Ferrovalleriite, 2(Fe,Cu)S · 1.5Fe(OH)2: Validation as a mineral species and new data

Ferrovalleriite, ideally 2(Fe,Cu)S · 1.5Fe(OH)₂, a layered hydroxide-sulfide of the valleriite group and an analog of valleriite with Fe instead of Mg in the hydroxide block, has been approved by the IMA Commission on New Minerals, Nomenclature and Classification as a valid mineral species. It was found in the Oktyabr’sky Mine, Noril’sk, Krasnoyarsk krai, Siberia, Russia. Ferrovalleriite occurs in cavities of massive sulfide ore mainly consisting of cubanite and mooihoekite. In different cases, it is associated with magnetite, Fe-rich chlorite-like phyllosilicate, ferrotochilinite, hibbingite, or rhodochrosite. Ferrovalleriite forms crystals flattened on [001] (from scaly to tabular; up to 5 mm across and up to 0.3 mm thick), typically split and curved. Occasionally, they are combined into aggregates up to 1.5 × 2 cm. Ferrovalleriite is dark bronze-colored, with a metallic luster and black streak. The Mohs’ hardness is ca. 1; VHN is 35 kg/mm². Cleavage is perfect parallel to {001}, mica-like. Individuals are flexible and inelastic. D(calc) = 3.72 g/cm³. In reflected light, ferrovalleriite is pleochroic from yellowish to gray; bireflectance is moderate. Anisotropy is strong, with bluish gray to yellowish beige rotation colors. Reflectance values [R ₁–R ₂ %, (λ, nm)] are: 15.6–16.6 (470), 14.8–20.5 (546), 14.7–22.3 (589), 14.5–24.1 (650). The IR spectrum shows the presence of (OH) groups bonded with Fe cations and the absence of H2O molecules. The chemical composition of the holotype (wt %; electron microprobe, H content is calculated) is as follows: 0.10 Al, 0.03 Mn, 45.31 Fe, 0.07 Ni, 18.29 Cu, 20.37 S, 15.62 O, 0.98 H, total is 100.77. The empirical formula calculated on the basis of 2 S atoms is: Al_0.01Fe_2.55Cu_0.91S₂(OH)_3.07 = (Fe_1.09Cu_0.91)_Σ2S₂ · (Fe _1.34 ²⁺ Fe _0.12 ³⁺ Al_0.01)_Σ1.47(OH)_3.07. The structure of ferrovalleriite is incommensurate (misfit); two sublattices are present: (1) sulfide sublattice, space group $R\bar 3m$ , R3m or R32; the unit-cell dimensions are: a = 3.792(2), c = 34.06(3) Å, V = 424(1) ų and (2) hydroxide sublattice, space group $P\bar 3m1$ , P3m1 or P321; the unit-cell dimensions: a = 3.202(3), c = 11.35(2)Å, V = 100.8(3) ų. Together with this main polytype modification with three-layer (R-cell, Z = 3) sulfide block, the holotype ferrovalleriite contains the modification with one-layer (P-cell, Z = 1) sulfide block (sulfide sublattice with $P\bar 3m1$ , P3m1 or P321, unit cell dimensions: a = 3.789(4), c = 11.35(1) Å, V = 141(5) ų). The strongest reflections in the X-ray powder pattern (d, Å-I) are: 5.69–100; 3.268–58; 3.163–36; 1.894–34; 1.871–45.     

Hydrogen bonding in coquimbite, nominally Fe2(SO4)3·9H2O, and the relationship between coquimbite and paracoquimbite

Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite ( $ P {\bar 3}1c $ , a?=?10.924(2)/10.882(2) Å, c?=?17.086(3) / 17.154(3) Å, V?=?1765.8(3)/1759.2(5) ų, at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O–H distances in coquimbite at 50 K vary between 0.98 and 1.01 Å. In addition to H₂O molecules coordinated to the Al³⁺ and Fe³⁺ ions, there are rings of six “free” H₂O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 Å. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its “dimorph” paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe₂(SO₄)₃·9H₂O. Coquimbite, on the other hand, has a composition approximating Fe_1.5Al_0.5(SO₄)₃·9H₂O. Hence, even a “simple” sulfate Fe_2-x Al _x (SO₄)₃·9H₂O may be structurally rather complex.     

