Volume 85, Issue 1

Mineral assemblages and microstructures of a newly identified (retrogressed) eclogite (sensu lato) in the North Dabie Complex (NDC), central-eastern China, indicate early very high-pressure metamorphic relics preserved in a dominant amphibolite-facies host, where no eclogite (sensu stricto) has been positively identified before. The investigated eclogitic rock shows distinct multistage recrystallization, with granulite- and amphibolite-facies assemblages overprinting eclogite-facies relics. The minimum temperature for the eclogite-facies metamorphism is estimated to be ~800-820 °C. A spectacular microstructure of oriented quartz needles (~2-20 μm wide, ~5-200 μm long) in matrix Ca-Na clinopyroxene implies the prior existence of a non-stoichiometric “supersilicic” omphacite stabilized at ultrahigh-pressure (UHP, ³25 kbar) conditions, although no coesite or coesite pseudomorphs have been found in the samples. The absence of coesite may be due to the lack of free silica at UHP conditions or the consumption of silica during retrograde reactions. The inferred UHP conditions metamorphism is further supported by Sm-Nd ages that are equivalent to Triassic metamorphic ages from UHP eclogites in the southeastern Dabie Mountains. This finding expands the UHP terrane northward about 50 km; spatial distribution of subduction/collision-related UHP rocks includes parts of the NDC.

A kyanite-sillimanite isograd is well-exposed in the Mica Creek area, British Columbia, and both pelitic and basic rocks occur within a few hundred meters of the isograd. This provides us with an opportunity to test geothermobarometry in pelitic and basic bulk compositions against an independent metamorphic equilibrium, kyanite-sillimanite. The kyanite-sillimanite isograd is modeled by a univariant P-T curve that passes through the following points (T °C, P kbar): 564/5; 610/6; 656/7; and 796/10. The quoted experimental uncertainty is 25 °C or about 350 J in ΔG. Samples that bracket the kyanite-sillimanite isograd in the Mica Creek area have the following mineral assemblages: quartzmuscovite- biotite-garnet-plagioclase-kyanite or sillimanite in pelitic rocks; and hornblende-plagioclase- clinopyroxene-garnet-quartz in basic rocks. Using the thermodynamic database of Berman (1988) and the INVEQ program of Gordon (1992), we have calculated metamorphic pressures and temperatures for 17 samples that bracket the kyanite-sillimanite isograd. For the basic rocks, the P-T estimate fell entirely within the kyanite stability field, even for samples containing sillimanite, and the 68.3% confidence region overlapped the ±25 °C uncertainty in the kyanite-sillimanite P-T curve. For the pelitic rocks, the P-T estimate and the 68.3% confidence region usually fell within the sillimanite stability field and the confidence region overlapped the ±25 °C uncertainty in the kyanite-sillimanite P-T curve.

The microstructures of magnetite crystals in three samples from a single outcrop of granulitefacies marble were characterized by transmission electron microscopy (TEM) to determine how exsolution history can affect physical properties and mineral reactivity during retrograde metamorphism. The microstructure of sample 90LP9 consists of dislocations, dislocation arrays (with dislocation spacings of 100 to 500 nm), and linear channels filled with layer silicates. Acid etching and ion milling of polished 90LP9 magnetite grains show dislocation arrays clustered near grain boundaries with rheologically hard magnetite, diopside, and monticellite, but rarely near grain boundaries with softer calcite. Sample LP204-1 magnetite grains contain coherent {100} Al-Mn-Fe-spinel precipitates (~40 nm diameter, ~1-3 nm thick, ~10 4 platelets/μm 3 ) and very few dislocations. Larger, more widely spaced spinel precipitates are present in a third sample, 94AK3. Extremely low dislocation densities in powders of LP204-1 and intermediate dislocation densities in powders of 94AK3, compared with extremely high dislocation densities in powders of 90LP9, are related to strong dislocation pinning effects by precipitates. The different exsolution behavior of the three magnetite samples is attributed to small, but important, differences in Al content. Because dislocations can provide fast pathways for exchange that enhance diffusion, especially in very slowly cooled rocks, these microstructural results may explain previously reported subgrain-scale oxygen isotopic heterogeneity in 90LP9 magnetite compared with relative isotopic homogeneity in LP204-1 magnetite

Monazite, a typical light rare-earth element (LREE) mineral of S-type granitoids in the Western Carpathians, was found in the peraluminous biotite granodiorite-tonalite in the Tribeč Mountains commonly containing polymineralic inclusions. These inclusions are dominated by anhedral allanite, although allanite also occurs rarely as discrete grains not enclosed by monazite. The monazite studied here is relatively homogeneous and characterized by high Th contents with proportions of huttonite (ThSiO 4 ) and brabantite [CaTh(PO 4 ) 2 ] up to 14.6 and 9.3%, respectively. The discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock. However, allanite included in monazite is extremely variable in LREE, Al, Fe, and Mg contents. This variation is interpreted to result from entrapment of allanite (+ melt) in monazite before local equilibrium was attained. The change from allanite to monazite as the stable LREE-rich phase is related to an overall decrease in Ca concentration caused by the onset of plagioclase crystallization. The early precipitation of allanite was possible because of the high LREE concentrations in the melt. The crystallization temperature of allanite must have been higher than monazite saturation (>856-845 °C and 798-790 °C for two analyzed samples). The Zr saturation temperature based on zircon solubility and REE thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton, which coincides with an increase in the huttonite content in monazite. The primary LREE assemblage is accompanied by small grains of late huttonite(?) replacing monazite and brabantite replacing allanite.

