P–T–X controls on phase stability and composition in LTMP metabasite rocks – a thermodynamic evaluation

The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H₂O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe₂O₃ bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe₂O₃ contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe₂O₃‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe₂O₃ (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe₂O₃‐saturated rocks, compositional isopleths for sodic amphibole (Al³⁺ and Fe³⁺ on the M2 site), epidote (Fe³⁺/Fe³⁺ + Al³⁺) and chlorite (Fe²⁺/Fe²⁺ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al³⁺ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe²⁺‐Mg ratios in chlorite may be dependent on pressure. The Fe³⁺/Al and Fe²⁺/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.     

Sulfur K- and L-edge X-ray absorption spectroscopy of sphalerite,chalcopyrite and stannite

Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and L-edge XANES of sphalerite (ZnS), chalcopyrite (CuFeS₂) and stannite (Cu₂FeSnS₄) have been recorded using synchrotron radiation. The K- and L-edge XANES features are interpreted using a qualitative MO/energy band structure model. The densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The SK- and L-edge XANES also indicate that, for sphalerite, the Fe²⁺ 3d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu⁺ 3d and Fe³⁺ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu⁺ 3d band strongly hybridizes with S 3p and S 3s orbitals, but the Fe²⁺ 3d band does not. The post-edge XANES features (15–50 eV above the edge) of sphalerite, chalcopyrite and stannite are similar. These features are related to the tetrahedral coordination of sulfur in all these structures, and interpreted by a multiple scattering model. The resonance energies from both the K-edge and L-edge XANES for these minerals are well correlated with reciprocal interatomic distances and lattice spaces. Sulfur K-edge EXAFS analyses using Fourier transform and curve fitting procedures are presented. Comparison of the structural parameters from EXAFS with x-ray structure data shows that the first shell bond distances (BD) from EXAFS are usually accurate to ±0.02 Å, and that coordination numbers (CN) are generally accurate to ±20 percent. For sphalerite, EXAFS analysis yields the structure parameters for the first three neighbour shells around a sulfur atom; the BD and CN even for the third shell are in close agreement with the x-ray structure, and the Debye-Waller term decreases from the first shell to the third shell. It is shown that sphalerite (ZnS) is a good model compound for EXAFS analysis of sulfur in chalcogenide glasses and metalloproteins.     

Phase transitions in ilvaite,a mixed-valence iron silicate

Ilvaite, Ca(Fe²⁺, Fe³⁺) Fe²⁺Si₂O₇O(OH), a mixed-valence iron silicate shows an insulator-semimetal transition with a band gap of 0.13 eV due to thermally induced charge delocalization between Fe²⁺ and Fe³⁺ ions (A sites) in double octahedral chains. The charge delocalization induces a second order crystallographic phase transition on heating from monoclinic (P2₁/a) to orthorhombic (Pnam) symmetry at 346 K. The unit cell dimensions within the 295–420 K range and the crystal structures at 295, 320, 340, 360, 380 and 400 K have been determined by high temperature single crystal X-ray diffraction. The degree of charge delocalization determined from the sizes of the Fe(Ao) and Fe(Am) octahedra is the primary order parameter, Q which couples linearly with the spontaneous strain component, ₁₃. The order parameter coupling and the associated free energy expression is given. The calculated normal modes of the space group symmetry change are consistent with the experimentally observed atomic displacements, which are parallel and antiparallel to c. Formation of antiphase lamellar twin domains parallel to (001) in the monoclinic phase is predicted to occur as a result of the phase transition. Above T_c (= 346 K), the slow asymptotic decrease of ₁₃ attaining a zero value at 380 K indicates the presence of fluctuating precursor clusters with considerable short-range order above T_c. A peak in the specific heat (C_p) measurements coincides with the onset of longrange order at 380 K, whereas ⁵⁷Fe Mössbauer measurements indicate the onset of charge localization at a considerbly higher temperature (470 K). The coupling of the d⁶ electron of the Fe²⁺ (A) ion with a longitudinal optic phonon with the polarization vector along c ^* is the likely mechanism to drive the phase transition. The electronphonon coupling also provides a charge conduction mechanism through electron hopping, whereby the short-bonded Fe²⁺-Fe³⁺ pair containing the d⁶ electron (intermediate polaron) will break up and re-form, thereby propagating the electron one step along the c axis.     

