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The response of the mesolite crystal structure ([Na.sub.16][Ca.sub.16][Al.sub.48][Si.sub.72][O.sub.240] x 64 [H.sub.2]O) to dehydration was evaluated as a function of temperature and partial pressure of water (i.e., [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] or relative humidity, RH) using laboratory X-ray powder diffraction (XRD; CuK[alpha] radiation) and synchrotron X-ray pair distribution function (PDF) methods. At 425[degrees]C under low-[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] [less than or equal to] ~1.3 mbar), dehydrated mesolite preserved the long-range ordered aluminosilicate framework structure, which has not been previously observed. This new dehydrated phase, x-metamesolite, has unit-cell parameters [a 16.731(3) [Angstrom], b = 17.822(2) [Angstrom], c = 6.312(1) [Angstrom], V 1882.5(5) [[Angstrom].sup.3], and possible space group Fdd2] similar to those of other dehydrated natrolite phases (either [alpha]1-or [alpha]2-metanatrolite). Conversely, under high-[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] > ~1.3 mbar), dehydrated mesolite became amorphous (amorphous [T.sub.5][O.sub.10]) at 425[degrees]C. The local structure of amorphous [T.sub.5][O.sub.10] was characterized by PDF analyses, which showed the formation of twisted [T.sub.5][O.sub.10] nano-fibers [with dimensions (LWH) of ~6.9 x 6.9 x 6.3 [Angstrom]] resulting from breakage of the mesolite aluminosilicate framework. The two distinct high-temperature [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-dependent phase transition paths illustrate the importance of considering the combined effects of temperature and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in mesolite. In addition, the low-temperature phase transition in mesolite, involving order-disorder of the extraframework cations, also showed a [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-dependent transition temperature. Although, there is no path dependence on [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] for this transition, the local arrangement of Na, Ca, and vacancies in disordered metamesolite (formed through the extraframework cation order-disorder phase transition) likely influences the thermal stability of the aluminosilicate framework during further heating. Keywords: Phase transition, mesolite, XRD data, crystal structure DOI: 10.2138/am.2010.3443

The crystal structure of the 2.5 hydrate MgSO4 phase was determined by simulated annealing from laboratory X-ray powder diffraction data measured from 2-140[degrees]2[theta] using CuK[alpha] radiation. The 2.5 hydrate is monoclinic, space group C2/c, with unit-cell parameters a - 18.8636(4) [Angstrom], b 12.3391(2) [Angstrom], c= 8.9957(2) [Angstrom], [beta] = 94.568(2)[degrees], V 2087.1(6) [[Angstrom].sup.3], and Z= 16. The model was refined using fundamental-parameters Rietveld refinement, converging to [R.sub.wp] = 8.89%, [R.sub.p] = 6.61%, [R.sub.exp] = 3.33%, [R.sub.Bragg] = 3.95%, and [chi square] = 2.67. The refined structure is consistent with a formula of 2.5 [H.sub.2]O. Bond-valence calculations for the refined model show that the structure is chemically sensible. In the refined structure, [[Mg(O,[H.sub.2]O).sub.6]] octahedra and [[SO.sub.4]] tetrahedra build up 2-D double-sheet slabs by sharing vertex O atoms, which are held together by inter-slab H-bonds involving [([SO.sub.4]).sup.2-] groups and [H.sub.2]O molecules coordinated with [Mg.sup.2+] cations to form the layer structure of the 2.5 hydrate phase. Keywords: Mg[SO.sub.4].2.5[H.sub.2]O, Mg[SO.sub.4].2.4[H.sub.2]O, crystal structure, simulated annealing, structure determination, powder diffraction, Rietveld refinement

