L. Cemič

Heat capacity measurements of synthetic pyrope-grossular garnets between 320 and 1000 K by differential scanning calorimetry

The heat capacity of synthetic periclase (MgO), pyrope (M93Al2Si3O12), grossular (Ca3Al2Si3O12), and four pyrope-grossular solid solutions (PY89.8Gr10.2, PY73.6Gr26.4, PY48.8Gr51.2, PY24.0Gr76.0) has been measured between 320 and 1000 K by differential scanning calorimetry. Measurements were made in both interval-scanning and step-scanning modes; with the latter method, Cp was determined to a precision of better than ± 1%. The results, together with previous calorimetric measurements on synthetic grossular and pyrope, now permit a complete description of the heat capacities for both phases between 10-1200 K and 10-1350 K, respectively. The following best-fit equations for the temperature range from 298 to 1200 K were obtained for pyrope and grossular, respectively (T in Kelvin and Cp in J/ mol*K): Cp(pyrope) = 542.01 - 1387.07·T-0.5 - 20.492×106·T-2+2500.67×106·T-3; Cp(grossular) = 607.81 - 3214.49·T-0.5 - 12.141×106·T-2+1269.92×106·T-3. For the solid solution garnets a model assuming ideal mixing (linear combination) of molar heat capacities of the endmember phases reproduces the experimental heat capacities within errors. This indicates that the vibrational entropies of mixing and enthalpies of mixing are temperature-independent between room temperature and 1000 K. It is expected that excess heat capacities of mixing are not present in most silicate solid solutions at temperatures between 320 and 1000 K.

Solubility of corundum in supercritical water

Abstract Isothermal (670–700°C) solubility of corundum in supercritical water, within the stability range of corundum as a phase of the system Al2O3-H2O, has been determined by the weight loss method. Experiments were performed in the pressure range 2.5 to 6 kbar in cold seal hydrothermal equipment at 670 ± 5°C using gold tubing. The overall uncertainty of the solubility values obtained was 8%. Experiments in the pressure range 10 to 20 kbar were performed in a piston cylinder apparatus at 700°C using sealed gold capsules held by supporting steel containers. The overall uncertainty of the solubility values obtained was estimated to be 10%. All data could be fitted by the linear equation S[ppm Al2O3] = −12.37 + 7.24 · p[kbar] with correlation coefficient r = 0.9963. Separate fits of hydrothermal and piston cylinder results yielded a small difference between the two data sets, which is due rather to the experimental uncertainty than to the small temperature difference.

New developments in two-feldspar thermometry

Abstract The thermodynamic model of the two-feldspar thermometer has been revised. From recent enthalpy and volume measurements in the (Na,Ca)- and (K,Ca)-feldspar binaries, new interaction parameters have been derived and previous ones have been updated. Entropy parameters have been fitted to the phase equilibrium data of Seck (1971) and Elkins and Grove (1990). The two data sets could be suitably combined into one. Ideal Ab, Or, and An activities have been expressed in terms of both the molecular mixing and Al-avoidance models. Two-feldspar pairs from high-grade metamorphic rocks that cooled slowly under dry conditions suffer from a distinct type of retrograde resetting. Whereas the original An content in both the plagioclase and the alkali feldspar is preserved because the intercrystalline Ca + Al ↔ (Na,K) + Si diffusion is sluggish, Na and K may be freely exchanged between phases. Mathematical reversal of the Na-K exchange at constant An yields the temperature at which the two feldspars originally coexisted. The shifts in Ab and Or contents obtained from the reversal reflect the relative plagioclase/alkali feldspar proportions observed in thin sections. Good agreement between calculated and measured ratios was found for feldspar pairs from Sri Lankan granulites. This observation represents a successful test of the reliability of the calculated Ab-Or shifts. In contrast to dry metamorphic rocks, similar application of chemical constraints is not indicated in the case of volcanic rocks. Then the two-feldspar thermometer delivers three, usually incongruent temperatures: T(Ab), T(Or), and T(An). From the abundance of temperatures, Fuhrman and Lindsley (1988) suggested adjusting compositions within assumed chemical uncertainties (e.g., ±2 mol%) so that congruent temperatures could be obtained. However, the result is not unique. Depending on minute variations in the starting compositions, the temperatures may vary by several tens of degrees. In addition, temperatures vary to a similar extent depending on the type of search algorithm. Therefore, we advise users to completely abandon this practice. Instead, a statistical procedure is suggested: Two-feldspar compositions are randomly generated according to Gaussian distributions with their means at the observed compositions and standard errors chosen according to the quality of the chemical analysis. This procedure returns normally distributed temperatures [T(Ab), T(Or), T(An)] together with means and standard deviations. From the overlap of the three Gaussian curves the question of equilibrium or non-equilibrium crystallization of feldspar pairs may be addressed.