Linked four-membered silicate rings: vibrational analysis of Gillespite

Raman spectra and lattice dynamics calculations are presented for the silicate mineral gillespite, BaFeSi₄O₁₀, which contains sheets of linked four-membered silicate rings. The results are analyzed in relation to earlier work done on the isolated four-membered ring silicate BaCuSi₂O₆ and to published claims that vibrational modes of four-membered rings are responsible for the sharp D ₁“defect line” observed in the Raman spectra of SiO₂ glass. The crystal structure of gillespite (space group P4/ncc or D ) consists of puckered Si₄O₁₂ rings, where each SiO₄ tetrahedron is linked to two neighboring tetrahedra within a ring and to a third tetrahedron within a different ring. The rings are linked to each other in a staggered configuration to form sheets, which are also bonded together by Ba²⁺ and Fe²⁺. The calculation adjusts the bond bending and bond stretching force constants so that calculated fundamental mode frequencies best fit observed fundamental frequencies in the Raman spectra; eigenmodes associated with each calculated fundamental mode are then generated. Some eigenmodes calculated are unique to the gillespite structure, but many of the more localized four-membered ring modes are similar to those calculated for the four-membered rings in BaCuSi₂O₆ and for the three- and six-membered rings in the cyclosilicates studied earlier. The results for gillespite indicate that the Raman-active A _1g mode at 450 cm⁻¹ is a four-membered ring bridging oxygen breathing mode that is mixed with other minor ring deformation displacements; because of this mixing, the calculated frequency of this mode is 45 to 61 cm⁻¹ lower than that calculated for bridging oxygen binding breathing modes of puckered four-membered rings in BaCuSi₂O₆ and in simulated glass structures. Received: 9 April 1997 / Revised, accepted: 19 August 1997     

Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy

X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe²⁺ and Fe³⁺ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe²⁺-O bond distance, closely matching octahedral Fe²⁺-O bonds obtained from solid oxide model compounds. Solution Fe²⁺-O bond distances decrease with chloride/iron ratio, pH, and total FeCl₂ concentration. A slight intensification of the $\text{1s → 3d}$ transition with increasing FeCl₂ concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe³⁺ solutions show a pronounced increase in the $\text{1s → 3d}$ transition intensities between 1.0 M FeCl₃/7.8 M Cl^? to 1.0 M FeCl₃/ 15 M Cl^?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe³⁺-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H₂O)₆³⁺ or trans-Fe(H₂O)₄Cl₂ or both; Fe³⁺-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe³⁺-Cl bond distances of 2.25 Å].     

Temperature dependence of the polarized electronic absorption spectra of olivines. Part I – fayalite

 Polarized electronic absorption spectra of orthorhombic fayalite, Fe₂SiO₄, [E || a(|| Z),E || b(|| X), E || c(|| Y)], space group Pbnm, have been studied in the temperature range 293 ≤T/K ≤ 1273. The spectra were analysed into component bands originating from spin-allowed dd transitions of iron(II) at the different sites, M1 and M2, in the structure. The assignments of bands, made on the basis of the polarization dependence of the spectra and considerations of transition energies, were confirmed by the analysis of the temperature-dependent spectra. The temperature dependencies of integral intensities, half band widths and energy positions of absorptions bands caused by Fe²⁺ on the different octahedral sites, M1 and M2, were evaluated for the individual transitions. Independent of the site symmetry, absorption bands shift to lower energies and half band widths increase on rising temperature. The temperature dependence of band intensities depends on site symmetry. The integral intensities are found to increase with temperature for the transition metal ion on a centrosymmetric site, or remain constant when the site is missing an inversion centre. This is consistent with the general conclusion of Taran et al. (1994). Received: 11 October 2001 / Accepted: 17 January 2002     