Structure and Mössbauer spectroscopy of barbosalite Fe2+Fe3+ 2 (PO4)2(OH)2 between 80 K and 300 K

Natural barbosalite Fe²⁺Fe³⁺ ₂ (PO₄)₂(OH)₂ from Bull Moose Mine, South Dakota, U.S.A., having ideal composition, was investigated with single crystal X-ray diffraction techniques, Mössbauer spectroscopy and SQUID magnetometry to redetermine crystal structure, valence state of iron and evolution of ⁵⁷Fe Mössbauer parameter and to propose the magnetic structure at low temperatures. At 298?K the title compound is monoclinic, space group P2₁/n, a _o ?= 7.3294(16)?Å, b _o ?=?7.4921(17)?Å, c _o ?=?7.4148 (18)?Å, β?=?118.43(3)°, Z?=?2. No crystallographic phase transition was observed between 298?K and 110?K. Slight discontinuities in the temperature dependence of lattice parameters and bond angles in the range between 150?K and 180?K are ascribed to the magnetic phase transition of the title compound. At 298?K the Mössbauer spectrum of the barbosalite shows two paramagnetic components, typical for Fe²⁺ and Fe³⁺ in octahedral coordination; the area ratio Fe³⁺/Fe²⁺ is exactly two, corresponding to the ideal value. Both the Fe²⁺ and the Fe³⁺ sublattice order magnetically below 173?K and exhibit a fully developed magnetic pattern at 160?K. The electric field gradient at the Fe²⁺ site is distorted from axial symmetry with the direction of the magnetic field nearly perpendicular to V_zz, the main component of the electric field gradient. The temperature dependent magnetic susceptibility exhibits strong antiferromagnetic ordering within the corner-sharing Fe³⁺-chains parallel to [101], whereas ferromagnetic coupling is assumed within the face-sharing [1?1?0] and [?1?1?0] Fe³⁺-Fe²⁺-Fe³⁺ trimer, connecting the Fe³⁺-chains to each other.     

Mössbauer effect study of anapaite, Ca2Fe2+(PO4)2 · 4H2O, and of its oxidation products

Mössbauer spectra (MS) of anapaite (Ca₂ Fe²⁺(PO₄)₂?·?4H₂O) and of a sample after being immersed in a 4% H₂O₂ solution at room temperature (RT) over 12 days (hereafter an4ox) were collected at temperatures in the range 4.2 to 420?K and 11 to 300?K respectively. All MS consist of symmetrical doublets, hence magnetic ordering was not observed. The temperature dependencies of the Fe²⁺ centre shifts of anapaite and an4ox were analysed with the Debye model for the lattice vibrations. The characteristic Mössbauer temperatures were found as 370?K?±?25?K and 340?K?±?25?K, and the intrinsic isomer shifts as 1.427?±?0.005?mm/s and 1.418?±?0.005?mm/s respectively. From the external-field (60?kOe) MS recorded at 4.2 and 189?K for the non-treated sample, the principal component V _zz of the electric field gradient (EFG) is determined to be positive and the asymmetry parameter η?≈?0.2 and 0.4 respectively. The temperature variations of the quadrupole splittings, ΔE _Q(T), cannot be interpreted on the basis of the thermal population of the ⁵ D electronic levels resulting from the tetragonal compression of the O₆ co-ordination. The low-temperature linear behaviour of ΔE _Q(T) is attributed to a strong orbit-lattice coupling. A field of 60 kOe applied to anapaite at 4.2?K produces magnetic hyperfine splitting with effective hyperfine fields of ?136, ?254 and ?171?kOe along the principal axes Ox, Oy and Oz of the EFG tensor respectively. Additional oxidation treatments in solutions with various H₂O₂ concentrations up to 20% and subsequent Mössbauer experiments at room temperature, have revealed that the anapaite structure is not sensitive to oxidation since eventually only a small amount of Fe²⁺ (～6.5%) is converted into Fe³⁺.     

Raman study of MgCr2O4–Fe2+Cr2O4 and MgCr2O4–MgFe2 3+O4 synthetic series: the effects of Fe2+ and Fe3+ on Raman shifts

Two synthetic series of spinels, MgCr₂O₄–Fe²⁺Cr₂O₄ and MgCr₂O₄–MgFe₂ ³⁺O₄ have been studied by Raman spectroscopy to investigate the effects of Fe²⁺ and Fe³⁺ on their structure. In the first case, where Fe²⁺ substitutes Mg within the tetrahedral site, there is a continuous and monotonic shift of the Raman modes A_1g and E_g toward lower wavenumbers with the increase of the chromite component into the spinel, while the F_2g modes remain nearly in the same position. In the second series, for low Mg-ferrite content, Fe³⁺ substitutes for Cr in the octahedral site; when the Mg-ferrite content nears 40 %, a drastic change in the Raman spectra occurs as Fe³⁺ starts entering the tetrahedral site as well, consequently pushing Mg to occupy the octahedral one. The Raman spectral region between 620 and 700 cm^?1 is associated to the octahedral site, where three peaks are present and it is possible to observe the Cr–Fe³⁺ substitution and the effects of order–disorder in the tetrahedral site. The spectral range at 500–620 cm^?1 region shows that there is a shift of modes toward lower values with the increase of the Mg-ferrite content. The peaks in the region at 200–500 cm^?1, when observed, show little or negligible Raman shift.     

