Calculated mineral equilibria in the greenschist-blueschist-eclogite facies in Na2O−FeO−MgO−Al2O3−SiO2−H2O

The new, greatly expanded internally-consistent dataset of Holland and Powell includes thermodynamic data for a wide range of mineral end-members in common rock-forming minerals, in particular, including FeMg_-1 substitutions in glaucophane, garnet, chloritoid and carpholite, and FeMg_-1 and MgSiAl_-1Al_-1 substitutions in talc and chlorite. Moreover, we have the uncertainties and correlations for these data. With the data, we have calculated the full pressure-temperature phase diagram for the system Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NFMASH) for quartz (or coesite) and H₂O in excess, in the range 300°–800°C and 5–50 kbars. By solving the set of non-linear equations formed by the equilibrium relationships for an independent set of equations between the end-members in an assemblage in NFMASH, the compositions of the minerals (and PT) can be calculated. Thus the changes in MgSiAl_-1Al_-1 along NMASH reactions, and FeSiAl_-1Al_-1 along NFASH reactions, are calculated, and the changes in FeMg_-1 and MgSiAl_-1Al_-1 along NFMASH reactions are calculated. From this information it is straightforward to generate PT diagrams for specific rock compositions. Mineral assemblages and mineral compositional changes in the phase diagram are discussed in relation to greenschist, blueschist and eclogite facies assemblages in metapelitic rocks. It is found that the correspondence between the predictions of the phase diagrams and the observations on rocks is remarkably good. When semiquantitative extensions of the phase diagram to include Ca(MgFe)_-1, NaSiCa_-1Al_-1, Fe³⁺Al_-1 and KNa_-1 substitutions are taken into account the agreement is essentially complete.     

镁铝对烧结矿中铁酸钙的矿物学特性影响

铁酸钙是高碱度烧结矿中的主要黏结相矿物,它的含量、结晶形态、化学成分及晶体结构等矿物学特性对烧结矿质量起着关键性作用,而烧结原料中各组分的含量直接影响着铁酸钙的生成。以Fe₃O₄、SiO₂、CaO、MgO、Al₂O₃的化学纯试剂为原料,在实验室进行微型烧结实验,运用XRD、偏光显微镜、电子探针等手段,定量分析研究了原料组分中MgO、Al₂O₃对烧结矿中铁酸钙的生成及其矿物学特性的影响。原料中MgO含量的增加对铁酸钙的生成有一定抑制作用,尤其在MgO含量为2.0%~3.0%时,烧结矿中铁酸钙含量明显减少,其晶体形态也从以板柱状和针状为主逐渐过渡为它形不规则状;原料中Al₂O₃的增加,对烧结矿中铁酸钙的形成具有促进作用,即随着Al₂O₃的增加铁酸钙含量呈明显增加趋势,且铁酸钙的形态也由以柱状和针状为主向板柱状变化。电子探针成分分析及矿物化学式计算结果表明,铁酸钙是由Fe₂O₃、CaO、SiO₂、Al₂O₃及MgO组成的复杂结晶体,其化学通式为Ca_2.60Mg_0.44Si_1.07Al_0.96Fe_8.92O₂₀。原料中MgO、Al₂O₃含量的变化,对铁酸钙的化学成分中Fe₂O₃/CaO摩尔分数比影响不大,均接近3∶2。上述研究结果对于深刻理解烧结工艺条件下铁酸钙晶相的晶体化学特征及其对烧结矿质量的影响具有重要指导意义。     

Ilmenite-type solid solutions between MgSiO3 and Al2O3 and some structural systematics among ilmenite compounds