Optical absorption and electron spin resonance in blue and green natural beryl

A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe³⁺ in blue beryl occupies a substitutional Al³⁺ site and in green beryl is localized in the structural channels between two O₆ planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe²⁺. Thermal treatments above 200° C in green beryl cause the reduction of Fe³⁺ into Fe²⁺ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe³⁺ into Fe²⁺ is composed of two first order processes; the first one has an activation energy ΔE ₁=0.30 eV and the second one has an activation energy ΔE ₂=0.46 eV.     

Temperature dependence of the polarized electronic absorption spectra of olivines. Part II—Cobalt-containing olivines

Polarized electronic absorption spectra of single crystalline Co₂[SiO₄] and (Co_0.64Mg_0.36)₂[SiO₄] (E|| a (|| Z), E || b (|| X), E || c (|| Y)) have been studied in the temperature range 293 T/K 1273. The three polarized spectra show a total of 15 bands. Five bands are caused by spin-allowed transitions in Co²⁺ ions at M1 sites which appear in all polarization directions. Seven polarization-dependent bands can be ascribed to spin-allowed transitions in Co²⁺ ions at M2 sites and three bands may be assigned to spin-forbidden transitions. The assignment of bands due to Co²⁺ ions at M1 and M2 sites has been made on the basis of transition energies and intensity ratios. Further arguments have been derived from the comparison of spectra of crystals with different cobalt content, from the analysis of the polarization dependence of the spectra, and from the evolution of band intensities with temperature.     

Optical absorption of electronic Fe–Ti charge-transfer transition in natural andalusite: the thermal stability of the charge-transfer band

Differently colored natural Brazilian andalusite crystals heat-treated under reducing and oxidizing conditions were analyzed by optical spectroscopy. The intensity of a broad intense band at around 20,500 cm⁻¹ in the optical absorption spectra of all color zones of the sample is proportional to the product of Ti- and Fe-concentrations and herewith proves its attribution to electronic Fe²⁺/Ti⁴⁺ IVCT transition. The band is strictly E||c-polarized, causing an intense red coloration of the samples in this polarization. The polarization of the Fe²⁺/Ti⁴⁺ IVCT band in andalusite, E||c, shows that the electronic charge-transfer process takes place in Al–O octahedral groups that share edges with neighbors on either side, forming chains parallel to the c-axis of the andalusite structure. Under thermal treatments in air, the first noticeable change is some intensification of the band at 800°C. However, at higher temperatures its intensity decreases until it vanishes at 1,000°C in lightly colored zones and 1,100°C in darkly colored ones. Under annealing in reducing conditions at 700 and 800°C, the band also slightly increases and maintains its intensity at treatments at higher temperatures up to 1,000°C. These results demonstrate that weakening and disappearance of the Fe²⁺/Ti⁴⁺ IVCT band in spectra of andalusite under annealing in air is caused by oxidization of Fe²⁺ to Fe³⁺ in IVCT Fe²⁺/Ti⁴⁺-pairs. Some intensification of the band at 800°C is, most probably, due to thermally induced diffusion of Fe²⁺ and Ti⁴⁺ in the structure that leads to aggregation of “isolated” Ti⁴⁺ and Fe²⁺ ions into Fe²⁺–Ti⁴⁺-pairs. At higher temperatures, the competing process of Fe²⁺ → Fe³⁺ oxidation overcomes such “coupling” and the band continues to decrease. The different thermal stability of the band in lightly and darkly colored zones of the samples evidence some self-stabilization over an interaction of Fe²⁺/Ti⁴⁺-pairs involved in IVCT process.     