Electronic state of Fe2+ in (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)SiO3 majorite at pressures up to 81 GPa and temperatures up to 800 K

Despite a large number of studies of iron spin state in silicate perovskite at high pressure and high temperature, there is still disagreement regarding the type and P–T conditions of the transition, and whether Fe²⁺ or Fe³⁺ or both iron cations are involved. Recently, our group published results of a Mössbauer spectroscopy study of the iron behaviour in (Mg,Fe)(Si,Al)O₃ perovskite at pressures up to 110 GPa (McCammon et al. 2008), where we suggested stabilization of the intermediate spin state for 8- to 12-fold coordinated ferrous iron (^[8–12]Fe²⁺) in silicate perovskite above 30 GPa. In order to explore the behaviour in related systems, we performed a comparative Mössbauer spectroscopic study of silicate perovskite (Fe_0.12Mg_0.88SiO₃) and majorite (with two compositions—Fe_0.18Mg_0.82SiO₃ and Fe_0.11Mg_0.88SiO₃) at pressures up to 81 GPa in the temperature range 296–800 K, which was mainly motivated by the fact that the oxygen environment of ferrous iron in majorite is quite similar to that in silicate perovskite. The ^[8–12]Fe²⁺ component, dominating the Mössbauer spectra of majorites, shows high quadrupole splitting (QS) values, about 3.6 mm s^?1, in the entire studied P–T region (pressures to 58 GPa and 296–800 K). Decrease of the QS of this component with temperature at constant pressure can be described by the Huggins model with the energy splitting between low-energy e _g levels of ^[8–12]Fe²⁺ equal to 1,500 (50) cm^?1 for Fe_0.18Mg_0.82SiO₃ and to 1,680 (70) cm^?1 for Fe_0.11Mg_0.88SiO₃. In contrast, for the silicate perovskite dominating Mössbauer component associated with ^[8–12]Fe²⁺ suggests the gradual change of the electronic properties. Namely, an additional spectral component with central shift close to that for high-spin ^[8–12]Fe²⁺ and QS about 3.7 mm s^?1 appeared at ~35 (2) GPa, and the amount of the component increases with both pressure and temperature. The temperature dependence of QS of the component cannot be described in the framework of the Huggins model. Observed differences in the high-pressure high-temperature behaviour of ^[8–12]Fe²⁺ in the silicate perovskite and majorite phases provide additional arguments in favour of the gradual high-spin—intermediate-spin crossover in lower mantle perovskite, previously reported by McCammon et al. (2008) and Lin et al. (2008).     

Shear modulus, heat capacity, viscosity and structural relaxation time of Na2O–Al2O3–SiO2 and Na2O–Fe2O3–Al2O3–SiO2 melts

The configurational heat capacity, shear modulus and shear viscosity of a series of Na₂O–Fe₂O₃–Al₂O₃–SiO₂ melts have been determined as a function of composition. A change in composition dependence of each of the physical properties is observed as Na₂O/(Na₂O + Al₂O₃) is decreased, and the peralkaline melts become peraluminous and a new charge-balanced Al-structure appears in the melts. Of special interest are the frequency dependent (1 mHz–1 Hz) measurements of the shear modulus. These forced oscillation measurements determine the lifetimes of Si–O bonds and Na–O bonds in the melt. The lifetime of the Al–O bonds could not, however, be resolved from the mechanical spectrum. Therefore, it appears that the lifetime of Al–O bonds in these melts is similar to that of Si–O bonds with the Al–O relaxation peak being subsumed by the Si–O relaxation peak. The appearance of a new Al-structure in the peraluminous melts also cannot be resolved from the mechanical spectra, although a change in elastic shear modulus is determined as a function of composition. The structural shear-relaxation time of some of these melts is not that which is predicted by the Maxwell equation, but up to 1.5 orders of magnitude faster. Although the configurational heat capacity, density and shear modulus of the melts show a change in trend as a function of composition at the boundary between peralkaline and peraluminous, the deviation in relaxation time from the Maxwell equation occurs in the peralkaline regime. The measured relaxation times for both the very peralkaline melts and the peraluminous melts are identical with the calculated Maxwell relaxation time. As the Maxwell equation was created to describe the timescale of flow of a mono-structure material, a deviation from the prediction would indicate that the structure of the melt is too complex to be described by this simple flow equation. One possibility is that Al-rich channels form and then disappear with decreasing Si/Al, and that the flow is dominated by the lifetime of Si–O bonds in the Al-poor peralkaline melts, and by the lifetime of Al–O bonds in the relatively Si-poor peralkaline and peraluminous melts with a complex flow mechanism occurring in the mid-compositions. This anomalous deviation from the calculated relaxation time appears to be independent of the change in structure expected to occur at the peralkaline/peraluminous boundary due to the lack of charge-balancing cations for the Al-tetrahedra.     