The sessile drop method has been used for measurements of the surface tension of haplogranite (HPG) melts containing an excess of alkalis and phosphorous (HPG8, HPG8 + 5 wt% Li 2 O, 5 wt% Na 2 O, 20 wt% Na 2 O, 5 wt% K 2 O, 5 wt% Rb 2 O, 5 wt% Cs 2 O, 10 wt% P 2 O 5 ) and of Armenian rhyolite in the temperature interval, 650-1665 °C, and at 1 bar pressure. Sessile drops were placed on graphite substrates in a Pyrox tube furnace purged with Ar. Drop shape was monitored with a videocamera and stored in a videorecorder. The surface tension was calculated by measuring the two principal radii of curvature of the drop shape in vertical cross section. The precision of the method was checked against the surface tension of water. The surface tension of HPG and rhyolite melt is ~280-300 ± 5 mN/m in the temperature interval 1200-1400 °C. Temperature dependence of the surface tension of haplogranite melts and rhyolite is weak and positive (dσ/dT = 0.06 to 0.09 mN/m/°C). Addition of 5 wt% of alkali oxides (except Li 2 O) results in a decrease of the surface tension of haplogranite melts. The HPG melts with 10 wt% P 2 O 5 have 30% higher surface tension than haplogranite melts with excess alkalis, and a negative temperature derivative (dσ/dT = -0.1 mN/m/°C). The HPG melts with 20 wt% Na 2 O and 5 wt% Li 2 O exhibit a decrease in surface tension with temperature (dσ/dT = -0.02 and -0.10 mN/m/°C, respectively). The surface tension of HPG8 melt saturated with water at 1-4 kbar was measured on sessile drops quenched at high pressure in an internally heated gas vessel at temperatures of 800-1200 °C. Water pressure significantly decreases the surface tension of melt from 270 mN/m at 1 bar (1000 °C) to 65 mN/m at 4 kbar. At 1 bar in “dry” conditions, dσ/dT = +0.056 mN/m/°C and at 3 kbar of water pressure, dσ/dT = +0.075 mN/m/°C. The decrease in the surface tension of HPG melt at a water pressure of several kbars is from -10 to -30 mN/m/wt% H 2 O. The increase of water content to more than 10 wt% in granite melts may not result in any significant decrease in the surface tension, which may be explained by formation of a surface sublayer having physical properties very distinct from those of the bulk.

Although recent thermodynamic models predict a low-temperature miscibility gap in the pyropealmandine- grossular garnet solid-solution series, coexistence of two immiscible garnets consistent with the calculated miscibility gap has not been demonstrated previously. Here we report the discovery of such a coexistence in nature, which provides direct evidence for immiscibility in the pyropealmandine- grossular system. Garnet inclusions were found near solidified fluid/melt inclusions in a single pyrope crystal collected from the ultramafic diatreme at Garnet Ridge, Arizona. Microprobe analyses close to the contact between the pyrope host and one garnet inclusion yield formulae of (Mg 1.98 Ca 0.52 Fe 0.50 Mn 0.02 )(Al 1.89 Cr 0.09 )Si 3 O 12 for the host and (Mg 1.28 Ca 1.31 Fe 0.41 Mn 0.02 )(Al 1.93 Cr 0.07 )Si 3 O 12 for the inclusion. High-resolution analytical traverses across the two-garnet boundary reveal that the compositional difference between the inclusion and the host increases as the contact is approached, implying that the two garnet grains represent an immiscible pair rather than a disequilibrium coexistence. The compositions of the coexisting garnets are in good agreement with those predicted from recent thermodynamic mixing models for pyrope-almandine-grossular garnet.

Cluster analysis has the potential to quantify the size and characteristics of groups or clusters of grains and crystals within a rock texture. We present two techniques: (1) the complete linkage hierarchical cluster analysis (CLHCA) technique that we use to analyze grain-center distributions, and (2) the density linkage cluster analysis (DLCA) technique that we use to analyze grain boundary distributions. CLHCA applies a cluster algorithm to the distance matrix calculated from grain-center coordinates. This produces a hierarchical cluster distribution that is characteristic of the spatial pattern of the texture. This cluster distribution is normalized to that of a random distribution of points to identify clustered and ordered patterns in the data. The technique successfully identified 3-D cluster sizes from 2-D sections of a modal data set. In the application to real rock textures, the CLHCA of a komatiite cumulate identified a glomerocryst population with cluster sizes in the range of 0.3-2.5 mm, and also a variation of cluster patterns up through the cumulate pile. The CLHCA of a polymodal texture from a pyroxene-scapolite-sphene granulite indicated a clustered distribution for the sphene, which was restricted in growth pattern by the pyroxene and scapolite in the texture. DLCA computes a single solution of clusters that conforms to the search criteria input into the algorithm. DLCA performed poorly when used to identify distributions of large, randomly packed clusters of small spheres, mainly identifying smaller cluster patterns within each large cluster. Application of DLCA to the komatiite cumulate example indicated cluster lengths of 0.72-0.80 mm similar to the CLHCA results, and an average aspect ratio of 1.52.