Actinolite-forming reaction at low pressure and the role of Fe2+-Mg substitution

The 6km-thick Karmutsen metabasites, exposed over much of Vancouver Island, were thermally metamorphosed by intrusions of Jurassic granodiorite and granite. Observation of about 800 thin sections shows that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion. The reaction leading to the appearance of actinolite, which is the facies boundary between prehnite-pumpellyite and prehnite-actinolite facies, was examined using calcite-free Karmutsen metabasites collected from the route along the Elk river. In the prehnite-pumpellyite facies, X _Fe3+[Fe³⁺/(Fe³⁺+Al)] in prehnite, pumpellyite and epidote buffered by the four-phase assemblage prehnite+pumpellyite+epidote+chlorite systematically decreases with increasing metamorphic grade. Such a trend is the reverse of that proposed by Cho et al. (1986); this may be related to the higher in the Mt. Menzies area. The actinolite-forming reaction depends on the value of X ^Fe3+ in pumpellyite. If using a low value of ^Fe3+, 3.89 Pr(0.06)+0.48 Ep(0.26)+0.60 Chl+H₂O=2.10 Pm (0.08)+0.17 Act+0.88 Qz is delineated. The number in parentheses stands for the X _Fe3+value in Ca-Al silicates. On the other hand, replacing the X _Fe3+ of 0.08 in pumpellyite with a higher X _Fe3+ value (0.24) changes the reaction to 0.41 Pm+0.02 Chl+0.42 Qz=0.11 Pr+0.62 Ep+0.10 Act+H₂O. The first (hydration) reaction forms pumpellyite and actinolite on the high-temperature side, whereas the second (dehydration) reaction consumes pumpellyite to form prehnite, epidote and actinolite. The former reaction seems to explain the textural relationship of Ca-Al silicates in the study area. However, actinolite-forming reaction changes to a different reaction depending on the compositions of the participating minerals, although in the other area even physical conditions may be similar to those in the study area. Chemographic analysis of phase relations in the PrA facies indicates that the appearance of prehnite depends strongly on the bulk FeO/MgO ratio: this may explain the rarity of prehnite in common metabasites in spite of the expected dominant occurrence in the conventional pseudo-quaternary (Ca-Al-Fe³⁺-FM) system. An increasing FeO/MgO ratio stabilizes the Pr+Act assemblage and reduces the stability of the Pm+Act one. Therefore, the definition of pumpellyite-actinolite facies should include not only Pm+Act but also the absence of Pr+Act assemblages. In addition to the possible role of high (Cho and Liou 1987) and/or high to mask the appearance of prehnite, the effect of the FeO/MgO ratio is emphasized.     

A 57Fe M?ssbauer spectroscopic study of sogdianite: an example of a symmetric electric field gradient around Fe3+

Sogdianite, a double-ring silicate of composition ( \textZr_{0. 7 6} \textTi_{0. 3 8}^{4 +} \textFe_{0. 7 3}^{3 +} \textAl_0.13 )_{\Upsigma = 2} ( \square _{1. 1 5} \textNa_{0. 8 5} )_{\Upsigma = 2} \textK[\textLi ₃ \textSi _{1 2} \textO ₃₀ ] ( {\text{Zr}}_{0. 7 6} {\text{Ti}}_{0. 3 8}^{4 + } {\text{Fe}}_{0. 7 3}^{3 + } {\text{Al}}_{0.13} )_{\Upsigma = 2} \left( {\square_{ 1. 1 5} {\text{Na}}_{0. 8 5} } \right)_{\Upsigma = 2} {\text{K}}[{\text{Li}}_{ 3} {\text{Si}}_{ 1 2} {\text{O}}_{ 30} ] from Dara-i-Pioz, Tadjikistan, was studied by the combined application of ⁵⁷Fe M?ssbauer spectroscopy and electronic structure calculations. The M?ssbauer spectrum confirms published microprobe and X-ray single-crystal diffraction results that indicate that Fe³⁺ is located at the octahedral A-site and that no Fe²⁺ is present. Both the measured and calculated quadrupole splitting, ΔE _Q, for Fe³⁺ are virtually 0 mm s⁻¹. Such a value is unusually small for a silicate and it is the same as the ΔE _Q value for Fe³⁺ in structurally related sugilite. This result is traced back to the nearly regular octahedral coordination geometry corresponding to a very symmetric electric field gradient around Fe³⁺. A crystal chemical interpretation for the regular octahedral geometry and the resulting low ΔE _Q value for Fe³⁺ in the M?ssbauer spectrum of sogdianite is that structural strain is largely “taken up” by weak Li–O bonds permitting highly distorted LiO₄ tetrahedra. Weak Li–O bonding allows the edge-shared more strongly bonded Fe³⁺O₆ octahedra to remain regular in geometry. This may be a typical property for all double-ring silicates with tetrahedrally coordinated Li.     