Jichengite 3CuIr2S4·(Ni, Fe)9S8, a New Mineral, and Its Crystal Structure

A new mineral, jichengite ideally 3CuIr2S4·(Ni,Fe)9S8, was found as a constituent of placer concentrates at a branch of the Luanhe River, about 220 km NNE of Beijing. Its associated minerals are chromite, magnetite, ilmenite, zircon, native gold, iridium, ferrian platinum and osmium. The placer is distributed at places around ultrabasic rock, which hosts chromite orebodies, from which PGM originated. Jichengite occurs commonly as massive or granular aggregates. No perfect morphology of jichengite was observed. It is steel gray and opaque with metallic luster and black streak. It has a Mohs hardness of 5, VHN (d) μm 21.65, Hm 4.465, Hv = 268.1 N/um2. It is brittle and weakly magnetic. Cleavage {010} is rarely observed. No fracture was observed. Density could not be measured because of its too small grain size. Density (calc.) is 7.003 g/cm3. Reflect light is reddish-brown, without internal reflections. Anisotropism is distinct with grayish or yellowish white in crossed nicols and bluish violet-copper red in uncrossed nicols. Jichengite shows weak pleochroism and strong bireflectance. The reflectance values in air at the Standard Commission on Ore Mineralogy wavelengths are: 38.9, 34.3 at 470 nm, 38.9, 34.5 at 546 nm, 39.1, 35.3 at 590 nm, 39.2, 36.8 at 650 nm, parallel-axial extinction. The six strongest lines in the X-ray powder-diffraction pattern [d in ?, (I), (hkl)] are: 3.00 (100) (116), 2.80 (50) (205), 2.48. (50) (208), 1.916 (40) (2, 1, 10), 1.765 (60) (220), 1.753 (50) (2, 0, 16). Five chemical analyses carried out, yielding the following results: S 25.76 (25.49-5.97), Fe 10.03 (9.78-10.31), Co 0.78 (0.75-0.81), Ni 12.48 (12.32-12.85), Cu 4.77 (4.69-4.83), Ir 46.98(46.14-47.89), sum 100.80wt%, which produced a formula (Cu1.556Fe0.976)2.532(Ir5.063S10.126)·(Fe2.7451Ni4.404Co0.273)7.422S6.517. The ideal formula is X10Ir5S17.5, which was calculated by single crystal structure analyses, where X = Cu(II) + Fe(II) + Ni(II) + Co(II). The single crystal data were collected using a diffractometer with Mo Ka radiation and a graphite monochromate. The crystal system is trigonal with space group R3m and unit cell parameters a=7.0745(14) ?, c=34.267(10) ? (The superstructure not found), and the final R Indices [with 564 observed reflections, I>2sigma (I)] are R1=0.0495, wR2=0.1349. The specimens are deposited in the Geological Museum of China.     

β(Mg0.9, Fe0.1)2SiO4: Single crystal structure,cation distribution,and properties of coordination polyhedra

The synthesis boundaries of the phase transformation; ++ in (Mg_0.9, Fe_0.1)SiO₄, have been clarified at temperatures to 2000° C and pressures up to 20 GPa in order to synthesize single crystals of high quality. A single crystal of (Mg_0.9, Fe_0.1)₂SiO₄ was grown successfully to a size of 500 m. The crystal structure has been refined from single-crystal X-ray intensities. The ferrous ions prefer M1 and M3 sites to over the larger M2 site. The volume change of all the occupied polyhedra does not contribute to the decrease of total volume in the transformation; rather it tends to increase the bulk volume through the expansion of occupied tetrahedra. The volume reduction in the phase transformations is accounted for by unoccupied polyhedra, with the octahedra contributory 60% and the tetrahedra 40% to the V of the transition. The volume change in the transformation is caused also partly by the volume decrease of MO ₆ (25%), partly the unoccupied tetrahedra (45%) and octahedra (30%).     