Experimental investigations in the system albite-water have been carried out to evaluate the accuracy of a new procedure to determine major-element solubilities in fluid phases and water solubilities in melts. The system albite-water had been examined previously and some consensus has been achieved with respect to phase relations, solidi, and water solubilities in the melts. Above 15 kbar the system approaches a second critical end point, where a distinction between fluid and melt no longer can be made and the term solidus has to be reconsidered. All experimental runs were carried out at near-solidus conditions at 5-17 kbar and 625-775 °C. Experimental charges contained albite, water, and a layer of diamond crystals (grain size 50 mm). The pore space between the diamonds is preserved during the run indicating that the fluid was able to circulate throughout the entire capsule. During quenching, the material dissolved in the fluid precipitated between the diamond crystals and, as a result, could so be separated from the solid residue. The recovered capsules were directly embedded in epoxy and the diamond layers were analysed by laser ablation microprobe (LAM-ICP-MS). The new method allows the determination of a broad range of silicate/water ratios in aqueous fluids and hydrous melts. In some experimental runs at 15 and 17 kbar, fluids with approximately 40 wt% solute could be trapped leading to the conclusion that supercritical conditions were reached. Results furthermore indicate that albite dissolves non-stoichiometrically at 5 kbar producing a higher Na/Al and Si/Al in the solute than in the melt. Dissolution and melting at 10 kbar and higher pressures appears to be congruent within the limits of this method.

Different estimation methods have been combined with calorimetric data for a natural Mg-rich tourmaline and dumortierite, a structurally similar borosilicate, to estimate thermodynamic properties of five tourmaline end-members (dravite, schorl, alkali-free dravite, aluminofoitite, and ferrifoitite). The results were tested by predicting the solubility of Fe-rich tourmaline in natural, low-variance hydrothermal mineral assemblages, for which the B concentration was independently known from Laser Ablation-ICP-MS analysis of coexisting fluid inclusions. Agreement within the uncertainty of experimental and thermodynamic data supports the validity of the estimation methods that used the fictive properties of the III B 2 O 3 polyhedron for calculating standard enthalpy and entropy, and reactions involving dumortierite to estimate the heat-capacity functions. This study indicates more generally that the development of new microanalytical techniques for measuring the composition of natural fluid inclusions in well-constrained mineral assemblages represents a promising approach to estimating or testing thermodynamic data for complex silicates.

Accurate determination of F in minerals is a difficult task even when high F concentrations are present. Fluorine usually is determined by means of electron micro-probe analysis (EMPA) standardized on non-silicate-matrix compounds (e.g., fluorite), and some previous work has revealed the difficulties in determining F at high concentrations such as found in the humite-group minerals. Moreover, when both single-crystal structure refinement (SREF) and EMPA are available for the same crystal, the two estimates do not always agree. On the other hand, the secondary ion mass spectrometry (SIMS) technique is not easily applied at high F concentrations due to the existence of matrix effects related to the chemical composition and structure of the sample as well as to the concentration of the element itself. We tested the agreement among these analytical techniques in the estimation of high F contents and propose an analytical procedure for the analysis of fluorine. Our results indicate that careful selection of working conditions for EMPA of F together with appropriate correction, can yield accurate fluorine concentrations in minerals. Fluorine data extracted from refined site occupancies are systematically overestimated. New accurate working curves have been worked out for SIMS analysis of F taking Si and Mg, in turn, as the reference element for the matrix. Humite-group minerals show SIMS matrix effects on the order of ~10%. In analyzing fluoborite in the most unfavorable cases, the difference in Ion Yield (F/Mg) between “disoriented” humite-group minerals and “oriented” fluoborite samples can reach ~27%. Finally, a lower than expected IY(F/Si) from the F/Si working curve (made with humite minerals) is shown by topaz, which can be ascribed to chemical matrix effects, as well as to the covalent-type bonding between F and the major element in the matrix (Al).

The crystal structure of fluoborite [Mg 3 F 3 (BO 3 )] was refined by Dal Negro and Tadini (1974) who provided a complete structural model. Previously, Takeuchi (1950) had refined an OH-dominant fluoborite (OH ~70%), but the limited quantity of data (extracted from two Weissenberg-Buerger photographs) did not permit the location of H atoms. Dal Negro and Tadini (1974) also could not locate H atoms because they used a crystal with near end-member composition. We have located the H bond in an OH-dominant fluoborite from the Betic Cordilleras (SE Spain). Excellent quality X-ray data on two crystals of fluoborite allowed discovery and refinement of the H position in this mineral. Electron microprobe (EMP) and secondary-ion mass spectrometry (SIMS) analyses of the light elements H, B, and F have resulted in the formulation of special procedures to obtain accurate, highquality quantitative data, which are presented in this paper. EMP, SIMS, and crystal structure refinement (SREF) data are in a good agreement. Linear equations are also presented to calculate the F content directly from cell parameters.