Complete solid solutions with the ilmenite structure from pure MgSiO₃ to 75% MgSiO₃·25% Al₂O₃ have been synthesized in the pressure region between 240 and 300 kbar at 1000–1400°C in a diamond-anvil cell coupled with laser heating. The results suggest that complete solid solutions with the ilmenite structure might be formed between MgSiO₃ and Al₂O₃ under high pressure-high temperature conditions. The lattice parameters for the ilmenite solid solutions between MgSiO₃ and Al₂O₃ deviate from ideality in the same manner as those found by Berry and Combs along the FeVO₃–Fe₂O₃ join. For the ordered A²⁺ B⁴⁺O₃ ilmenite-type compounds, c_o is determined primarily by the size of the relatively large A²⁺ cation, whereas a_o depends strongly on the radii of both A²⁺ and B⁴⁺ cations. Such systematics might account for the fact that c_o and a_o for the ilmenite-type MgSiO₃ are respectively smaller and larger than those for Al₂O₃. The lattice parameters _ao and molar volumes for the A₂³⁺O₃ corundum-type compounds and the disordered A²⁺B⁴⁺O₃ ilmenites follow a different trend and are, therefore, readily distinguished from the ordered A²⁺B⁴⁺O₃ ilmenites.     

Melting and subsolidus reactions in the system K2O−CaO−MgO−Al2O3−SiO2−H2O: experiments and petrologic application

Experiments with synthetic starting materials of muscovite, phlogopite, zoisite, kyanite and quartz were performed in the pressure temperature range 10–25 kbar, 640–780° C under water excess conditions. The reaction muscovite+zoisite+quartz+vapor=liquid+kyanite was bracketed at 10.5 kbar/689–700° C, 15.5 kbar/709–731° C and 20 kbar/734–745° C. The equivalent reaction in the Mg-bearing system muscovite_ss +zoisite+quartz+vapor=liquid+kyanite+phlogopite_ss lies at the same temperature around 10 kbar and approximately 10° C higher around 20 kbar, compared with the Mg-free reaction. At slightly higher temperatures formation of melt and tremolite_ss was reversibly observed from the assemblage phlogopite_ss+zoisite +kyanite+quartz around 10.5 kbar/690–710° C, 15.5 kbar/720–750° C and 20.5 kbar/745–760° C. In the subsolidus region, the reaction muscovite_ss+talc_ss+ tremolite_ss=phlogopite_ss+zoisite+quartz+vapor were located in the range 700° C/16.7–19.0 kbar and 740° C/19.7–20.8 kbar. From these data, a wedge shaped stability field of phlogopite_ss+zoisite+quartz appears with a high P, T termination around 21 kbar/755° C. Muscovite+tremolite+talc or kyanite comes in at higher pressures. These phase relations are in qualitative accord with petrographic observations from high pressure metamorphic areas. Formation and crystallization of melts in rocks of a wide compositional range involving zoisite/epidote has been ascribed to relatively high pressures and is consistent with experimentally determined stability fields in the simplified KCMASH system.     

Study on Al2O3 content and phase stability of aluminous-CaSiO3 perovskite at high pressure and temperature

 In order to clarify Al₂O₃ content and phase stability of aluminous CaSiO₃-perovskite, high-pressure and high-temperature transformations of Ca₃Al₂Si₃O₁₂ garnet (grossular) were studied using a MA8-type high-pressure apparatus combined with synchrotron radiation. Recovered samples were examined by analytical transmission electron microscopy. At pressures of 23–25 GPa and temperatures of 1000–1600 K, grossular garnet decomposed into a mixture of aluminum-bearing Ca-perovskite and corundum, although a metastable perovskite with grossular composition was formed when the heating duration was not long enough at 1000 K. On release of pressure, this aluminum-bearing CaSiO₃-perovskite transformed to the “LiNbO₃-type phase” and/or amorphous phase depending on its Al₂O₃ content. The structure of this LiNbO₃-type phase is very similar to that of LiNbO₃ but is not identical. CaSiO₃-perovskite with 8 to 25 mol% Al₂O₃ was quenched to alternating lamellae of amorphous layer and LiNbO₃-type phase. On the other hand, a quenched product from CaSiO₃-perovskite with less than 6 mol% consisted only of amorphous phase. Most of the inconsistencies amongst previous studies could be explained by the formation of perovskite with grossular composition, amorphous phase, and the LiNbO₃-type phase. Received: 11 April 2001 / Accepted: 5 July 2002     