Petrographic and Fe Isotopic Constraints on the Genesis of the Shilu Fe Ore Deposit in Hainan Province, China

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Mineralogy, 57Fe Mössbauer spectra and magnetization of chalcolithic pottery

Three chalcolithic pottery sherds, paint removed from the surface of each sherd, and an unheated red pigment (Tell-Halaf culture, Turkey) were analysed within the frame of archaeometric studies using mineralogical methods, ⁵⁷Fe Mössbauer spectroscopy, magnetization and rotational hysteresis data. From mineralogical results, the individual minerals forming the cores of the sherds were determined. It was found that the sherds are lime-rich. High temperature X-ray analysis on comparable Ca-rich material showed that the established composition is consistent with a firing temperature of 750-950°C. Apart from the pigment, each Mössbauer spectrum of Fe-bearing components consists of dominating paramagnetic doublets, arising mostly from silicate phases, and of a six-line pattern with reduced intensity, due to ferri- and/or antiferromagnetic Fe-oxide phases. For three samples, an Fe³⁺ silicate component of the spectra is clearly dominating, which points to oxidizing conditions during firing. For the others Fe²⁺ and Fe³⁺ components occur in about equal intensities. For the pigment, the magnetic sextet is of similar intensity to the Fe³⁺ silicate component. From magnetic analysis of ferrimagnetic phases it follows that a low percentage of particles of solid solutions -Fe₂O₃ – Fe₃O₄ exist, probably in part 0.1 m in diameter. The ferrimagnetic particles of at least one paint are probably covered by a thin layer of hematite as found from rotational hysteresis data. An attempt is made to draw conclusions from the experimental results, regarding the firing conditions of the sherds and paints.     

Geochemical constraints on Cu–Fe and Fe skarn deposits in the Edong district,Middle–Lower Yangtze River metallogenic belt,China

Copper and iron skarn deposits are economically important types of skarn deposits throughout the world, especially in China, but the differences between Cu and Fe skarn deposits are poorly constrained. The Edong ore district in southeastern Hubei Province, Middle–Lower Yangtze River metallogenic belt, China, contains numerous Fe and Cu–Fe skarn deposits. In this contribution, variations in skarn mineralogy, mineralization-related intrusions and sulfur isotope values between these Cu–Fe and Fe skarn deposits are discussed.The garnets and pyroxenes of the Cu–Fe and Fe skarn deposits in the Edong ore district share similar compositions, i.e., dominantly andradite (Ad_29–100Gr_0–68) and diopside (Di_54–100Hd_0–38), respectively. This feature indicates that the mineral compositions of skarn silicate mineral assemblages were not the critical controlling factors for variations between the Cu–Fe and Fe skarn deposits. Intrusions associated with skarn Fe deposits in the Edong ore district differ from those Cu–Fe skarn deposits in petrology, geochemistry and Sr–Nd isotope. Intrusions associated with Fe deposits have large variations in their (La/Yb)_N ratios (3.84–24.6) and Eu anomalies (δEu = 0.32–1.65), and have relatively low Sr/Y ratios (4.2–44.0) and high Yb contents (1.20–11.8 ppm), as well as radiogenic Sr–Nd isotopes (εNd(t) = − 12.5 to − 9.2) and (⁸⁷Sr/⁸⁶Sr)_i = 0.7067 to 0.7086. In contrast, intrusions associated with Cu–Fe deposits are characterized by relatively high Sr/Y (35.0–81.3) and (La/Yb)_N (15.0–31.6) ratios, low Yb contents (1.00–1.62 ppm) without obvious Eu anomalies (δEu = 0.67–0.97), as well as (⁸⁷Sr/⁸⁶Sr)_i = 0.7055 to 0.7068 and εNd(t) = − 7.9 to − 3.4. Geochemical evidence indicates a greater contribution from the crust in intrusions associated with Fe skarn deposits than in intrusions associated with Cu–Fe skarn deposits. In the Edong ore district, the sulfides and sulfates in the Cu–Fe skarn deposits have sulfur isotope signatures that differ from those of Fe skarn deposits. The Cu–Fe skarn deposits have a narrow range of δ³⁴S values from − 6.2‰ to + 8.7‰ in sulfides, and + 13.2‰ to + 15.2‰ in anhydrite, while the Fe skarn deposits have a wide range of δ³⁴S values from + 10.3‰ to + 20.0‰ in pyrite and + 18.9‰ to + 30.8‰ in anhydrite. Sulfur isotope data for anhydrite and sedimentary country rocks suggest that the formation of skarns in the Edong district involved the interaction between magmatic fluids and variable amounts of evaporites in host rocks.     