The crystal structure of sanderite, MgS[O.sub.4] x 2[H.sub.2]O, was determined from laboratory X-ray powder diffraction data measured from 2-140 [degrees]2[theta] using CuK[alpha] radiation. Sanderite is orthorhombic, space group P[2.sub.1][2.sub.1][2.sub.1], with unit-cell parameters a = 8.8932(1) [Angstrom], b = 8.4881(1) [Angstrom], c = 12.4401(2) [Angstrom], V = 939.16(3) [Angstrom], Z = 8. The crystal structure model was determined using the charge flipping method and was refined using fundamental parameters Rietveld refinement method to [R.sub.wp] = 6.52%, [R.sub.exp]: 1.89%, and [chi square] = 3.43. Bond-valence calculations for the refined model show that the structure is chemically reasonable. In the refined structure, [Mg.sup.2+] cations are coordinated by four O atoms from [[S[O.sub.4]].sup.2-] groups and by two [H.sub.2]O molecules, forming distorted octahedra. By sharing vertex O atoms, [S[O.sub.4]] tetrahedral and [Mg[O.sub.4]([[H.sub.2]O).sub.2]] octahedra build up a 3-D framework. Keywords: Sanderite, MgS[O.sub.4] x 2[H.sub.2]O, crystal structure, charge flipping, structure determination, powder diffraction

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.12.006 Byline: Jochen Janchen (a)(b), Richard V. Morris (c), David L. Bish (d), Mareike Janssen (a), Udo Hellwig (a) Keywords: Mars; surface; Mineralogy Abstract: The recent detection of up to [approximately equal to]10 wt% water-equivalent H heterogeneously distributed in the upper meter of the equatorial regions of the martian surface and the presence of the 3-[mu]m hydrations feature across the entire planet raises the question whether martian surficial dust can account for this water-equivalent H. We have investigated the H.sub.2O and CO.sub.2 adsorption properties of palagonitic dust ( Author Affiliation: (a) TFH Wildau (University Applied Sciences), BahnhofstraAe, 15745 Wildau, Germany (b) TFH Wildau, c/o ZeoSolar e.V., Volmerstr. 13, 12489 Berlin, Germany (c) NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA (d) Indiana University, 1001 E. 10th Street, Bloomington, IN 47405, USA Article History: Received 3 December 2007; Revised 15 September 2008; Accepted 2 December 2008

The crystal structures of natrolite and its dehydrated high-temperature phases ([alpha]1- and ([alpha]2-metanatrolite) have been determined from powder X-ray diffraction measurements as a function of temperature and partial pressure of water ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) to characterize the phase transition behavior. The evolution of crystal structure as a function of temperature shows two different phase transitions, depending on the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], with [alpha]1-metanatrolite occurring at elevated [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [alpha]2-metanatrolite occurring at low [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Our discovery of [alpha]2-metanatrolite implies the existence of more than one transition mechanism, which we correlate with the migration of [Na.sup.+] ions and the rate of evolution of [H.sub.2]O molecules. The transition behavior is rationalized in terms of two cooperating mechanisms: (1) dehydration-induced processes, which determine the phase transition temperature; and (2) thermally induced processes, which determine how the framework and its extraframework cations are modified. Keywords: Phase transition, natrolite, crystal structure, Rietveld refinement, TGA

Rietveld refinements using synchrotron powder X-ray diffraction data were used to study the crystal structure and dehydration behavior of sepiolite from Durango, Mexico. The room-temperature (RT) sepiolite structure in air compares well with previous models but reveals an additional zeolitic [H.sub.2]O site. The RT structure under vacuum retained only ~1/8 of the zeolitic [H.sub.2]O and the volume decreased by 1.3%. Real-time, temperature-resolved synchrotron powder X-ray diffraction data and Rietveld refinements were used to investigate the behavior of the sepiolite structure from 300 to 925 K. Rietveld refinements revealed that most of the zeolitic [H.sub.2]O is lost by ~390 K, accompanied by a decrease in the a and c unit-cell parameters. Above ~600 K the sepiolite structure folds as one-half of the crystallographically bound [H.sub.2]O is lost. Rietveld refinements of the 'anhydrous' sepiolite structure reveal that, in general, unit-cell parameters a and b and volume steadily decrease with increasing temperature; there is an obvious change in slope at ~820 K suggesting a phase transformation coinciding with the loss of the remaining bound [H.sub.2]O molecule. Keywords: Sepiolite, Rietveld, synchrotron, powder diffraction