The structure of glasses in the K 2 O-Al 2 O 3 -TiO 2 -SiO 2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent the addition of Al 2 O 3 to a base of composition K 2 TiSi 4 O 11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al 2 O 3 . We infer depletion of fivefold-coordinated titanium (possibly as alkali titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V] Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation.

The primary objective of this study is to determine the effect of substrate type on the coordination environments of Cu 2+ adsorbed on amorphous SiO 2 , γ-Al 2 O 3 , and anatase at a surface coverage of approximately 1 mmol/m2. We also collected X-ray absorption fine structure (XAFS) data for several Cu 2+ -containing model compounds, including tenorite ( VI CuO), spertiniite [ VI Cu(OH) 2 ], dioptase ( VI CuSiO 2 ·H 2 O), shattuckite [ VI Cu 5 (SiO 3 ) 4 (OH) 2 ], chrysocolla [ VI (Cu,Al) 2 H 2 Si 2 O 5 (OH) 4 ·nH 2 O], and Cu 2+ acetate monohydrate [ VI Cu(CH 4 CO 2 )2·H 2 O], for comparison with the sorption sample data. Detailed analysis of these model compounds indicates that the bonding of second neighbors surrounding a central Cu absorber determines whether these second neighbors can be detected by XAFS. The XAFS results of Cu 2+ sorption samples are consistent with the presence of Jahn-Teller distorted Cu 2+ (O,OH) 6 octahedra, with four equatorial Cu-O bonds (1.95 Å) and two longer axial bonds; the axial Cu-O bonds are difficult to characterize quantitatively by XAFS spectroscopy. Cu 2+ sorbed on amorphous SiO 2 was found to have Cu second and third neighbors at 2.95 Å, 3.30 Å, and 5.72 Å, but no Cu-Si correlation was detected for these sorption products associated with amorphous SiO 2 . Based on XAFS and wet chemical results, it seems likely that a Cu(OH) 2 precipitate has formed in the Cu 2+ /amorphous SiO 2 system. Cu 2+ sorbed on γ-Al 2 O 3 is present as a mixture of monomeric, dimeric, and perhaps a small number of oligomeric hydroxo-bridged Cu(O,OH) 6 species with a Cu-Cu distance of approximately 2.95 Å. Sorbed Cu 2+ on anatase is present predominantly as hydroxo-bridged Cu dimers. At similar sorption densities, Cu 2+ cluster sizes on amorphous SiO 2 are significantly larger than those on g-Al 2 O 3 or anatase, indicating that the substrate has an important effect on the type of Cu 2+ sorption complex or precipitates formed.

The crystal chemistry of Fe in four nontronites (Garfield, Panamint Valley, SWa-1, and NG-1) was investigated by chemical analysis, X-ray goniometry, X-ray absorption pre-edge spectroscopy, powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) spectroscopy, and X-ray diffraction. The four reference nontronites have Fe/(Fe + Al + Mg) ratios ranging from 0.58 to 0.78, and are therefore representative of the different chemical compositions of dioctahedral ferruginous smectites. Pre-edge and powder EXAFS spectroscopy indicate that NG-1 contains 14 to 20% of tetrahedrally coordinated Fe 3+ , whereas the other three samples have no detectable IV Fe 3+ . The partitioning of VI Fe 3+ between cis (M2) and trans (M1) sites within the octahedral sheet was determined from the simulation of X-ray diffraction patterns for turbostratic nontronite crystallites by varying the site occupancy of Fe. Based on this analysis, the four nontronite samples are shown to be trans-vacant within the detection limit of 5% of total iron. The in-plane and out-of-plane local structure around Fe atoms was probed by angular P-EXAFS measurements performed on highly oriented, self-supporting films of each nontronite. The degree of parallel orientation of the clay layers in these films was determined by texture goniometry, in which the half width at half maximum of the deviation of the c* axis of individual crystallites from the film plane normal, was found to be 9.9° for Garfield and 19° for SWa-1. These narrow distributions of orientation allowed us to treat the self-supporting films as single crystals during the quantitative analysis of polarized EXAFS spectra. The results from P-EXAFS, and from infrared spectroscopy (Madejova et al. 1994), were used to build a two-dimensional model for the distribution of Fe, and (Al,Mg) in sample SWa-l. In this nontronite, Fe, Al, and Mg atoms are statistically distributed within the octahedral sheet, but they exhibit some tendency toward local ordering. Fe-Fe and (Al, Mg)-(Al,Mg) pairs are preferentially aligned along the [010] direction and Fe-(Al,Mg) pairs along the [31̅0], and [3̅1̅0] directions. This distribution is compatible with the existence of small Fe domains separated by (Al,Mg), and empty octahedra, which segregation may account for the lack of magnetic ordering observed for this sample at low temperature (5K) (Lear and Stucki 1990).

