Search

The Mossbauer spectra of an albite glass and melt doped with [sup.57]Fe (bulk composition Na[Fe.sub.0.04]Al[Si.sub.3][O.sub.8]) were measured in situ to 1100[degrees]C. A specially designed furnace was used where a disk of the sample was enclosed in a capsule of graphite. The furnace was purged with argon to avoid oxidation of the graphite, and shields of boron nitride (BN) were used to protect both the [gamma]-ray source and the detector from thermal radiation. Room-temperature spectra show contributions from paramagnetic [Fe.sup.2+], paramagnetic [Fe.sup.3+], and magnetic [Fe.sup.3+]. The signal of [Fe.sup.2+] disappears around 400 [degrees]C, and the signal of [Fe.sup.3+] disappears at about 1100 [degrees]C. This behavior cannot be due to changes in the oxidation state of iron because in situ visible and near infrared spectra of the same sample show that Fez+ persists to at least 1000 [degrees]C and the quenched samples do not show any change in oxidation state. Rather, the disappearance of [Fe.sup.2+] and [Fe.sup.3+] in the spectra is likely due to the structural relaxation time around these cations becoming comparable to the M6ssbauer timescale (lifetime of excited state [approximately equal to] [10.sup.-8] s). We show that the temperature at which the [Fe.sup.3+] contribution disappears corresponds to the expected glass transition temperature of a 'ferrialbite' NaFe[Si.sub.3][O.sub.8] melt for a timescale of [10.sup.-8] s. Our data therefore confirm that viscous flow is due to the rearrangement of the chemical bonds between oxygen anions and network-forming cations. Furthermore, our data show that M6ssbauer spectroscopy can be used to study the speciation and relaxation time around iron in silicate melts even at temperatures above the macroscopic glass transition temperature as defined from viscosimetry and calorimetry. Keywords: Glass transition, iron, Mossbauer spectroscopy, silicate glasses, structural relaxation, relaxation time DOI: 10.2138/am.2010.3490

[Fe.sup.3+] and [Mn.sup.3+] distributions on octahedral M1, M2, and M3 sites in synthetic epidote/piemontite from [Ca.sub.2][Al.sub.2][Fe.sup.3+.sub.l-q][Si.sub.3][O.sub.12.5] starting material and their effects on the crystal structure were investigated using X-ray Rietveld and [sup.57]Fe Mossbauer methods. Epidote and piemontite were crystallized as almost single phases from q = 1.0, 0.75, 0.5, and 0.25 starting materials at [P.sub.fluid] of 200-400 MPa and a temperature of 500[degrees]C, using standard cold-seal pressure vessels. The [Mn.sub.2][O.sub.3]-Mn[O.sub.2] buffer was used to produce fog adequate to maintain [Fe.sup.3+] and [Mn.sup.3+]. The Rietveld refinements converged to goodness-of-fit ranges from 1.21 to 1.60. At this temperature, site preferences of [summation]([Fe.sup.3+] + [Mn.sup.3+]) for octahedral sites are M3>MI(>>M2). [K.sub.D] values of [summation]([Fe.sup.3+] + [Mn.sup.3+]), where [K.sub.D] = [([Fe.sup.3+] + [[Mn.sup.3+] )/Al].sup.M1] /[([Fe.sup.3+] + [[Mn.sup.3+])/Al].sup.M3], (0.05-0.13) are similar to those of individual [Mn.sup.3+] and [Fe.sup.3+] vs. [Al.sup.3+], respectively. However, the [K.sub.D] values of [Fe.sup.3+] and [Mn.sub.3+] for M1 and M3, where [K.sub.D] = [([Fe.sup.3+]/[Mn.sup.3+]).sup.M1]/ [([Fe.sup.3+]/[Mn.sup.3+]).sup.M3], vary with [Fe.sup.3+.sub.Total]: [Mn.sup.3+.sub.Total] ratios. In epidote with [Fe.sup.3+] content larger than 0.4 atoms per formula unit (apfu) and [Mn.sup.3+] < 0.6 apfu, [Fe.sup.3+] has a stronger preference for M1 than [Mn.sup.3+]. In piemontite with 0.12 [Fe.sup.3+] and 0.73-0.78 [Mn.sup.3+] apfu, the preference of [Mn.sup.3+] for M1 is greater than that of [Fe.sup.3+]. The site occupancies of individual [Mn.sup.3+] and [Fe.sup.3+] are governed by the individual [K.sub.D] values and the [Mn.sup.3+] and [Fe.sup.3+] concentrations in corresponding epidote and piemontite. Variations of the unit-cell parameters indicate the combined result of linear variation due to Al [left and right arrow] [Fe.sup.3+] substitution and nonlinear variation due to Al [left and right arrow] [Mn.sup.3+] substitution. Keywords: Epidote, piemontite, synthesis, Rietveld refinement, M6ssbauer spectroscopy DOI: 10.2138/am.2010.3418