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.10.010 Byline: Jochen Janchen (a), David L. Bish (b), Diedrich T.F. Mohlmann (c), Helmut Stach (d) Keywords: Mars; surface; Mineralogy; Geochemistry Abstract: Encouraged by recent results of the Mars Odyssey spacecraft mission and the OMEGA team (Mars Express) concerning water in equatorial latitudes between [+ or -]45[degrees] on Mars and the possible existence of hydrated minerals, we have investigated the water sorption properties of natural zeolites and clay minerals close to martian atmospheric surface conditions as well as the properties of Mg-sulfates and gypsum. To quantify the stability of hydrous minerals on the martian surface and their interaction with the martian atmosphere, the water adsorption and desorption properties of nontronite, montmorillonite, chabazite and clinoptilolite have been investigated using adsorption isotherms at low equilibrium water vapor pressures and temperatures, modeling of the adsorption equilibrium data, thermogravimetry (TG), differential scanning calorimetry (DSC), and proton magic angle spinning nuclear magnetic resonance measurements (.sup.1H MAS NMR). Mg-sulfate hydrates were also analyzed using TG/DSC methods to compare with clay mineral and zeolites. Our data show that these microporous minerals can remain hydrated under present martian atmospheric conditions and hold up to 2.5-25 wt% of water in their void volumes at a partial water vapor pressure of 0.001 mbar in a temperature range of 333-193 K. Results of the.sup.1H MAS NMR measurements suggest that parts of the adsorbed water are liquid-like water and that the mobility of the adsorbed water might be of importance for adsorption-water-triggered chemistry and hypothetical exobiological activity on Mars. Author Affiliation: (a) Technische Fachhochschule Wildau (University of Applied Sciences), BahnhofstraAe, D-15754 Wildau, Germany (b) Indiana University, 1001 E 10th St., Bloomington, Indiana 47405, USA (c) DLR Institute of Planetary Research, Rutherfordstr. 2, D-12489 Berlin, Germany (d) ZeoSys GmbH, Volmerstr. 13, D-12489 Berlin-Adlershof, Germany Article History: Received 11 May 2005; Revised 20 October 2005

Encouraged by recent results of the Mars Odyssey spacecraft mission and the OMEGA team (Mars Express) concerning water in equatorial latitudes between [+ or -] 45[degrees] on Mars and the possible existence of hydrated minerals, we have investigated the water sorption properties of natural zeolites and clay minerals close to martian atmospheric surface conditions as well as the properties of Mg-sulfates and gypsum. To quantify the stability of hydrous minerals on the martian surface and their interaction with the martian atmosphere, the water adsorption and desorption properties of nontronite, montmorillonite, chabazite and clinoptilolite have been investigated using adsorption isotherms at low equilibrium water vapor pressures and temperatures, modeling of the adsorption equilibrium data, thermogravimetry (TG), differential scanning calorimetry (DSC), and proton magic angle spinning nuclear magnetic resonance measurements ([sup.1]H MAS NMR). Mg-sulfate hydrates were also analyzed using TG/DSC methods to compare with clay mineral and zeolites. Our data show that these microporous minerals can remain hydrated under present martian atmospheric conditions and hold up to 2.5-25 wt% of water in their void volumes at a partial water vapor pressure of 0.001 mbar in a temperature range of 333-193 K. Results of the [sup.1] H MAS NMR measurements suggest that parts of the adsorbed water are liquid-like water and that the mobility of the adsorbed water might be of importance for adsorption-water-triggered chemistry and hypothetical exobiological activity on Mars. Keywords: Mars, surface; Mineralogy; Geochemistry

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.04.020 Byline: Claire I. Fialips (a)(b), J. William Carey (b), David T. Vaniman (b), David L. Bish (c), William C. Feldman (b), Michael T. Mellon (d) Keywords: Mars surface; Mineralogy; Geochemistry Abstract: Thermodynamic data for several clays, zeolites, and MgSO.sub.4 salts were combined with calculated yearly mean temperatures and water-vapor pressures on the martian surface to predict mineral hydration states from low to middle latitudes. These predictions were used to evaluate whether the necessary amount and distribution of hydrous minerals were compatible with the Mars Odyssey observations of water-equivalent hydrogen (WEH). Our results indicate that zeolites like chabazite or clay minerals like Ca-montmorillonite would have to be unrealistically abundant in the martian soil (as much as 55 wt%) while Mg-sulfate hydrates at concentrations between 2 and 11 wt% could account for the WEH. However, the geographic distribution of WEH is incompatible with a uniformly distributed mineralogy in equilibrium with the annual mean P-T environment. A heterogeneous distribution of a mixture of different hydrous minerals, reflecting a heterogeneous Mars surface geology, may better explain a significant portion of the observed near-equatorial WEH. Author Affiliation: (a) University of Newcastle upon Tyne, School of Civil Engineering and Geosciences, Drummond Building, Newcastle upon Tyne, NE1 7RU, UK (b) Los Alamos National Laboratory, Los Alamos, NM 87545, USA (c) Indiana University, Bloomington, IN 47405, USA (d) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA Article History: Received 10 January 2005; Revised 22 April 2005