The crystallochemical structure of reduced Garfield nontronite was studied by X-ray absorption pre-edge and infrared (IR) spectroscopy, powder X-ray diffraction, polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy, and texture goniometry. Untreated and highly reduced (>99% of total Fe as Fe 2+ ) nontronite samples were analyzed to determine the coordination number and the crystallographic site occupation of Fe 2+ , changes in in-plane and out-of-plane layer structure and mid-range order between Fe centers, and to monitor the changes in structural and adsorbed OH/H 2 O groups in the structure of reduced nontronite. Contrary to earlier models predicting the formation of fivefold coordinated Fe in the structure of nontronites upon reduction, these new results revealed that Fe maintains sixfold coordination after complete reduction. In-plane PEXAFS evidence indicates that some of the Fe atoms occupy trans-sites in the reduced state, forming small trioctahedral domains within the structure of reduced nontronite. Migration of Fe from cisto trans sites during the reduction process was corroborated by simulations of X-ray diffraction patterns which revealed that about 28% of Fe 2+ cations exist in trans sites of the reduced nontronite, rather than fully cis occupied, as in oxidized nontronite. Out-of-plane P-EXAFS results indicated that the reduction of Fe suppressed basal oxygen corrugation typical of dioctahedral smectites, and resulted in a flat basal surface which is characteristic of trioctahedral layer silicates. IR spectra of reduced nontronite revealed that the dioctahedral nature of the nontronite was lost and a band near 3623 cm -1 formed, which is thought to be associated with trioctahedral [Fe 2+ ] 3 OH stretching vibrations. On the basis of these results, a structural model for the reduction mechanism of Fe 3+ to Fe 2+ in Garfield nontronite is proposed that satisfies all structural data currently available. The migration of reduced Fe ions from cis-octahedra to adjacent trans-octahedra is accompanied by a dehydroxylation reaction due to the protonation of OH groups initially coordinated to Fe. This structural modification results in the formation of trioctahedral Fe 2+ clusters separated by clusters of vacancies in which the oxygen ligands residing at the boundary between trioctahedral and vacancy domains are greatly coordination undersaturated. The charge of these O atoms is compensated by the incorporation of protons, and by the displacement of Fe 2+ atoms from their ideal octahedral position toward the edges of trioctahedral clusters, thus accounting for the incoherency of the Fe-Fe1 and Fe-Fe2 distances. From these results, the ideal structural formula of reduced Garfield nontronite is Na 1.30 [Si 7.22 Al 0.78 ] [Fe 2+ 3.65 Al 0.32 Mg 0.04 ]O 17.93 (OH) 5 in which the increased layer charge due to reduction of Fe 3+ to Fe 2+ is satisfied by the incorporation of protons and interlayer Na.

We report here the first nuclear magnetic resonance (NMR) spectroscopic study of the dynamical and structural behavior of nitrate on the surface and in the interlayer of hydrotalcite-like compounds ( 15 NO 3 - -HT). Spectroscopically resolvable surface-absorbed and interlayer NO 3 - have dramatically different dynamical characteristics. The interlayer nitrate shows a well defined, temperature independent uniaxial chemical shift anisotropy (CSA) powder pattern. It is rigidly held or perhaps undergoes rotation about its threefold axis at all temperatures between -100 °C and +80 °C and relative humidities (R.H.) from 0 to 100% at room temperature. For surface nitrate, however, the dynamical behavior depends substantially on temperature and relative humidity. Analysis of the temperature and R.H. dependences of the peak width yields reorientational frequencies which increase from essentially 0 at -100 °C to 2.6 × 10 5 Hz at 60 °C and an activation energy of 12.6 kJ/mol. For example, for samples at R.H. = 33%, the surface nitrate is isotropically mobile at frequencies greater than 10 5 Hz at room temperature, but it becomes rigid or only rotates on its threefold axis at -100 °C. For dry samples and samples heated at 200 °C (R.H. near 0%), the surface nitrate is not isotropically averaged at room temperature. In contrast to our previous results for 35 Cl - -containing hydrotalcite ( 35 Cl - -HT), no NMR detectable structural phase transition is observed for 15 NO - 3 -HT. The mobility of interlayer nitrate in HT is intermediate between that of carbonate and chloride.

By combining data from oxide melt solution calorimetry and 29 Si MAS-NMR spectroscopy, the enthalpy for short-range Si,Al ordering in b-eucryptite (LiAlSiO 4 ) was determined for a series of samples prepared by glass annealing. Si,Al ordered b-eucryptite gives a single 29 Si NMR peak for two unresolved crystallographic sites, but two distinct 27 Al resonances that differ in both chemical shift and quadrupolar coupling. Samples of β-eucryptite crystallized from glass contain additional 29 Si NMR peaks, indicating significant levels of short-range Si,Al disorder. This disorder decreases exponentially with annealing time at 1173 K, from 0.55(±0.04) Al-O-Al linkages per formula unit after one hour to 0.05(±0.01) after 70.5 hours. The decrease in the concentration of Al-O-Al linkages with annealing time correlates linearly with enthalpies of drop-solution in molten lead borate, giving an enthalpy of ΔH ord = -26 ± 3 kJ/mol for the reaction: Al-O-Al + Si-O-Si → 2(Si-O-Al). Additional NMR results are presented for samples synthesized along the SiO 2 -LiAlSiO 4 join (quartz to b-eucryptite). Solid solution samples with compositions 20 and 69 mol% quartz appear to have very few Al-O-Al linkages (<0.04 per formula unit).

