one sees a quadrilateral bounded by data for: (1) AlSi3 feldspars whose expansion behavior is governed largely by the size of the monovalent alkali-site occupant; (2) Al2Si2 feldspars whose expansion is uniformly limited by divalent cations having stronger, partially covalent, bonds to bridging Al-O-Si O atoms; and (3,4) plagioclase and Ba-K feldspars where expansion behavior across the series transitions from one control to the other. Overall, the coefficient of thermal expansion in any binary feldspar series is a linear function of VRT between the pertinent end-members. This makes it possible to predict the thermal expansion behavior of any feldspar simply from knowledge of its chemical system and room-temperature volume

A novel technique for fluorapatite synthesis and the thermodynamic mixing behavior of F-OH apatite crystalline solutions

Abstract Successful synthesis of fluorapatite has been achieved through ion-exchange between NIST hydroxlyapatite SRM 2910a and optical-grade fluorite. Additional intermediate F-OH apatite compositions were made through ion-exchange between the newly synthesized fluorapatite and the original hydroxylapatite. Based on solution calorimetric data collected on seven fluorapatite-hydroxlyapatite crystalline solutions at 50 °C in 20.0 wt% HCl under isoperibilic conditions, fluorine-rich series members display ideal thermodynamic behavior, whereas hydroxyl-rich compositions show negative enthalpies of F-OH mixing. Unit-cell volumes for the series are linear with composition. Relative to enthalpy and volume, therefore, there are no energy barriers to complete solid solution between the F and OH end-members.

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

Abstract 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.