Thermodynamic data for several clays, zeolites, and MgS[O.sub.4] salts were combined with calculated yearly mean temperatures and water-vapor pressures on the martian surface to predict mineral hydration states from low to middle latitudes. These predictions were used to evaluate whether the necessary amount and distribution of hydrous minerals were compatible with the Mars Odyssey observations of water-equivalent hydrogen (WEH). Our results indicate that zeolites like chabazite or clay minerals like Ca-montmorillonite would have to be unrealistically abundant in the martian soil (as much as 55 wt%) while Mg-sulfate hydrates at concentrations between 2 and 11 wt% could account for the WEH. However, the geographic distribution of WEH is incompatible with a uniformly distributed mineralogy in equilibrium with the annual mean P-T environment. A heterogeneous distribution of a mixture of different hydrous minerals, reflecting a heterogeneous Mars surface geology, may better explain a significant portion of the observed near-equatorial WEH. Keywords: Mars surface; Mineralogy; Geochemistry

Yucca Mountain, Nevada, has been approved by the U.S. Congress as the site of the first high-level radioactive waste repository in the U.S., paving the way for the Department of Energy to submit a license application to the Nuclear Regulatory Commission. The U.S. concept of a radioactive waste repository involves both engineered (man-made) barriers and geologic barriers to minimize the long-term migration of waste from the repository. The minerals at Yucca Mountain are an integral part of the geologic barrier to the migration of radionuclides, and understanding the distribution of reactive and potentially sorptive minerals is crucial to modeling the long-term performance of the repository. We have determined the three-dimensional distribution of minerals at Yucca Mountain by analyzing a number of drill cores using quantitative X-ray diffraction methods. Clinoptilolite and mordenite are locally abundant in the rhyolitic tuffs in both the unsaturated and saturated zones. They formed by the alteration of glassy rhyolitic tuffs, and their occurrence is stratified, often following the distribution of nonwelded, initially vitric tuffs near and below the water table. Because of their large cation-exchange capacities, these minerals have traditionally been considered to be important ] in retardation of the migration of cationic radionuclides such as [sup.137]Cs and [sup.90]Sr However, analysis of radionuclides that may be contained at Yucca Mountain reveals that, for the most part, the radioactive species of concern are not those that would be readily sorbed by natural zeolites. Although clinoptilolite and mordenite are highly selective for Cs and Sr, the half-lives of these radionuclides ([Tilde]30 years) are sufficiently short that the man-made containment at any radioactive waste repository is expected to outlive these radionuclides. The most important radionuclides are thus anionic species and long-lived actinides that often form large, complex aqueous species, both of which are not strongly sorbed by cation exchangers such as zeolites. However, zeolites play an important role in a repository at Yucca Mountain for several other reasons. Emplacement of radioactive waste will change the temperature and [MATHEMATICAL EXPRESSION CANNOT BE REPRODUCIBLE IN AS II] conditions in large volumes of rock, and zeolites will be important sources and sinks of water and thermal energy during heating and cooling in the vadose zone. In addition, dehydration/hydration reactions are accompanied by volumetric changes that can affect macroscopic rock properties. Longer-term reactions, e.g., of clinoptilolite to analcime, would give rise to larger volume reductions and production of water and silica. These studies show that the importance of zeolites extends far beyond cation exchange by zeolites to phenomena affecting the entire thermohydrologic system.