Thermodynamics of (Fe2+, Mn2+, Mg, Ca)3− Al2Si3O12 garnet: a review and analysis

Summary The thermodynamic properties of garnets in the system (Fe²⁺, Mn²⁺, Mg, Ca)₃A1₂Si₃O₁₂ are reviewed. The thermodynamic properties of the three end-member garnets pyrope, almandine and grossular, including their volume, enthalpy of formation, entropy, compressibility and thermal expansion have been well determined. For spessartine enthalpy of formation and heat capacity at low temperatures are needed. Pyrope's unusual behavior in some of its properties is probably related to the presence of the small, light Mg cation, which has a large anisotropic thermal vibration. The thermodynamic mixing properties of the six binaries are also discussed. Good volume of mixing data exist now for all of the binaries, but much work is still required to determine the enthalpies and third-law vibrational entropies of mixing. It is shown that the magnitude of the positive deviations in the volumes of mixing is related to the volume difference between the two end-member components. It is probable that excess entropies, if present, originate at low temperatures below 200 K. Recent²⁹Si NMR experiments have demonstrated the presence of short-range ordering (SRO) of Ca and Mg in pyrope-grossular solid solutions. Short-range order will have to be considered in new models describing the entropies of mixing. Its possible presence in all garnet solid solutions needs to be examined. The mixing properties of pyrope-grossular garnets, which are the best known for any garnet binary, can, in part, be described by the Quasi-Chemical approximation, which gives insight into the microscopic interactions which determine the macroscopic thermodynamic mixing properties. Microscopic properties are best investigated by spectroscopic and computational approaches. Hard mode IR measurements on binary solid solutions show that the range of local microscopic structural distortion is reflected in the macroscopic volumes of mixing. The nature of The contents of this contribution was presented at the IMA Meeting in Toronto in August, 1998. It precedes issues of Mineralogy and Petrology containing thematic sets of IMApapers strain tiields and site relaxation needs to be studied in order to obtain a better understanding of the solid-solution process and energetics in garnet. Critical areas for future experimentation are also addressed.[/p]

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Eine kritische Zusammenstellung und Analyse der thermodynamischen Daten der (Fe²⁺, Mn²⁺, Mg, Ca)₃Al₂Si₃O₁₂ granate

Zusammenfassung In dieser Studie werden die thermodynamischen Eigenschaften der Granate im System (Fe²⁺,Mn²⁺, Mg, Ca)₃Al₂Si₃O₁₂ kritisch zusammengestellt. Die thermodynamischen Eigenschaften der drei Endglied-Granate Pyrop, Almandin und Grossular, einschließlich ihrer Volumina, Bildungswärmen, Entropien, Kompressibilitäten und thermischen Ausdehnungen wurden bereits hinreichend gut bestimmt. Dagegen müssen die Bildungswärme und Tieftemperatur-Wärmekapazität von Spessartin noch gemessen werden. Die Eigenschaften des Pyrops sind wahrscheinlich mit den großen anisotropen Schwingungen des kleinen, leichten Mg-Kations verbunden. Die thermodynamischen Mischungseigenschaften der sechs binären Mischkristallreihen werden ebenfalls diskutiert. Während die Mischungs-Volumendaten der binären Mischreihen gut bekannt sind, müssen ihre Mischungs-Enthalpien und Standard-Mischungsentropien noch ermittelt werden. Es wurde gezeigt, daß die Größe der positiven Exzeß-Volumina mit dem Volumen-Unterschied der zwei Endglied-Komponenten der jeweiligen Mischreihe verknüpft ist. Es ist wahrscheinlich, daß Exzeß-Entropien, wenn vorhanden, erst bei Tieftemperaturen unter 200 K auftreten. Neue²⁹Si NMR-Experimente belegen, daß in Pyrop-Grossular-Mischkristallen Nahordnung von Mg und Ca vorliegt. Der Effekt der Nahordnung muß in künftigen thermodynamischen Modellen berücksichtigt werden. Hieraus ergibt sich die Notwendigkeit, alle Granat-Mischreihen auf mögliche Nahordnung hin zu untersuchen. Die Mischungseigenschaften der Pyrop-Grossular-Mischreihe, die von sämtlichen Granat-Mischreihen am besten bestimmt wurden, können teilweise mit dem Quasi-Chemical-Model beschrieben werden. Dieses Modell ermöglicht die Beschreibung der mikroskopischen Wechselwirkungen, die die makroskopischen thermodynamischen Eigenschaften bestimmen. Mikroskopische Eigenschaften werden am besten mit spektroskopischen Messungen und theoretischen Berechnungen untersucht. Hard-mode IR-Spektroskopie-Messungen an binären Mischreihen zeigen, daß die lokalen mikroskopischen strukturellen Verzerrungen in den makroskopischen Mischungs-Volumina widergespiegelt werden. Die Art der Spannungsfelder und Platz-Relaxationen muß detaillierter untersucht werden, um ein besseres Verständnis des Mischkristall-Bildungsprozsses und der Energetik der Granate zu erreichen. Darüber hinaus werden wichtige künftige Forschungsgebiete diskutiert.