The structure of Fe(OH 0.86 ,OD 0.14 ) 2 was refined by applying the Rietveld technique to neutron powder diffraction data collected at 300, 150, and 10 K. The nuclear structure, of the CdI 2 type (P3̅ m1) and isostructural with Mg, Ca, Mn, Ni, and β-Co(OH) 2 , is maintained over the range of temperatures studied. At 10 K, the magnetic structure is ordered antiferromagnetically, with the magnetic moments aligned in the basal plane. The refined Fe 2+ magnetic moment magnitude is 3.50(4) μB. This magnetic structure (space group P 2c 1̅) is unique amongst those encountered in the transition metal dihydroxides, for which the moment is either directed along the c axis [b-Co(OH) 2 and Ni(OH) 2 ], or at least has a considerable component along this direction [Mn(OH) 2 ]. The dependence of the strength of potential hydrogen bonds with temperature is discussed.

The R3̅ - to R3̅c cation ordering phase transition in the ilmenite (FeTiO 3 ) - hematite (Fe 2 O 3 ) solid solution has been investigated using in-situ time-of-flight neutron powder diffraction. Four synthetic samples of the solid solution containing 70, 80, 90, and 100% FeTiO 3 (ilm70, ilm80, ilm90, and ilm100, respectively) were heated under vacuum to a maximum of 1350 °C. Powder diffraction patterns were collected at several temperatures on heating and cooling, with a Rietveld refinement performed in each case. Samples ilm80, ilm90, and ilm100 were fully ordered after quenching from the synthesis temperature to room temperature. Sample ilm70 had a higher degree of quenched in disorder, which is the result of chemical heterogeneities produced during quenching and subsequent heating in the neutron experiments. The degree of order in all samples decreased smoothly at high temperatures, with second-order transitions to the R3̅c phase being observed at 1000, 1175, and 1325 °C in ilm70, ilm80, and ilm90, respectively. The transition temperature in ilm100 was higher than the maximum temperature reached in the neutron experiments, and is estimated as ~1400 °C. The character of the transition is typical of that predicted by three-dimensional Ising models and appears to become more first-order in character with increasing Ti-content. The temperature-dependence of the cell parameters reveals that components of the spontaneous strain tensor, e 11 and e 33 , are negative and positive, respectively. Little volume strain is associated with long-range ordering. A small negative volume strain due to short-range ordering within the R3̅c phase is identified. The variations in cell parameters and cation-cation distances can be understood in terms of the competing effects of long- and short-range ordering as a function of temperature and composition.

The P-V-T behavior of synthetic lawsonite [CaAl 2 (Si 2 O 7 )(OH) 2 ×H 2 O] and synthetic zoisite [Ca 2 Al 3 (SiO 4 )(Si 2 O 7 )O(OH)] has been determined under high pressures and high temperatures up to 7 GPa and 1000 °C using a MAX 80 cubic anvil high-pressure apparatus. Several experimental runs were carried out for both minerals, first compressing the samples at room temperature and then heating them until a breakdown reaction occurred. Generally, the samples were mixed with vaseline to ensure hydrostatic pressure transmitting conditions, NaCl served as an internal standard for pressure calibration. The pressure data at room temperature were fitted to a Birch-Murnaghan equation of state (K' T₀ assumed to be 4), yielding isothermal bulk moduli K T₀ = 106.7 ± 1.3 GPa for lawsonite and 125.1 ± 2.1 GPa for zoisite, respectively. These values agree well with recent results. The hightemperature high-pressure data were fitted to the high-temperature Birch-Murnaghan equation of state. In contrast to other investigations, (∂K T /∂T) P is slightly negative for lawsonite. The sign is not affected by considering the thermal expansion data at ambient pressure. Similar calculations for zoisite yielded a positive (∂K T /∂T) P without using high-temperature data at atmospheric pressure whereas a negative sign resulted from calculations taking into account the 1 bar thermal expansivity. Calculations of several mineral equilibria involving zoisite and lawsonite with the Berman data set supplemented by the P-V-T data presented here correspond well to experimentally determined pressure stability limits of these phases.