We report the pressure dependence of the Mossbauer spectra and the electrical resistivity up to 9.1 GPa at room temperature for a native iron sample collected from the Precambrian Chaibasa shales, Singhbhum Craton, Eastern India. The Mossbauer spectroscopy of the sample at ambient conditions yields isomer shifts and magnetic hyperfine field values that confirm the presence of [Fe.sup.O] oxidation state. Many theories have been put forward to explain the origin of this native iron including a Precambrian meteoritic impact. High-pressure Mossbauer spectroscopic measurements using diamond anvil cell (DAC) showed a constant isomer shift up to 5.6 GPa with a subtle variation of -1.07 x [10.sup.-3] mm/s/GPa followed by sharper change -4.3 x [10.sup.-3] mm/s/GPa above 6.3 GPa, a pressure much lower than the usual value reported for metallic iron. Further increase of pressure to 9.1 GPa results in the emergence of a tiny peak at ~0 isomer shift indicating the onset of the martensitic phase transition of iron from the body-centered-cubic (bcc) to hexagonal-close-packed (hcp) transition 4 GPa lower than the transition pressure normally observed for pure iron. This phase transition in the native iron is confirmed by high-pressure electrical resistivity study. Lowering of the transition pressure could be due to nucleation of hcp by stacking faults caused by shock metamorphism resulting from the Precambrian impact in the region. Keywords: Mossbauer spectroscopy, high pressure, diamond anvil cell, native iron, Precambrian impact, electrical resistivity, XPS spectroscopy DOI: 10.2138/am.2010.3479

EPMA analyses of uvarovite from the chromite-hearing ultramafic intrusive of Sukinda, India, show that uvarovite containing 9.83 to 17.68 wt% [Cr.sub.2][O.sub.3] represents a solid solution of the binary uvarovite-andradite series with [Uva.sub.34-58] [And.sub.42-58]. Associated chromite grains with 57-59 wt% [Cr.sub.2] [O.sub.3] do not show any compositional variation from core to rim. [sup.57]Fe Mossbauer spectroscopic studies on the uvarovite samples undertaken both at room temperature (298 K) and low temperature (l10 K) indicate the presence of [Fe.sup.3+] only in octahedral coordination. Uvarovite grew from chromite (Cr source) and pyroxenes (Al, Fe, and Ca source) of the ultramafic host rocks. Keywords: Natural uvarovite, chromite, ultramafic intrusive, Eastern India DOI: 10.2138/am.2010.3328

Fe-rich 'oxydravite' and dravite from the Late Proterozoic ophiolitic melange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mossbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and [Fe.sup.2+] over the Y and Z sites, and an ordering of [Fe.sup.3+] at Y. The disordering can be explained by the substitution mechanisms [2.sup.Y] Mg+[sup.Z]+[sup.W]OH = [2.sup.Y]Al+[sup.Z]Mg+[sup.W][O.sup.2-] and [2.sup.Y][Fe.sup.2+]+[sup.2][Fe.sup.3+]+[sup.W]OH = [2.sup.Y][Fe.sup.3+]+[sup.2][Fe.sup.2+]+[sup.W][O.sup.2], which are consistent with reducing the mismatch in dimensions between Y[O.sub.6] and Z[O.sub.6] octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three [YO.sub.6] octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing Mg[O.sub.6] octahedra and the smaller [Si.sub.6][O.sub.18] tetrahedral tings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between [YO.sub.6], Z[O.sub.6], and T[O.sub.4] polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of [sup.Z]Al combined with the occurrence of B at T in Al-rich tourmaline. [sup.Z]Al has a greater effect than [sup.Y]Al as long as Al does not fully occupy the Z site. Keywords: Chemical analysis, tourmaline, crystal structure, Mossbauer spectroscopy, order-disorder, XRD data DOI: 10.2138/am.2010.3357