Forsterite-diopside-spinel-liquid equilibria in the system CaO−MgO−Al2O3−SiO2 at 20 kbar and petrological applications

Liquidus phase relationships in the CaAlAl–SiO₆–Mg₂SiO₄–CaMgSi₂O₆–CaAlSi₂O₈ portion of the simplified basalt tetrahedron in the CaO–MgO–SiO₂–Al₂O₃ system have been experimentally determined at 20 kbar pressure. The fo+di _ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di _ss+ en_ss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di _ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo₁₁Di₅₆An₃CaTs₃₀, very close to and slighthly to the SiO₂-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e _ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di _ss+ sp. Compositions of di _ss in equilibrium with the liquids from the temperature maximum to the fo+di _ss+en_ss+ sp+li invariant point range from Di₆₆En₉CaTs₂₅ to Di₃₆En₄₀CaTs₂₄. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di _ss-sp-li univariant curve onto the sp-di _ss divariant surface. Crystallization of di _ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al₂O₃–CaSiO₃–MgSiO₃ plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO₂-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO₂-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al₂O₃ join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations di CaMgSi₂O₆ - fo Mg₂SiO₄ - an CaAl₂Si₂O₈ - CaTs CaAlAlSiO₆ - sp MgAl₂O₄ - en MgSiO₃ - wo CaSiO₃ - gt Ca₃Al₂Si₃O₁₂–Mg₃Al₂Si₃O₁₂ - qz SiO₂ - li Liquid - gl glass - ss Solid Solution - A An mxn matrix - X A column vector - kbar kilobar     

Calculated greenschist facies mineral equilibria in the system CaO−FeO−MgO−Al2O3−SiO2−CO2−H2O

The kinetic problems associated with the experimental determination of reactions among complex solidsolution phases at low temperatures have hindered our understanding of the phase relations in greenschist facies rocks. In the absence of reliable experimental data, we have used the new, expanded internally-consistent thermodynamic dataset of Holland and Powell (1990), to present calculated phase equilibria for the system CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ (CaFMASCH) with quartz in eccess, in the range 400°–500°C at low to intermediate pressures, involving the minerals amphibole, chlorite, anorthite, clinozoisite, dolomite, chloritoid, garnet, margarite, andalusite, and calcite. By solving independent sets of non-linear equations formed from equilibrium relationships, we calculate not only the loci of reactions in pressuretemperature-x(CO₂) space, but also the compositions of coexisting minerals in terms of the substitutions, FeMg_-1 and (Fe,Mg)SiAl_-1Al_-1. Invariant, univariant and divariant equilibria are calculated and discussed in relation to naturally-occurring greenschist facies metabasic and siliceous dolomitic mineral assemblages. We thus avoid the use of activity-corrected curves so commonly presented in the literature as a substitute for genuine univariant phase diagram boundaries.     

The electrical conductivity of some hydrous and anhydrous molten silicates as a function of temperature and pressure

Electrical conductivity of the following molten silicate systems (in mol%): 60SiO₂-40Na₂O; 65SiO₂-35Na₂O; 75SiO₂-25Na₂O; 78SiO₂-22Na₂O; 72SiO₂-24Na₂O-4CaO; 66SiO₂-19Na₂O-15H₂O; and an anhydrous and hydrous (4 wt% H₂O) Mt. Erebus lava, have been measured as a function of temperature (to 1000°C) and pressure (to 1.3 kbar). The anhydrous soda-rich melts have a positive pressure coefficient of conductivity to ~200 bars and beyond this pressure the pressure coefficient is small and negative. Addition of water lowers the conductivity and gives rise to a negative pressure coefficient at the highest temperatures. The conductivity of hydrous Mt. Erebus lava passed through a maximum with increasing temperature at constant pressure. These phenomena are interpreted in terms of explanations of similar phenomena found in molten salt and aqueous electrolyte solutions.     