The electronic polarization of oxygen ions has been explicitly incorporated in a shell model to better simulate the structure of calcite and related rhombohedral carbonate minerals. Pair-potentials for Ca 2+ ions and C and O comprising the carbonate molecular ion were simultaneously fitted to experimental lattice, elastic, dielectric, and vibrational data for calcite, and the structure and elastic properties of aragonite. The resulting potential parameters for the CO 3 2- group were then transferred to models for the structures and bulk moduli of the carbonate minerals incorporating Mn, Fe, Mg, Ni, Zn, Co, Cd, and thus a fully consistent set of interaction parameters for calculating the properties of the carbonate minerals was obtained. Defect energies for doping the divalent cations into the calcite structure, and for calcium and carbonate ion vacancies were calculated. In addition, various disorder types for dolomite, including anti-site defects, stacking defects, and the energy related to increasing the Ca/Mg ratio in the dolomite structure were simulated. The theoretical enthalpy for dolomite ordering (34.4 kJ/mol) compares very well with experimental measurements.

The principal refractive indices, 2V, and optical orientation were measured for a series of monovalent and divalent cation-exchanged heulandite samples (Si/Al = 3.19) from Nasik, India, by use of the spindle stage and double variation method. For monovalent cations, Na + , K + , Rb + , and Cs + , the mean refractive indices were 1.4881, 1.4841, 1.4874, and 1.5039, respectively; and for divalent cations, Mg 2+ , Mn 2+ , Cu 2+ , Sr 2+ , and Cd 2+ , the mean refractive indices were 1.4968, 1.5053, 1.4726, 1.5086, and 1.4860, respectively. Lower mean refractive indices correspond to changes in optical orientation from b||Z in the natural sample to b||Y in the K sample to b||X in the Cu sample. Samples exchanged with divalent cations generally have higher channel water contents than samples exchanged with monovalent cations. Water has a high refractivity and samples with high water contents would be expected to have high mean refractive indices. Monovalent samples with high water contents relative to other monovalent samples have low mean refractive indices. Consequently, the refractive indices of the monovalent samples are governed by a factor other than water content. Despite their higher water contents than the monovalent samples, not all divalent samples have higher mean refractive indices than monovalent samples, apparently because full cation exchange did not occur in these samples. Most of the divalent samples exhibit a positive charge deficit based on calculations made from microprobe analyses and crystal structure refinements. The low pH of the exchange solutions and the low mean refractive indices for most of the divalent samples in this study support the claim that H+ exchanged into the framework channels for charge balance.

The discovery of unusually well-crystallized samples of gearksutite allowed determination of its crystal structure. The crystals belong to space group P1̅ , a = 4.940(1), b = 6.810(1), c = 6.978(1) Å; a = 101.12(1), β = 94.86(1), γ = 110.07(1)°; V = 213.43(6) Å 3 ; Z = 2. The structure consists of layers of eightfold-coordinated calcium polyhedra connected through pairs of [Al 2 F 8 (OH) 2 ] octahedra which are oriented identically, similar to those observed in other hydroxyl aluminum fluorides. A comparison with other minerals of similar composition is made. The structure data allowed us to reliably index the powder pattern and to show that the reflection intensities are strongly affected by preferred orientation.

Obertiite is a new amphibole species from Bellerberg, Laccher See district, Eifel, Germany. It occurs with tridymite, fluororichterite, hematite, rutile, aegirine-augite, kinoshitalite, and fluor-apatite in vugs in volcanic rocks, and crystallized from late-stage hydrothermal fluids associated with recent volcanism. Obertiite occurs as pale-pink elongated blades and divergent aggregates. It is brittle, H = 5, D calc = 3.16 g/cm 3 , has a colorless streak, vitreous luster, and does not fluoresce; it has perfect cleavage on {110} and conchoidal fracture. In plane-polarized light, obertiite is slightly pleochroic in shades of pink to red-orange; X ^ a = 2° (in β obtuse), Z = b, Y ^ c = 12° (in b obtuse) with absorption X ~ Y ~ Z. It is biaxial negative, α = 1.643(1), β =1.657(1), γ = 1.670(3), 2V x = 81(1)°, no dispersion visible. Obertiite is monoclinic, space group C2/m, a = 9.776(2), b = 17.919(3), c = 5.292(1) Å, b = 104.05(2)°, V = 899.3(3) Å 3 , Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I,hkl)]: 8.414(10,110), 2.705(7,331,151), 3.390(6,131), 4.467(5,040), 3.117(5,310), 2.531(5,202), 3.255(3,240), 2.577(3,061), 2.163(3,171,261), 4.013(2,111). Analysis by a combination of electron microprobe, SIMS, and crystal-structure refinement gives SiO 2 54.53, Al 2 O 3 0.15, TiO 2 7.75, Fe 2 O 3 2.61, Mn 2 O 3 3.27, FeO 3.36, ZnO 0.08, MgO 14.13, Li 2 O 0.05, CaO 0.52, Na 2 O 9.51, K 2 O 0.98, F 0.55, H 2 O 0.20, O ≡ F -0.23, Ni, Cr, V, Cl not detected, sum 97.46 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is (K 0.18 Na 0.84 )(Na 1.86 Ca 0.08 Fe 2+ 0.06 ) (Mg 3.09 Zn 0.01 Li 0.03 Fe 3+ 0.29 Mn 3+ 0.37 Fe 2+ 0.41 Ti 4+ 0.86 Al 0.03 )Si 8.00 O 22 [(OH) 0.20 F 0.26 O 1.54 ], and is close to the ideal endmember composition Na Na2 (Mg 3 Fe 3+ Ti 4+ ) Si 8 O 22 O 2 . The crystal structure of obertiite was refined to an R index of 2.6% using MoKa X-ray intensity data. The M1 site is split into two subsites along the b axis, M1 and M1A; the M1 site is occupied by Mg, and M1A is occupied predominantly by Ti 4+ and Mn 3+ ; M2 is occupied by Mg, Fe 2+ , and Fe 3+ , and M3 is occupied by Mg. Local bond-valence considerations suggest that O 2- at O3 is linked to Ti 4+ Mg or Mn 3+ Mn 3+ at the adjacent M1 and/or M1A sites, and that OH or F at O3 is linked to MgMg at the adjacent M1 sites.