A suite of basaltic glasses were examined to determine how subtle compositional changes affect mid-infrared spectra (650 to 5400 [cm.sup.1]). Glasses with different SI[O.sub.2], Fe[O.sub.total], [Fe.sup.3+]/[Fe.sup.2+], and alkali contents were synthesized in a gas-mixing furnace and analyzed using electron probe microanalysis, Mossbauer spectroscopy, and micro-reflectance Fourier transtbrm infrared spectroscopy. The major mid-infrared spectral feature in silicate glasses is a broad peak located at ~900 to 1100 [cm.sup.-1] arising from Si-(AI-)O asymmetric stretching vibrational modes. To accurately compare spectra of different glass compositions, we have applied the Kramers-Kronig (KK) transform to our spectra and examined the resulting absorption peak (KK abs. peak). The location of the KK abs. peak shifts to higher wavenumbers as Si[O.sub.2] content increases (1031-1054 [cm.sup.1] with Si[O.sub.2] from 47.18 to 55.57 wt%). For basaltic glasses with near-constant AI/(AI+Si), the full-width half maximum of the KK abs. peak decreases as alkali content increases (235-188 [cm.sup.-1] with [Na.sub.2]O+[K.sub.2]0 contents from 0.07 to 3.74 wt%). In contrast, the location and shape of the KK abs. peak are not affected by variations in total FeO (6.06-16.30 wt%) and [Fe.sup.3+]/[Fe.sup.2+] (0.05-1.17). Our results show that KK transformed mid-infrared spectra of basaltic glasses may be used to determine the Si[O.sub.2] contents in basaltic glasses, irrespective of Fe[O.sub.total] and [Fe.sub.3+]/[Fe.sup.2+], and tile alkali contents if AI/(AI+Si) is known. These observations will aid in the interpretation of laboratory and remotely sensed IR spectra. Keywords: IR peak location and shape, glass composition, Mossbauer spectroscopy, oxygen fugacity DOI: 10.2138/am.2009.3113.

A complete crystal-chemical characterization of erionite-K from Rome, Oregon, was obtained by combining field emission scanning electron microscopy, laboratory parallel-beam transmission powder diffraction, and [sup.57]Fe Mossbauer spectroscopy. Rietveld refinement results evidenced that the most striking difference in comparison with the structure of erionite-Ca is significant K at a K2 site (1/2, 0, 0), which is empty in erionite-Ca. In addition, site Cal shows low occupancy and Ca3 is vacant. The oxidation and coordination state of Fe, whose occurrence was revealed by chemical analysis, have been clarified by exploiting room- and low-temperature [sup.57]Fe Mossbauer spectroscopy. The majority of Fe (95%) was attributed to [Fe.sup.3+]-bearing, superparamagnetic, oxide-like nanoparticles with dimensions between 1 and 9 nm, and the remaining 5% was attributed to hematite particles with size [greater than or equal to] 10 nm, both located on the crystal surface. Keywords: Oregon erionite, erionite-K, crystal structure, iron topochemistry, field emission, scanning electron microscopy, laboratory parallel-beam transmission powder diffraction, Rietveld refinement, [sup.57]Fe Mossbauer spectroscopy

High-pressure and -temperature multianvil experiments were performed to test the effect of varying oxygen fugacity on the olivine to wadsleyite transformation. Two capsules, containing samples of [(Fe,Mg).sub.2]Si[O.sub.4], were placed in each experiment; the first buffered the oxygen fugacity with an assemblage of Re and Re[O.sub.2], whereas the second ensured the lowest possible ferric iron concentration through the presence of excess Fe metal. Measurements of coexisting olivine, wadsleyite, and ringwoodite compositions from the Fe metal saturated experiments were used to accurately determine the pressure in each experiment using established phase relations. Under the more oxidizing conditions of the Re-Re[O.sub.2] buffer, the stability field of wadsleyite was found to expand with respect to both the olivine and ringwoodite stability fields. Mossbauer spectroscopy measurements reveal [Fe.sup.3+]/ [SIGMA]Fe ratios for wadsleyite buffered by Re-Re[O.sub.2] of 0.1-0.25, while olivine appears to be [Fe.sup.3+]-free. A thermodynamic model that employs the wadsleyite end-members ([Fe.sup.3+.sub.5/3][[].sub.1/3])[Fe.sup.3+][O.sub.4]-[Fe.sub.2] Si[O.sub.4]-[Mg.sub.2]Si[O.sub.4] is used to examine the effect of varying bulk mantle [Fe.sup.3+]/[SIGMA]Fe ratio on the depth and depth interval of the 410 km seismic discontinuity. [Fe.sup.3+]/[SIGMA]Fe ratios in the range 0.02-0.12 would cause the depth interval or thickness of the 410 km discontinuity to increase from ~8 to 15 km but would have very little effect on the seismically observable absolute depth. Very large bulk mantle [Fe.sup.3+]/[SIGMA]Fe ratios (>0.2), unrepresented in recovered mantle samples, would be required to explain recent seismic observations that the depth interval of the 410 km may be >20 km beneath certain regions. Such observations are more likely to be explained by moderate local enrichments in both ferric iron and [H.sub.2]O in the mantle, most likely as a result of slab interaction. Keywords: Ferric iron, multianvil, transition zone, seismic discontinuity