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With 11 Figures     

Fe43+FeTiO9,一个由2Fe2O3-FeTiO3组成的铁-钛氧化物矿物

Fe－Ti氧化物矿物产于新疆阿尔泰伟晶岩中．与之共生的有铌-钽铁矿、细晶石、锆石、白云母、长石等。Fe－Ti氧化物是不规则粒状,黑褐色．条痕浅褐红色．金属光泽,无解理,硬度5．5～6．0．比重5.30～5．50。在反光镜下矿物的反射色为灰白色,反射多色性弱,在MPV－3型裂隙光度计上其反射率为27．37～22．65（400～700nm）。化学分析结果是：Fe2O362．23,MgO0．22．Mno1．24．FeO13．98,TiO217．96,SiO23．87,CaO0．20,总计9970．扣除杂质后：Fe2O364．87,MgO0．24．MnO1．29．FeO14．58．TiO218．73,总计99．70,其化学式为FeFeTiO9。矿物的强X射线值：2．72（10,014）．2．52（10．110）．1．85（8．115）,1．70（9,008）．1．457（8,027）,三方晶系a=0．504,c=1．355nm．Z=2．空间群P63／m（？）。     

Fe–Mg Interdiffusion in (Mg,Fe)O

We have performed a series of interdiffusion experiments on magnesiowüstite samples at room pressure, temperatures from 1,320° to 1,400°C, and oxygen fugacities from 10^?1.0 Pa to 10^?4.3 Pa, using mixed CO/CO₂ or H₂/CO₂ gases. The interdiffusion couples were composed of a single-crystal of MgO lightly pressed against a single-crystal of (Mg_1-x Fe _x )_1-δO with 0.07<x<0.27. The interdiffusion coefficient was calculated using the Boltzmann–Matano analysis as a function of iron content, oxygen fugacity, temperature, and water fugacity. For the entire range of conditions tested and for compositions with 0.01<x<0.27, the interdiffusion coefficient varies as $$\tilde D\, =\,2.9\times10^{ - 6}\,f_{{\text{O}}_2 }^{0.19}\,x^{0.73}\,{\text{e}}^{ - (209,000\, -\,96,000\,x)/RT}\,\,{\text{m}}^{\text{2}} {\text{s}}^{ -1} $$ These dependencies on oxygen fugacity and composition are reasonably consistent with interdiffusion mediated by unassociated cation vacancies. For the limited range of water activity that could be investigated using mixed gases at room pressure, no effect of water on interdiffusion could be observed. The dependence of the interdiffusion coefficient on iron content decreased with increasing iron concentration at constant oxygen fugacity and temperature. There is a close agreement between our activation energy for interdiffusion extrapolated to zero iron content (x=0) and that of previous researchers who used electrical conductivity experiments to determine vacancy diffusivities in lightly doped MgO.     

Heat capacity and thermodynamic properties of andradite garnet,Ca3Fe2Si3O12, between 10 and 1000 K and revised values for ΔfGom (298.15 K) of hedenbergite and wollastonite

The heat capacity of synthetic andradite garnet (Ca₃Fe₂Si₃O₁₂) was measured between 9.6 and 365.5 K by cryogenic adiabatic calorimetry and from 340 to 990 K by differential scanning calorimetry. At 298.15 K C^o_p,m and S^o_m are 351.9 ± 0.7 and 316.4 ± 2.0 J/(mol·K), respectively.Andradite has a λ-peak in C^o_p,m with a maximum at 11.7 ± 0.2 K which is presumably associated with the antiferromagnetic ordering of the magnetic moments of the Fe³⁺ ions. The Gibbs free energy of formation, Δ_fG^o_m (298.15 K) of andradite is −5414.8 ± 5.5 kJ/mol and was obtained by combining our entropy and heat capacity data with the known breakdown of andradite to pseudowollastonite and hematite at ≈ 1410 to 1438 K. From a reexamination of the calcite + quartz = wollastonite equilibrium data we obtained Δ_fH^o_m (298.15 K) = − 1634.5 ± 1.8 kJ/mol for wollastonite.Between 300 and 1000 K the molar heat capacity of andradite can be represented by the equation C^o_p,m = 809.24 - 7.025 × 10−2T− 7.403 × 10³T−0.5 − 6.789 × 10⁵T−2. We have also used our thermochemical data for andradite to estimate the Gibbs free energy of formation of hedenbergite (CaFeSi₂O₆) for which we obtained Δ_fG^o_m (298.15 K) = −2674.3 ± 5.8 kJ/mol.     