Quantitative determination of iron oxidation states in minerals using Fe L 2,3 -edge electron energy-loss near-edge structure spectroscopy

The Fe L _2,3-edge spectra for a range of natural minerals and synthetic solid solutions have been measured using the technique of parallel electron energy-loss spectroscopy (PEELS) recorded in a transmission electron microscope (TEM). The Fe L _2,3 -edges of the minerals are characterised by two white-line features and exhibit electron energy-loss near-edge structure (ELNES) characteristic of Fe valence state. For divalent iron, the Fe L ₃ -edge spectra are dominated by a sharp peak (white-line) at ca. 707.8 eV, followed by a broader and less intense peak at ca. 710.5 eV. The ELNES on the Fe L ₃ -edge of trivalent iron consists of a white-line with its maximum at ca. 709.5 eV and a preceeding peak at ca. 708.0 eV. Mineral solid solutions that contain both Fe²⁺ and Fe³⁺ exhibit an Fe L ₃ -edge shape that is composed of Fe L ₃ -edges from the respective Fe²⁺- and Fe³⁺-bearing end members. The integral Fe L _2,3 -edge white-line intensity ratios I(L ₃ )/I(L ₂ ) show clear differences for Fe²⁺ and Fe³⁺. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by determining the integral Fe L _2,3 -edge white-line intensity ratios I(L ₃ )/I(L ₂ ) as a function of the ferric iron concentration resulting in an universal curve within the experimental errors. The application of the universal curve combined with the high spatial resolution using the PEELS/TEM allows the quantification of the ferric iron concentration on a scale down to 10 nm, which is illustrated from a sample of ilmenite containing hematite exsolution lamellae that shows different Fe L _2,3 -edge shapes consistent with variations in the Fe²⁺-Fe³⁺ ratio over distances of ca. 100 nm. Received: 30 July 1997 / Revised, accepted: 26 October 1997     

A Fe K-edge XAS study of amethyst

An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO₂) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe²⁺ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell:  = 1.78(2) Å, the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe³⁺ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe⁴⁺ is stabilised by the favourable local structural arrangement.     

Coordination of Fe,Ga and Ge in high pressure glasses by Mössbauer,Raman and X-ray absorption spectroscopy,and geological implications

Mössbauer spectra of glasses of NaFeSi₃O₈ and 3NaAlSi₂O₆ · NaFeSiO₄ starting compositions consist of a dominant Fe³⁺ and subordinate Fe²⁺ quadrupole-split doublet, in agreement with previous work. Fe³⁺ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe³⁺ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO₂, NaGaSi₃O₈ and NaGaSiO₄ glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge⁴⁺ in GeO₂ from 1 bar to 14 kbar, of Ga³⁺ in NaGaSi₃O₈ from 1 bar to 28 kbar and of Ga³⁺ in NaGaSiO₄ from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses.XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO₄ glasses and crystalline NaGaSiO₄ were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga³⁺ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga³⁺ is four-coordinated with oxygen in all three samples with a Ga³⁺-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO₄ (nepheline-type structure) and vitreous NaGaSiO₄ beyond the first coordination shell of Ga³⁺. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga³⁺ in NaGaSiO₄ glasses with pressures up to 25 kbar.Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al³⁺ and Si⁴⁺. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.     