Spectroscopic measurements have been made of two suites of olivine minerals synthesized under slightly different conditions in 5 10 mol% increments across the solid solution from forsterite to fayalite. Here, we present Mossbauer results for the entire Fe-Mg olivine suite, as well as the results for only the fayalite end-member as an introduction to our team's other diverse spectral-analysis techniques and data that will be presented in forthcoming papers. Experimental methods used to synthesize both suites of samples are discussed here in detail, along with specifics of the analytical techniques used to study them. Electron microprobe data and Mossbauer spectra acquired at 293 K across the solid solution are presented first to characterize and address the presence of impurities in the broad suite of samples that may affect other spectroscopic methods. We then focus specifically on the fayalite end-member to illustrate its properties using multiple techniques. Fayalite is an especially important phase for different types of spectroscopy because, by definition, it contains an equal distribution of [Fe.sup.2+] cations between the M1 and M2 octahedral sites. Thus, features associated with each of the two sites must represent equal numbers of [Fe.sup.2+] cations, removing uncertainties associated with assumptions about order/disorder of [Fe.sup.2+] and other cations. Mossbauer, Raman, thermal emission, attenuated total reflectance (ATR), specular reflectance, and visible to mid-infrared total reflectance studies are presented for fayalite. These include calculation of mid-infrared optical constants (n and k) and fundamental Mossbauer parameters: intrinsic isomer shift ([[delta].sub.1]), Mossbauer temperature ([[theta].sub.M]), and recoil-free fraction (f). Data from the different techniques are described and related, demonstrating the importance of multi-wavelength data to provide a complete characterization and understanding of the spectroscopic features in fayalite. Keywords: Fayalite, olivine, Mossbauer, Raman, thermal emission, reflectance, visible region, infrared, optical constant, attenuated total reflectance

The electronic spin state of [Fe.sup.2+] in ferropericlase, ([Mg.sub.0.75][Fe.sub.0.25])O, transitions from a high-spin (spin unpaired) to low-spin (spin paired) state within the Earth's mid-lower mantle region. To better understand the local electronic environment of high-spin [Fe.sup.2+] ions in ferropericlase near the transition, we obtained synchrotron Mossbauer spectra (SMS) of ([Mg.sub.0.75],[Fe.sub.0.25])O in externally heated and laser-heated diamond anvil cells at relevant high pressures and temperatures. Results show that the quadrupole splitting (QS) of the dominant high-spin [Fe.sup.2+] site decreases with increasing temperature at static high pressure. The QS values at constant pressure are fitted to a temperature-dependent Boltzmann distribution model, which permits estimation of the crystal-field splitting energy ([[DELTA].sub.3]) between the [d.sub.xy] and [d.sub.xz] or [d.sub.zy] orbitals of the [t.sub.2g] states in a distorted octahedral [Fe.sup.2+] site. The derived [[DELTA].sub.3] increases from approximately 36 meV at 1 GPa to 95 meV at 40 GPa, revealing that both high pressure and high temperature have significant effects on the 3d electronic shells of [Fe.sup.2+] in ferroperielase. The SMS spectra collected from the laser-heated diamond cells within the time window of 146 ns also indicate that QS significantly decreases at very high temperatures. A larger splitting of the energy levels at high temperatures and pressures should broaden the spin crossover in ferropericlase because the degeneracy of energy levels is partially lifted. Our results provide information on the hyperfine parameters and crystal-field splitting energy of high-spin [Fe.sup.2+] in ferropericlase at high pressures and temperatures, relevant to the electronic structure of iron in oxides in the deep lower mantle. Keywords: Ferroperielase, diamond-anvil cell, spin transition, Mossbauer spectroscopy, high pressures, high temperatures

Combined experimental, modeling, and analytical results indicate that the rapid acidification of dilute waters in contact with nominally [Fe.sup.2+]-sulfate minerals (FeS[O.sub.4] x n[H.sub.2]O) is caused by [Fe.sup.3+] hydrolysis, which occurs when low levels ( Keywords: Mars, AMD, melanterite, pH, Mossbauer