An isosynmetric displacive structural transformation in the hydrous layer silicate dickite [[Al.sub.2][Si.sub.2][O.sub.5]([OH).sub.4], monoclinic Cc, a = 5.161(3), b = 8.960(6), c = 14.459(10) [Angstrom], [beta] = 96.77(1)[degrees]], occurring under hydrostatic compression above 2.0 GPa, has been studied using single-crystal X-ray diffraction and diamond-anvil cell techniques. The structure of the high-pressure phase, determined in situ, is monoclinic with space group Cc with unit-cell parameters a = 5.082(3), b = 8.757(6), c = 13.771 (9) [Angstrom], and [beta] = 89.60(2)[degrees] at 4.1 GPa. The positions of all hydrogen atoms at both ambient and high pressure have been determined by a combination of simulated annealing and energy minimization. The mechanism of the transformation, which may be general for other hydrous layered materials, involves a shift of the 1:1 layers with respect to each other by the vector [1/6, 1/6, 0] and is accompanied by the formation of new hydrogen bonds.

The presence of water-bearing minerals on Mars has long been discussed, but little or no data exist showing that minerals such as smectites and zeolites may be present on the surface in a hydrated state (i.e., that they could contain [H.sub.2]O molecules in their interlayer or extraframework sites, respectively). We have analyzed experimental thermodynamic and X-ray powder diffraction data for smectite and the most common terrestrial zeolite, clinoptilolite, to evaluate the state of hydration of these minerals under martian surface conditions. Thermodynamic data for clinoptilolite show that water molecules in its extra-framework sites are held very strongly, with enthalpies of dehydration for Ca-clinoptilolite up to three times greater than that for liquid water. Using these data, we calculated the Gibbs free energy of hydration of clinoptilolite and smectite as a function of temperature and pressure. The calculations demonstrate that these minerals would indeed be hydrated under the very low-P ([H.sub.2]O) conditions existing on Mars, a reflection of their high affinities for [H.sub.2]O. These calculations assuming the partial pressure of [H.sub.2]O and the temperature range expected on Mars suggest that, if present on the surface, zeolites and Ca-smectites could also play a role in affecting the diurnal variations in martian atmospheric [H.sub.2]O because their calculated water contents vary considerably over daily martian temperature ranges. The open crystal structure of clinoptilolite and existing hydration and kinetic data suggest that hydration/dehydration are not kinetically limited. Based on these calculations, it is possible that hydrated zeolites and clay minerals may explain some of the recent observations of significant amounts of hydrogen not attributable to water ice at martian mid-latitudes. Keywords: Mars surface; Mineralogy; Geochemistry

Isothermal vapor sorption experiments under controlled partial pressures of [H.sub.2]O (between 0.1 and 30 mbar, at 23.4, 30.1, 49.5, 64.5, and 79.3 [degrees]C) and liquid water immersion calorimetry experiments at 25.0 [degrees]C were conducted to determine standard molar thermodynamic properties of hydration of the W1 and W5 sites in laumontite that host hydrogen-bonded water. A Langmuir adsorption model was used in a thermodynamic analysis of the isothermal adsorption data for the W5 site together with a symmetrical regular solution model. Resulting values for the standard molar Gibbs energy and entropy of hydration of the W5 site relative to liquid water are -8430 [+ or -] 113 J/mol and -16.7 [+ or -] 2.1 J/(mol*K), respectively, and the Margules parameter, [W.sub.G], is 1590 [+ or -] 63 J/mol. The standard enthalpy of hydration of the W1 site was determined by liquid-water immersion calorimetry experiments on laumontite containing vacant W1 and fully occupied W5, W2, and W8 sites. Discontinuous hydration and dehydration of W1 at 23.4 [+ or -] 0.7 [degrees]C and 24 [+ or -] 1 mbar [P.sub.[H.sub.2]O] was used to constrain the molar Gibbs energy of hydration of this site. Resulting values for standard molar Gibbs energy of hydration and enthalpy of W1 relative to liquid water are -380 [+ or -] 170 and -8800 [+ or -] 1150 J/mol, respectively. Isothermal adsorption at 23.4 [degrees]C and isobaric thermogravimetric experiments indicate that during dehydration of W1, only 0.83 moles of water are released from the crystal structure and 0.17 moles are relocated to a disordered site that has energetic properties similar to the W8 site. Calculations using the thermodynamic data determined in this study indicate that the water content of laumontite in equilibrium with liquid water ranges from ~4.5 [H.sub.2]O per 12 framework O atoms at room temperature and one bar pressure to ~3.5 [H.sub.2]O at 250 [degrees]C and at liquid-vapor saturation pressure for water.