Kinoshitalite, ideally Ba(Mg) 3 (Al 2 Si 2 )O 10 (OH,F) 2 , was studied in a granulite-facies manganese ore associated with calcium-bearing magnesian tephroite, hausmannite, calcite and manganoan clinochlore, and in a second assemblage together with hausmannite, altered tephroite, manganoan diopside, and calcite. The metamorphic rocks occur enclosed in peridotites of the Semail Ophiolite, Sultanate of Oman and were metamorphosed during ophiolite obduction. Single-crystal X-ray data, collected on an inclusion-free kinoshitalite of Ba 0.99 K 0.06 Na 0.01 (Mg 2.64 Mn 0.31 Al 2.01 Si 2.03 )O 10 (OH 1.61 F-Cl 0.02 ) composition, yielded a C-centered lattice of monoclinic symmetry with a = 5.316(1), b = 9.230(2), c =10.197(2) Å, β = 100.06(1)°, V = 492.6 Å 3 , and Z = 1 characteristic of the 1 M polytype. The structure was refined in two models assuming complete Si, Al ordering in the space groups C2 and C1 - allowing for additional twinning. The data clearly suggest that in spite of the Si/Al ratio of 1 assumption of complete Si, Al ordering can be rejected. Two models, both in agreement with space group C2/m, causing Si, Al disorder, are discussed. Three-dimensional Si, Al disorder would lead to violation of the Al avoidance rule and substantial Ba displacement. The rather well defined Ba position capped by two six-membered rings of Si 3 Al 3 O 18 composition suggests one dimensional disorder where completely Si, Al ordered layers exist parallel to (001). The disorder occurs perpendicular to (001) and may be interpreted as random stacking faults.

The crystal structure of parisite-(Ce) was solved and refined to R = 0.044, Rw = 0.037 using three-dimensional X-ray diffraction data (XRD). In contrast to the putative hexagonal cell, weak XRD maxima on precession films demonstrate that the parisite-(Ce) is monoclinic with a space group C2/c or Cc. The unit cell was refined as a = 12.305(2), b = 7.1053(5), c = 28.250(5) Å, and β = 98.257(14)°. The structure refinement confirmed the space group Cc. Like bastnäsite and synchysite, parisite possesses a (001) layer structure, with layers of (Ca) and (CeF) separated by layers of carbonate groups. The [001] layer stacking sequence is … (Ca), (CO3), (CeF), (CO3), (CeF), (CO 3 ), (Ca), (CO 3 ), (CeF), (CO 3 ), (CeF), (CO 3 )…. The parisite-(Ce) structure can be considered as two portions of the bastnäsite structure connected by a (Ca) layer. The insertion of a (Ca) layer would create long [001]* structure voids in the F columns if the bastnäsite portions stacked hexagonally; therefore, the two bastnäsite portions are offset along [1̅10] by a/6(sin60°) (or [1̅00] in hexagonal cells, by a/3) such that the oxygen atoms of the vertical edges of the carbonate groups occupy the voids. Polytypism results because a bastnäsite portion can shift equally in two possible directions, ±120° from the previous offset vector. The parisite-(Ce) structure elucidated herein is the simplest and most common polytype possible, 2M. The polytype 6R, which was previously believed to be the most common, has not been found during our extensive study

A new hydrous phase, Na 2 Mg 6 Si 6 O 18 (OH) 2 , which is isostructural with aenigmatite, was synthesized at 10 GPa and 1250 °C and its structure studied with X-ray diffraction data collected from a twinned crystal on a CCD diffractometer. The unit-cell parameters are a = 10.2925(9), b = 10.7052(9), c = 8.8027(10) Å, α = 105.280(2), β = 96.712(2), γ = 125.256(2)°, and V = 718.1(5) Å 3 . The structure refinement indicates that O4 and O14, the only 2 O atoms that are not bonded to Si, are protonated. The presence of OH in the structure is confirmed by an unpolarized Raman spectrum. Compared to anhydrous sodic phases with the aenigmatite-type structure, silicate chains in hydrous Na 2 Mg 6 Si 6 O 18 (OH) 2 are more kinked, resulting in relatively long average Na-O distances.