Experimental determination of the upper stability of scorzalite, FeAl2[OH/PO4]2, and the occurrence of minerals with a composition intermediate between scorzalite and lazulite(ss) up to the conditions of the amphibolite facies

SummaryThe stability field of scorzalite (FeAl2[OH/PO4]2) was investigated in the P-T range from 487 to 684 °C and 0.1 to 0.3 GPa. in hydrothermal experiments. The oxygen fugacity was fixed by the Ni/NiO buffer. Scorzalite shows a decomposition according to the reaction: FeAl2[OH/PO4]2) → FeAlPO5 + AlPO4 (berlinite) + H2O. The mean standard enthalpy and standard entropy of reaction were determined as ΔHR0 = 94(13) kJ, ASR = 180(16) JK−1. A57Fe-Mößbauer spectroscopic examination showed that about 4 atomic % of the total Fe in scorzalite is trivalent.ZusammenfassungDas Stabilitätsfeld von Skorzalith (FeAl2[OH/PO4]2) wurde im P-T-Bereich zwischen 487 und 684 °C und zwischen 0.1 und 0.3 GPa in Hydrothermalexperimenten unter der Sauerstoffugazität des Ni/NiO-Puffers untersucht. Skorzalith zerfällt unter diesen Bedingungen gemäß der Reaktion: FeAl2[OH/PO4]2) → FeAlPO5 + AlPO4 (Berlinit) + H2O. Die Reaktionsenthalpie und -entropie für Standardbedingungen wurden zu ΔHR0 = 94(13) kJ und ASR = 180(16) JK−1 bestimmt.57 Fe-Mößbaueruntersuchungen ergaben, daß ungefähr 4% des Gesamteisens in Skorzalith dreiwertig vorliegen.

THE HEAT CAPACITY OF THE SERPENTINE SUBGROUP MINERAL BERTHIERINE (Fe2.5Al0.5)[Si1.5Al0.5O5](OH)4

The serpentine subgroup mineral berthierine was synthesized as a metastable precursor of the chlorite group mineral chamosite in cold seal pressure vessels at 575°C, 0.5 GPa and fO2-conditions of the NNO buffer from a glass of almandine bulk composition. The run products were investigated with X-ray powder diffraction (XRD), Mossbauer spectroscopy and electron microprobe analysis. One run product was also investigated by high-resolution transmission electron microscopy (HRTEM) and its heat capacity measured by heat pulse calorimetry and by differential scanning calorimetry in the temperature range 5–323 K. The XRD and HRTEM investigations clearly showed that the periodicity along the c axis of this sample is 7 Å demonstrating that the serpentine subgroup mineral berthierine of composition (Fe2+1.83Fe3+0.33Al0 67)[Si1.33Al0.67O5](OH)4 has formed in the synthesis experiments.Integration of our heat capacity data, corrected to the composition (Fe2.5Al0.5)[Si1.5Al0.5O5](OH)4 for end-member berthierine, yields a standard entropy of 284.1±0.3 J mol−1 K−1. The Cp polynomial Cp = 610.72 − 5140.0 × T−0.5 − 5.8848 × 106T−2 + 9.5444 × 108T–3 is recommended for thermodynamic calculations above 298 K involving berthierine.