Mineralogy,Mössbauer spectra and electrical conductivity of triphylite Li(Fe<Superscript>2+</Superscript>,Mn<Superscript>2+</Superscript>) PO<Subscript>4</Subscript>

The electrical conductivity of monocrystalline triphylite, Li(Fe²⁺,Mn²⁺)PO₄, with the orthorhombic olivine-type structure was measured parallel (∥) to the [010] direction and ∥ [001] (space group Pnma), between ～400 and ～700 K. Electrical measurements on triphylite are of technological interest because LiFePO₄ is a promising electrode material for rechargeable Li batteries. Triphylite was examined by electron microprobe, ICP atomic emission spectroscopy, X-ray diffraction, Mössbauer spectroscopy and microscopic analysis. The DC conductivity σ_DC was determined from AC impedance data (20 Hz–1 MHz) extrapolating to zero frequency. Triphylite shows σ_DC with activated behavior measured ∥ [010] between ～500 and ～700 K during the first heating up, with activation energy of E _A = 1.52 eV; on cooling E _A = 0.61 eV was found down to ～400 K and extrapolated σ_DC (295 K) ～10^?9 Ω^?1cm^?1; ∥ [001] E _A = 0.65 eV and extrapolated σ_DC(295 K) ～10^?9 to 10^?10 Ω^?1cm^?1, measured during the second heating cycle. The enhanced AC conductivity relative to σ_DC at lower temperatures indicates a hopping-type charge transport between localized levels. Conduction during the first heating up is ascribed to ionic Li⁺ hopping. DC polarization experiments showed conduction after the first heating up to be electronic related to lowered activation energy. Electronic conduction appears to be coupled with the presence of Li⁺ vacancies and Fe³⁺, formed by triphylite alteration. For comparison, σ_DC was measured on the synthetic compound LiMgPO₄ with olivine-type structure, where also an activated behavior of σ_DC with E _A ～1.45 eV was observed during heating and cooling due to ionic Li⁺ conduction; here no oxidation can occur associated with formation of trivalent cations.     

Correlation of Fe4+ optical anisotropy,Brazil twinning and channels in the basal plane of amethyst quartz

The biaxial absorption bands in amethyst quartz, with peaks at 2.28 eV and 3.54 eV related to Fe⁴⁺ and a peak at 3.02 eV—which is the A₃ band related to the [AlO₄]° trapped hole center, have orientations of maximum light absorption in the basal plane of Brazil-twinnedr-growth sectors paralleling the planes of Brazil optical twinning. Absorption minima are at 90° to the maxima in all cases. The Brazil twinning planes always parallel thea-axes(1210,etc) of quartz and in many cases also parallel planes perpendicular to ther-faces(1011,etc.). These are directions of channels in the quartz structure. The anisotropy ratio,σ_max/σ_min, of the Fe⁴⁺ band is that of the A₂ absorption band in smoky quartz as would be expected if Fe³⁺ furnishes electrons to quench the trapped holes causing this absorption band. In the absence of the A₁ and A₂ absorption bands, the A₃ absorption band width at half-maximum decreases from 1.43 to 0.36 eV indicating decreased charge-transfer character of the [AlO₄]° center in the absence of the other types of Al trapped-hole centers in quartz. The key to the Brazil twinning in α-quartz are the channels which fill with large Fe³⁺ ions that force twinning to relieve strain in the structure. Amethyst color results only if aluminum is present substitutionally in the quartz as well as the interstitial iron plus ionizing radiation.     