Spinel single crystals of 19 compositions along the magnetite-ulvospinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations, the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mossbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate, and tetrahedral bond length increase with increasing ulvospinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation, the Akimoto model [sup.T] [([Fe.sup.3+.sub.1-X][Fe.sup.2+.sub.X].sup.M][Fe.sup.2+][Fe.sup.3+.sub.1-X] [Ti.sub.X])[O.sub.4] describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction [sup.T] [Fe.sup.2+] + [sup.M][Fe.sup.3+] = [sup.T][Fe.sup.3+] + [sup.M][Fe.sup.2+], which causes [sup.M][Fe.sup.2+] [not equal to] 1 and [sup.T][Fe.sup.2+]/Ti [not equal to] 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti [approximately equal to] 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the [sup.T][Fe.sup.3+] + [sup.M][Fe.sup.3+] = [sup.T][Fe.sup.2+] + [sup.M][Ti.sup.4+] chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvospinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs. Keywords: Spinel, Mossbauer spectroscopy, crystal synthesis, crystal structure

Dark-brownish, euhedral crystals of an 'allanite-like' mineral occur in a hematite-impregnated Mn-silicate rock at Kesebol, Vastra Gotaland, Sweden, associated with gasparite-(Ce), chernovite(Y), rhodonite, andradite, manganoan calcite, and quartz. A structural study was carried out on single crystals--untreated, heated in air, and heated under inert atmosphere--combined with Mossbauer spectroscopy and TEM investigation. In all the untreated crystals the mean distance indicates that [Me.sup.2+] (Me = Mn, Fe) prevails at this site ( in the range 2.169-2.180 [Angstrom]), in contrast with chemical data obtained by EPMA that yield a simplified formula Ca([REE.sup.3+.sub.2/3][[??].sub.1/3])[Me.sup.3+.sub.3](Si[O.sub.4]) ([Si.sub.2][O.sub.7]) O(OH), when normalized to Si = 3.00 apfu. Moreover, when a crystal is heated in air, all geometrical and structural variations indicate the development of an oxidation-dehydrogenation reaction, thus confirming that M3 is occupied by divalent cations before heating. The corresponding dehydrogenation is confirmed by a dramatic lengthening of the donor-acceptor distance. A crystal was annealed under inert atmosphere to verify possible effects of radiation damage on the polyhedral volumes. After prolonged annealing at 700[degrees]C, a slight decrease of the unit-cell parameters is observed, suggesting restoring of crystallinity from a 'partially metamict' state. Nonetheless, even in the annealed crystal, the distance is still consistent with a dominance of divalent cations at the M3 site. For all the examined crystals, structural data point to an octahedral cation population as follows: M1 = ([Me.sup.3+], Al); M2 = (Al, [Me.sup.3+]); M3 = ([Me.sup.2+], [Me.sup.3+]). This assumption is also in agreement with the Mossbauer spectrum, which was fitted to two Lorentzian quadrupole doublets for [Fe.sup.3+] and one for [Fe.sup.2+]. Values of the isomer shifts (0.36 and 0.37 mm/s for [Fe.sup.3+]; 1.11 mm/s for [Fe.sup.3+]) and the quadrupole splitting (1.96 and 1.02 for [Fe.sup.3+]; 1.90 for [Fe.sup.2+]) show that [Fe.sup.2+] ([approximately equal to]12% of the total iron) is located in M3, while [Fe.sup.3+] occupies M1 and, to lesser extent, M2. TEM-EDS investigations have revealed chemical heterogeneities related to different degree of radiation damage. In particular, areas showing poor crystallinity are relatively enriched in Si and O with respect to the highly crystalline areas, thus suggesting that EPMA chemical data are biased by the presence of metamict areas enriched in Si[O.sub.2] and likely in [H.sub.2]O. EPMA data were therefore corrected for the excess of silica. The cation population after correction is in keeping with the structural and spectroscopic data. Disregarding minor substitutions, the ideal chemical formula for the epidote-group mineral from Kesebol is CaREE[Fe.sup.3+] Al[Mn.sup.2+]([Si.sub.2][O.sub.7])(Si[O.sub.4])O(OH), which is related to ferriallanite-(Ce) by the substitutional vector [sup.M3]([MN.sup.2+] [right arrow] [sup.M3]([Fe.sup.2+]. Keywords: Epidote-group, allanite-subgroup, single-crystal X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy, metamictization