Multistage growth of Fe–Mg–carpholite and Fe–Mg–chloritoid,from field evidence to thermodynamic modelling

We provide new insights into the prograde evolution of HP/LT metasedimentary rocks on the basis of detailed petrologic examination, element-partitioning analysis, and thermodynamic modelling of well-preserved Fe–Mg–carpholite- and Fe–Mg–chloritoid-bearing rocks from the Afyon Zone (Anatolia). We document continuous and discontinuous compositional (ferromagnesian substitution) zoning of carpholite (overall X _Mg = 0.27–0.73) and chloritoid (overall X _Mg = 0.07–0.30), as well as clear equilibrium and disequilibrium (i.e., reaction-related) textures involving carpholite and chloritoid, which consistently account for the consistent enrichment in Mg of both minerals through time, and the progressive replacement of carpholite by chloritoid. Mg/Fe distribution coefficients calculated between carpholite and chloritoid vary widely within samples (2.2–20.0). Among this range, only values of 7–11 correlate with equilibrium textures, in agreement with data from the literature. Equilibrium phase diagrams for metapelitic compositions are calculated using a newly modified thermodynamic dataset, including most recent data for carpholite, chloritoid, chlorite, and white mica, as well as further refinements for Fe–carpholite, and both chloritoid end-members, as required to reproduce accurately petrologic observations (phase relations, experimental constraints, Mg/Fe partitioning). Modelling reveals that Mg/Fe partitioning between carpholite and chloritoid is greatly sensitive to temperature and calls for a future evaluation of possible use as a thermometer. In addition, calculations show significant effective bulk composition changes during prograde metamorphism due to the fractionation of chloritoid formed at the expense of carpholite. We retrieve P–T conditions for several carpholite and chloritoid growth stages (1) during prograde stages using unfractionated, bulk-rock XRF analyses, and (2) at peak conditions using compositions fractionated for chloritoid. The P–T paths reconstructed for the Kütahya and Afyon areas shed light on contrasting temperature conditions for these areas during prograde and peak stages.     

Spinels,Fe–Ti oxide minerals,apatites, and carbonates hosted in the ophiolites of Eastern Desert of Egypt: mineralogy and chemical aspects

Spinels, Fe–Ti oxide minerals, apatites, and carbonates hosted in ophiolitic serpentinites and metagabbros of Gabal Garf (southern ED) and Wadi Hammariya (central ED) of Egypt are discussed. Microscopic and electron probe studies on these minerals are made to evaluate their textural and compositional variations. Alteration of chromites led to form ferritchromite and magnetite; rutile–magnetite intergrowths and martite are common in serpentinites. Fine trillis exsolution of ilmenite–magnetite and ilmenite–hematite and intergrowth of rutile–magnetite and ilmenite–sphene are recorded. Composite intergrowth grains of titanomagnetite–ilmenite trellis lamellae are common in metagabbros. The formation of ilmenite trellis and lamellae in magnetite and titanomagnetite indicate an oxidation process due to excess of oxygen contained in titanomagnetite; trapped and external oxidizing agents. This indicates the high P _H2O and oxygen fugacity of the parental magma. The sulfides minerals include pyrrhotite, pyrite and chalcopyrite. Based on the chemical characteristics, the Fe–Ti oxide from the ophiolitic metagabbros in both areas corresponds to ilmenite. The patites from the metagabbros are identified as fluor-apatite. Carbonates are represented by dolomites in serpentinites and calcite in metagabbros. Spinel crystals in serpentinites are homogenous or zoned with unaltered cores of Al-spinel to ferritchromit and Cr-magnetite toward the altered rims. Compared to cores, the metamorphic rims are enriched in Cr# (0.87–1.00 vs. 0.83–0.86 for rims and cores, respectively) and impoverished in Mg# (0.26–0.48 vs. 0.56–0.67) due to Mg–Fe and Al (Cr)–Fe³⁺ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. The Fe–Ti oxides have been formed under temperature of ~800 °C for ilmenite. Al-spinels equilibrated below 500–550 °C, while the altered spinel rims correspond to metamorphism around 500–600 °C. Geochemical evidence of the podiform Al-spinels suggest a greenschist up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the host rocks. Al-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, suggesting relic magmatic composition unaffected by metamorphism. The composition of Al-spinel grains suggest an ophiolitic origin and derivation by crystallization of boninitic magma that belonging to a supra-subduction setting could form either in forearcs during an incipient stage of subduction initiation or in back-arc basins.