Electron delocalization in ilvaite,a reinterpretation of its 57Fe Mössbauer spectrum

The Mössbauer spectra of ilvaite CaFe ₂ ²⁺ Fe³⁺[Si₂O₇/O/OH] and their temperature dependence between 298 K and 455 K can be satisfactorily least-squares fitted by a superposition of the resonances for Fe²⁺(8d), Fe³⁺(8d) and Fe²⁺(4c). The relative areas under the three resonances are nearly equal and vary only weakly with temperature. No additional resonances or line broadenings have to be introduced, if we assume that the hyperfine interactions of Fe²⁺(8d) and Fe³⁺(8d) fluctuate between their values due to electron hopping between the iron ions at the 8d sites. Hopping can be assumed to occur homogeneously among nearly equivalent sites. The fluctuation rate is described by an Arrhenius law with a pre-exponent of about 9 × 10⁸ s^?1 and an activation energy of 0.11 eV indicating non-adiabatic hopping. In addition to the intersite hopping process, the strong decrease of the quadrupole splitting and the isomer shift of Fe²⁺(8d) between 298 K and 360 K suggests the occurrence of intrinsic charge delocalization from Fe²⁺(8d) which does not involve the neighbouring Fe³⁺(8d) ions.     

Microanalysis of Fe3+/ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M 2,3 edge

The Fe M _2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe₃(Al_1–xFe_x)₂[SiO₄]₃ and andradite-skiagite (Fe_1–xCa_x)₃Fe₂[SiO₄]₃), pyroxenes (acmite-hedenbergite (Ca_1–xNa_x)(Fe²⁺ _1−xFe³⁺ _x)Si₂O₆), and spinels (magnetite-hercynite Fe(Al_1–xFe_x)₂O₄) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M _2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe³⁺/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe³⁺/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M _2,3 edge is observed. Since the partial cross sections of the Fe M _2,3 edges are some orders of magnitude higher than those of the Fe L _2,3 edges, the Fe M _2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M _2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe³⁺/ΣFe ratio over distances on the order of 100 nm. Received: 14 April 1998 / Revised, accepted: 8 March 1999     

Mössbauer spectra of kyanite,aquamarine, and cordierite showing intervalence charge transfer

The blue colors of several minerals and gems, including aquamarine (beryl, Be₃Al₂Si₆O₁₈) and cordierite (Al₃(Mg, Fe)₂Si₅AlO₁₈), have been attributed to charge transfer (CT) between adjacent Fe²⁺ and Fe³⁺ cations, while Fe²⁺→Ti⁴⁺ CT has been proposed for blue kyanites (Al₂SiO₅). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe²⁺ and Fe³⁺ were detected with MS, indicating that Fe²⁺→Fe³⁺ CT, Fe²⁺→Ti⁴⁺ CT, and Fe²⁺ and Fe³⁺ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe²⁺ and Fe³⁺ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm^?1 in E∥c spectra to Fe²⁺→Fe³⁺ CT between Fe cations replacing Al³⁺ ions 4.6Å apart along c. A band at 17,500 cm^?1 in E⊥c spectra of cordierite is generally assigned to Fe²⁺ (oct)→Fe³⁺ (tet) CT between cations only 2.74 Å apart. However, no Fe³⁺ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm^?1 band and reported to contain Fe³⁺. A quadrupole doublet with parameters consistent with tetrahedral Fe³⁺ appears in 77K MS, but the Fe³⁺/Fe²⁺ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe³⁺/Fe²⁺ ratios for minerals exhibiting Fe²⁺→Fe³⁺ CT.     

Vesuvianite-epidote paragenesis as a product of greenschist facies of regional metamorphism in the Western Alps

Occurences of stable vesuvianite-epidote paragenesis within greenschist facies of metamorphism are described. It is suggested that this paragenesis gives way to grossularite-diopside (plus vesuvianiteor epidote) assemblage in the almandine-amphibolite facies. This interpretation conforms best to the results of petrographic observations and also satisfies the topological requirements of the CaO-(Mg, Fe²⁺, Mn)O-(Al, Fe³⁺)₂O₃-SiO₂H₂O-CO₂ system, to which these assemblages belong. Contrary to the ideas in vogue, it is suggested that vesuvianite can very well originate within the greenschist facies, whereas the appearence of grossularite and diopside (consequent on which vesuvianite and epidote becomesmutually incompatible) ushers the almandine-amphibolite facies in.