The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the [Fe.sup.3+]/[Fe.sup.2+] ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360[degrees]C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 [+ or -] 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5-3.0 GPa, 1100-1500[degrees]C). Under those conditions, the amount of [Fe.sup.3+] in silicate phases (pyroxenes, olivine, glass) and spinel is negligible ([Fe.sup.3+]/ [SIGMA]Fe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] cannot be achieved using Pt-graphite or graphite only capsules, but [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] can be tuned to lower values by using small pieces of PtFe alloys. The potential range of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as [+ or -]10[degrees]C. Finally, unless capsules are dried overnight at 400 [degrees]C before the experiment, small amounts of [H.sub.2]O are always present in nominally dry experiments. These small amounts of [H.sub.2]O should not, however, significantly change phase relations. Keywords: Experimental petrology, oxygen fugacity, piston-cylinder, graphite, phase equilibria, reproducibility, Mossbauer spectroscopy, pyroxenes

The partitioning of iron among octahedrally coordinated sites in tourmaline, and its stereochemical consequences, were investigated in a Fe-rich dravite in a skarn rock from Uto, Sweden. A multi-analytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mossbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and [Fe.sup.2+] over the Y and Z sites, and ordering of [Fe.sup.3+] at the Y site. Two Fe-rich tourmalines from the literature, re-examined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Uto tourmaline with [Fe.sup.2+] disordered over the octahedral sites. This is best explained by disordered [Fe.sup.2+] distributions that minimize the strain state of the Y-O bonds and provide a shielding effect reducing Y-Z repulsion. This is consistent with predictions from bond-valence theory and Pauling's rules. An indication of Z-site occupancy by [Fe.sup.2+] in tourmaline may be signaled by a significant correlation between and the c lattice parameter ([r.sup.2] = 0.96). The c value for a very [Fe.sup.2+]-rich tourmaline and an ideal end-member schorl, with [Fe.sup.2+] and Al ordered at Y and Z (respectively), yielded values larger than 1.907 [Angstrom] (the likely bond length for ). These large lengths indicate that [Fe.sup.2+] occurs at the Z site. The hypothesis of a dragging effect from to explain lengthening of is not supported by experimental evidence. Keywords: Chemical analysis, tourmaline, crystal structure, Mossbauer spectroscopy, order-disorder, XRD data

The effects of heat treatment on stilpnomelane from granite pegmatites at Zolkiewka (Lower Silesia, Poland) were studied using [sup.57]Fe Mossbauer spectroscopy. Samples of naturally altered stilpnomelane and chlorite were also studied. The Mossbauer spectrum of an untreated sample was fitted to two [Fe.sup.2+] doublets and two [Fe.sup.3+] doublets. The [Fe.sup.2+] doublets had similar isomer shifts (1.13 and 1.16 mm/s for the octahedral M1 and M2 sites), different quadrupole splitting values (QS = 2.28 and 2.66 mm/s, respectively), and an M1:M2 area ratio of nearly 1:3. For ferric iron, the first doublet was assigned to the combined M1 + M2 + M4 octahedral positions and had IS = 0.41 and QS = 1.18 mm/s. The second doublet was assigned to the M3 site, with parameters IS = 0.36 and QS = 2.07 mm/s. This is characteristic for stilpnomelane. The oxidation of [Fe.sup.2+] is fast at low temperatures, with a maximum at 360[degrees]C. Near 1050[degrees]C, the stilpnomelane structure broke down completely, and the spectrum consisted of two sextets and two ferric iron doublets. The predominant phase in this new material is hematite, as represented by the two sextets. The abnormally high fractional [Fe.sup.2+] content in the altered stilpnomelane (0.70) as compared to the untreated sample (0.45) indicates that the sample was subjected to local hydrothermal processes. Nearly identical divalent iron contents and hyperfine parameters for the [Fe.sup.2+] and [Fe.sup.3+] doublets (except [Fe.sup.3+] in M3) in the altered stilpnomelane and associated chlorite strongly suggest that chlorite is transformed into stilpnomelane. Keywords: Stilpnomelane, Mossbauer spectroscopy, iron, oxidation state, heat treated