The response of the laumontite crystal structure to hydration/dehydration was evaluated using Rietveld refinements with XRD data collected under controlled [P.sub.[H.sub.2]O] conditions at ~28.5[degrees]C. Refined water contents per unit cell (unit-cell formula: [Ca.sub.4][Al.sub.8][Si.sub.16][O.sub.48]*n[H.sub.2]O) ranged between 12.5 [H.sub.2]O at 0.11 mbar [P.sub.[H.sub.2]O] and 17.3 [H.sub.2]O at 37.6 mbar. The occupancy of the two water sites hosting hydrogen-bonded water molecules, W5 and W1, ranged from 13% to 100% and from 2% to 86%, respectively. During hydration of W5, between 0.11 and 5 mbar, the unit cell expanded continuously and reversibly from 1327 to 1348 [[Angstrom].sup.3]. The unit-cell volume remained nearly constant between 5 and 28 mbar. The hydration/dehydration of W1 exhibited hysteresis; hydration occurred at ~29 mbar and dehydration at ~24 mbar. During hydration of W1 at ~29 mbar the unit cell expanded from 1351 to 1384 [[Angstrom].sup.3]. Further hydration of W1 above 29 mbar resulted in gradual and reversible unit-cell expansion to 1386 [[Angstrom].sup.3] at 37.6 mbar. Hydration/dehydration of W5 is a continuous reaction typical for zeolites. In contrast, the hydration/dehydration of W1 at room temperature is discontinuous, as manifested by the presence of two laumontite phases during hydration and dehydration. Unit-cell parameters of the two coexisting laumontite phases observed under these conditions are consistent with a vacant W1 site and ~80%-occupied W1 site, respectively. Gradual unit-cell expansion above 29 mbar due to increased [P.sub.[H.sub.2]O] and increased occupancy of W1 indicate that hydration of the remaining 20% of the W1 site proceeds continuously.

Water-vapor sorption isotherms, relative humidity (RH) controlled powder X-ray diffraction (XRD) data, and new and previously published multi-nuclear NMR spectroscopic data for a wide range of layered double hydroxides (LDHs) provide greatly increased understanding of the effects of hydration state on the structure and dynamical behavior of interlayer and surface anions and the factors controlling the expansion behavior of this group of minerals. Li,Al and Mg,Al LDH phases containing S[O.sup.2-.sub.4], Se[O.sup.2-.sub.4], P[O.sup.3-.sub.4], HP[O.sup.2-.sub.4], Mo[O.sup.2-.sub.4], Cl[O.sup.-.sub.4], Se[O.sup.2-.sub.3], C[O.sup.2-.sub.3], [F.sup.-], [Cl.sup.-], [Br.sup.-] [I.sup.-], O[H.sup.-], and N[O.sup.-.sub.3] were examined. The phases studied can be grouped into three types based on basal spacing expansion, water sorption, and interlayer anion dynamics: Type 1, significantly expandable (1.5-3.0 [Angstrom]); Type 2, slightly expandable (expansion

Raman spectra were obtained from the (001), (010), and (100) faces of a St. Claire dickite specimen of known orientation. Raman spectra collected from the (010) and (100) faces of dickite are reported for the first time and reveal vibrational features significantly different from the (001) spectra. Variations in intensities of the v(OH) bands in polarized spectra were used to confirm previous band assignments, to determine the shape and orientation of the local Raman tensors for the OH1 and OH3 groups. The most striking polarization effect observed in the v(OH) region of dickite was the behavior of the a(c'c')[bar]a spectrum relative to Raman and IR spectra of other orientations. Unlike previously reported spectra, the dominant feature in this spectrum was the 3643 [cm.sup.-1] band. This large increase in intensity of the 3643 [cm.sup.-1] band in comparison with the other v(OH) bands was related to the fact that Raman spectra were recorded from the edge faces of dickite with the electric vector of the incident laser polarized along the c axis. This permitted observation of vibrational modes polarized along the c axis. Raman frequencies of the v(OH) bands assigned to the OH2 and OH4 groups differ from their IR counterparts by 12 [cm.sup.-1], suggesting that these groups may be related by a center of symmetry. For comparison, Raman spectra in the v(OH) region were also obtained from individual micro-crystals of kaolinite that were approximately 5 [micro] m across the (001) face.

A thermodynamic formulation for the sorption of [H.sub.2]O in clinoptilolite has been obtained from analysis of equilibrium data collected by thermogravimetry on near end-member Ca-, Na-, and K-exchanged natural clinoptilolite (Fish Creek Mountains, Nevada). Temperature and pressure of the experiments ranged from 25 to 250 [degrees] C and 0.2 to 35 mbar [H.sub.2]O vapor pressure. Equilibrium of three clinoptilolite species was successfully formulated with the following expression for the Gibbs free energy of hydration as a function of temperature and pressure: [Mathematical Expression Omitted] where R is the gas constant, P is the vapor pressure of [H.sub.2]O, [W.sub.1] and [W.sub.2] are the excess mixing parameters, and [Theta] is the ratio [H.sub.2]O/(maximum [H.sub.2]O) with maximum water contents for the K, Na, and Ca end-members of 13.49, 15.68, and 16.25 wt%, respectively. The molar Gibbs free energy of hydration for calcium, sodium, and potassium clinoptilolite is -36.13 [+ or -] 3.02, -29.68 [+ or -] 3.77, and -25.53 [+ or -] 1.37 kJ/mol [H.sub.2]O, respectively. The molar enthalpy of hydration for these phases is -76.92 [+ or -] 2.88, -74.19 [+ or -] 3.46, and -67.78 [+ or -] 1.25 kJ/mol [H.sub.2]O. The thermodynamic formulation is applied to the occurrence of clinoptilolite at Yucca Mountain, Nevada, where the proposed emplacement of nuclear waste would lead to heating of clinoptilolite-bearing tuffs. Rock units with abundant clinoptilolite (or by analogy other hydrous phases) would remain significantly cooler than units with anhydrous minerals and would evolve a substantial volume of water.

Opal-CT x-ray diffraction patterns may be projected using mathematical models based on a recursive calculation method. The model is based on a hypothetical trigonal silica sheet which was then developed models of interstratified layers of cristobalite and tridymite. The new method predicts particle size, allows for intergrowths of ordered and disordered planar units, and can yield data about fine structure from weak bandwidths. Results compared well with observed x-ray diffraction measures.

The Rietveld method is used for the quantitative phase analysis of a number of multicomponent standard and natural mineral compositions, hematite, ilmenite, magnetite, biotite, analcime, mordenite, clenoptiolite and quartz with binary mixtures of corundum are investigated using digital powder X-ray diffraction data. Refined individual scale filters and unit-cell volumes yield the quantitative information. The Rietveld refinement program, modified to investigate the ten phases, possesses considerable advantage over other traditional methods.

Silicon neutron magnetic resonance spectroscopy and neutron powder diffraction analysis of fibrolitic sillimanite samples from Pays de Leon, France, is presented. The results revealed partial disorder in the sillimanite tetrahedral double chains with silicon-aluminum ordering. However, the effects of chain disorder are insignificant when compared with the normal site disorder. The study also demonstrates the added advantages of the analysis of both the short range and long range characteristics of the mineral structure.

Neutron powder diffraction data and the Rietveld method were used to refine the crystal structure of fibrolitic sillimanite. Lattice and structural parameters of the refined structure were found to correlate with previously reported data for sillimanite. The fibrolite was further observed to have tertrahedral site occupancies. A small amount of isotropic strain broadening was also seen. Results also indicate that the observed tetrahedral disorder is not caused by an equilibrium disordering process.

Author(s): David T. Vaniman (corresponding author) [1]; David L. Bish [2]; Steve J. Chipera [1]; Claire I. Fialips [1]; J. William Carey [1]; William C. Feldman [3] Recent reports of [...]

The interaction of hydrazine with kaoline was studied using a combination of infrared (IR) spectroscopy, inelastic neutron scattering (INS), and X-ray powder diffraction. The combined application of crystallographic methods has provided new insights into the structure and bonding of the KH intercalation complex.