This study is part of a broad research project devoted to the 'amphibole fibers environmental problem' as related to the proposed excavation of the Susa Valley railway tunnel. In this locality, tunnel excavations are planned through metamorphic formations containing amphibole asbestos minerals, and this may give rise to worker health and public environmental issues. The Susa Valley tremolite shows a marked fibrous character, a small reduction of fiber size under grinding, and a consistent increase of the surface area. From the toxicological point of view, such tremolite fibers have been shown to be very effective in the generation of reactive oxygen species. They exhibit a very high cellular reactivity asa consequence of their morphology, structure, and crystal chemistry. Results of combined electron microprobe analysis, Mossbauer spectroscopy, and parallel-beam X-ray powder diffraction are reported for fibrous tremolite from a serpentine-schist from the 'Unita. Oceanica della Bassa Val di Susa' collected near Condove, Susa Valley, Italy. Data indicate that [Fe.sup.2+] (84% of [Fe.sub.tot]) is located at both the (M1 + M3) and M2 sites and that [Fe.sup.3+] is at M2, in an approximate 3:2:1 ratio, respectively. No evidence of a split M4 site has been observed. The presence of [sup.M1+M3][Fe.sup.2+] is confirmed by FTIR spectroscopy to be distributed 70% at M1 and 30% at M3. Both the composition ([Ca.sub.1.95][K.sub.0.01][Na.sub.0.05])[summation]2.01 ([sup.VI][Al.sub.0.01][Fe.sup.3+.sub.002][Fe.sup.2+.sub.0.11] [Mg.sub.4.84][Mn.sub.0.02])[summation]5.00[Si.sub.8.00] [O.sub.22](O[H.sub.1.96][F.sub.0.03][Cl.sub.0.01])[summation]2.00 and the cell volume 907.37(1) [[Angstrom].sup.3] of the fibers are close to those expected for end-member (Ca/Mg = 2/5) tremolite. Keywords: Fibrous tremolite, amphibole crystal chemistry, Mossbauer spectroscopy, Rietveld method, electron microprobe analysis, Susa Valley, Italy, Trans European Network

We synthesized hematite spherules whose mineralogic, chemical, and crystallographic properties are strikingly similar to those for the hematite-rich spherules in lag deposits on the surface and embedded in outcrops at Meridiani Planum, Mars. The spherules were synthesized in the laboratory along with hydronium jarosite and minor hydronium alunite from Fe-Al-Mg-S-Cl acid-sulfate solutions under hydrothermal conditions. The reaction sequence was (1) precipitation of hydronium jarosite; (2) jarosite dissolution and precipitation of hematite spherules; and (3) precipitation of hydronium alunite upon depletion of hydronium jarosite. The spherules exhibit a radial growth texture with the crystallographic c axis aligned along the radial direction, so that thermal emission spectra have no hematite emissivity minimum at ~390 [cm.sup.-1]. Our experiments show that hydrothermal, acid-sulfate solutions are a pathway for formation of jarosite and the hematite spherules at Meridiani Planum, Mars. Keywords: Hematite, jarosite, spherule, synthesis, diagenesis, Mars, hydrothermal, concretions, thermal emission spectroscopy, X-ray diffraction, Mossbauer spectroscopy, scanning electron microscopy, transmission electron microscopy

Spinel single crystals (up to 4 mm in size) of eight compositions along the Fe[Fe.sub.2][O.sub.4]-[Fe.sub.2] Ti[O.sub.4] solid-solution join, with more than 75 mol% ulvospinel, were synthesized by use of a flux-growth method. The crystals were characterized by electron microprobe and Mossbauer spectroscopy. Results demonstrated that these ulvospinels are statistically stoichiometric. The atomic proportions of [Ti.sup.4+] and [Fe.sup.2+] progressively increase from 0.75 to 0.94 and from 1.75 to 1.94 apfu, respectively. Concomitantly the [Fe.sup.3+] content decreases from 0.49 to 0.13 apfu. Consequently, the cation substitutions are restricted to the ideal classic substitution 2[Fe.sup.3+] [left and right arrow] [Fe.sup.2+] + [Ti.sup.4+]. An average equilibrium temperature from coexisting spinel-ilmenite pairs of about 950[degrees]C was estimated using the QUILF95 and Ghiorso-Sack models. In contrast to previous studies based on non-stoichiometric samples sintered at higher temperatures, the present stoichiometric ulvospinel samples were grown from a melt under moderate temperatures. This fact indicates that the formation of vacancies is related to high-equilibration temperatures or growth mechanisms, whereas natural ulvospinel may very well be stoichiometric under normal magmatic conditions. Keywords: Ulvospinel, ilmenite, Mossbauer spectroscopy

The low-temperature heat capacities of natural near end-member minerals (about 95 mol%, except tinzenite of about 34 mol% on average) of the axinite group, previously characterized in detail by means of powder and single-crystal X-ray diffraction, electron microprobe, and Mossbauer spectroscopy, were measured by heat-pulse calorimetry using the Physical Properties Measurement System (Quantum Design) at temperatures between 5(2) and 300 K. From these data, the following entropy values [in J/(mol*K)] of the natural samples at 298.15 K were derived: [S.sub.298,magnesiaxinite] = 696.3 [+ or -] 1.1, [S.sub.298,ferroaxinite] = 743.5 [+ or -] 3.5, [S.sub.298,manganaxinite] 737.5 [+ or -] 2.6, and [S.sub.298,tinzenite] = 758.1 [+ or -] 2.8. For the end-member compositions, the corrected heat capacities at 298.15 K and standard third-law entropies of the axinites are [all in J/(mol*K)]: [C.sup.o.sub.Pmagnesioaxinite] = 827.5 [+ or -] 1.5 and [S.sup.o.sub.magnesioaxinite] = 693.7 [+ or -] 1.1, [C.sup.o.sub.Pferroaxinite] = 841.8 [+ or -] 3.3 and [S.sup.o.sub.ferroaxinite] = 749.6 [+ or -] 3.5, [C.sup.o.sub.Pmanganaxinite] = 849.1 [+ or -] 2.5 and [S.sup.o.sub.manganaxinite] = 737.8 [+ or -] 2.6, and [C.sup.o.sub.Ptinzenite] = 841.6 [+ or -] 2.6, [S.sup.o.sub.tinzenite] = 754.0 [+ or -] 2.8. The standard entropies of manganaxinite and tinzenite include contributions of 1.9 and 4.3 J/(mol*K) for the range 0-5 K evaluated based on a Schottky anomaly fitted to the low-T [C.sub.p] values of these axinites. The lowest measured heat capacities of ferroaxinite indicate that a lambda-type [C.sub.p] anomaly should exist between 0 and 2 K. Its likely contribution to the standard entropy was estimated as ~5.2 J/(mol*K). A low-temperature [C.sub.p], anomaly below 15 K for ferroaxinite is well-explained by ferromagnetic ordering, whereas for manganaxinite by uncompensated antiferromagnetic ordering, and for tinzenite by pure antiferrolnagnetic ordering. Keywords: Axinite group minerals, chemical composition, heat capacity, heat-pulse calorimetry, magnetic ordering, low-temperature anomaly

Accurate crystal-chemical analysis of complex minerals such as tourmalines belonging to the elbaite-schorl-dravite series was obtained by combining Mossbauer spectroscopy (MS) and structural intbrmation. Well-defined relationships were established between the two approaches, leading to a close match of results obtained and a strong link between observed parameters. Although MS information is a powerful tool for quantifying the amount of [Fe.sup.2+] and [Fe.sup.3+], it is not always useful in determining their site distribution. In particular, both for [Fe.sup.3+] and for ([Fe.sup.2+]-[Fe.sup.3+]) interactions structural information is still essential. [Fe.sup.3+] MS doublets were identified and assigned to iron in Y and Z sites on the basis of structural information. In a few spectra, [Fe.sup.3+] doublets with very low parameters ([sigma] ~ 0.2 mm/s) were observed and, in contrast with the T-site assignment of previous works, were assigned to [Fe.sup.3+] in octahedral coordination. Electron delocalization between [Fe.sup.2+] and [Fe.sup.3+] was observed and related to three different interactions (Y-Y, Y-Z, and Z-Z). Notably, MS hyperfine parameters of [Fe.sup.2+] were self-consistent and particularly reliable in determining Feet site partitioning. [Fe.sup.2+] at Y was modeled by three doublets ([DELTA][E.sub.Q] - 2.45, 2.19, and 1.72 mm/s). The sum of their absorption areas perfectly matches the [sup.Y][Fe.sup.2+] populations derived from structural data ([r.sup.2] = 0.97). The fourth doublet observed ([DELTA][E.sub.Q]= 1.38 mm/s) is consistent with [Fe.sup.2+] at Z, which is an octahedron smaller and less distorted than Y([[lambda].sub.Z] = 1.014, [[lambda].sub.Y] = 1.024). The absorption area of this doublet is highly correlated with the amount of [sup.Z][Fe.sup.2+] obtained from site-occupancy refinement ([r.sup.2] = 0.95). For [sup.Y][Fe.sup.2+] a link between the quadrupole splitting parameter [DELTA][E.sub.Q] and variations in the chemical/ structural environment surrounding Fe nucleus was observed. The [DELTA}[E.sub.Q] of [sup.Y][Fe.sup.3+] increases with Z[O.sub.6] volume ([r.sub.2] = 0.84) and is linked to the variation of electrical field gradient generated by the [sup.Z][R.sup.2+] [right arrow] [sup.Z]Al substitution. Since the Z skeleton completely surrounds Y islands, [DELTA][E.sub.Q] of [sup.Y][Fe.sup.2+] shows much more susceptibility to inductive effects from the second rather than the first coordination sphere. Keywords: Tourmaline, Mossbauer spectroscopy, structural formula, Fe disorder