X射线荧光光谱法快速分析镁质耐火材料中硅铝铁钛钙镁

采用X射线荧光光谱法快速测定镁质耐火材料（包括原料镁砂及其制品镁砖）中的SiO₂、Al₂O₃、Fe₂O₃、TiO₂、CaO、MgO等6种组分。利用粉末直接压片法制取试样,确定仪器最佳参数,系列标样建立工作曲线,经验系数法对基体效应进行校正。对于主次组分,方法精密度低于5%（n=10）。方法用于实际样品的测定,结果与实验室化学法和其他单位的X射线荧光光谱法相符。方法适用于厂矿企业大批量生产的镁质耐火材料化学组成分析。     

The stability of sapphirine-quartz and hypersthene-sillimanite-quartz assemblages: an experimental investigation in the system FeO−MgO−Al2O3−SiO2 under H2O and CO2 conditions

Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al₂O₂−SiO₂) under low fO₂ and for various H₂O/CO₂ conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H₂O, CO₂ and H₂O−CO₂ (±CH₄) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO₂-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO₂. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H₂O and CO₂ are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg₂Al₄SiO₁₀ due to AlAl=MgSi substitutions [100(Al₂O₃/Al₂O₃+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.     

Thermal and physical properties of reconsolidated crushed rock salt as a function of porosity and temperature

Crushed salt can be used as backfill to bury and conduct heat away from radioactive waste in salt repositories. As the crushed salt compacts during reconsolidation, its thermal, mechanical and hydrologic properties will change in a manner related to the porosity. Measurements of crushed salt thermal properties are conducted to evaluate such relationships. A simple mixture theory model is presented to predict thermal conductivity of consolidating salt in repository conditions. Experimental work was completed to evaluate the model by measuring thermal conductivity, thermal diffusivity and specific heat of crushed salt as a function of porosity and temperature. Sample porosity ranged from 0 to 46 %, and measurements were made at ambient pressure, from room temperature to 300 °C. These are the temperature conditions expected in a radioactive waste storage facility. Crushed salt thermal conductivity decreases with increasing porosity and temperature. Thermal diffusivity showed little porosity dependence but decreased with increasing temperature. Specific heat also shows little porosity dependence but increases with increasing temperature. Fracture porosity in deformed bedded salt cores appears to reduce thermal conductivity more dramatically than inter- and intra-granular porosity in consolidated salt. A long-term effort to dry crushed salt at high temperatures resulted in a 0.48 weight-percent loss of water that had resided at grain boundaries and in intra-granular fluid inclusions. While this loss does not significantly affect thermal properties, the release of this water volume could impact the mechanical response of the reconsolidating salt and host rock.     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln = Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1100 K) and SmCaAl[Al₂O₇] (up to T=1024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data were obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶×K⁻¹+3.6(7)×10⁻⁹ΔT×K⁻² and α₃=15.0(1)×10⁻⁶×K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶×K⁻¹+2.0(2)×10⁻⁹ΔT×K⁻² and α₃=8.5(2)×10⁻⁶×K⁻¹+2.0(3)×10⁻⁹ΔT×K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)×10⁻⁶×K⁻¹+1.7(2)×10⁻⁹ΔT×K⁻² and α₃=9.344(5)×10⁻⁶×K⁻¹. The expansion mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ‘proper’ layer structure, the aluminates show increased framework structure behavior. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)–(Ln ³⁺+Al³⁺) in the melilite-type structure. This article has been mistakenly published twice. The first and original version of it is available at .     

Decomposition of kyanite and solubility of Al2O3 in stishovite at high pressure and high temperature conditions

In order to constrain the high-pressure behavior of kyanite, multi-anvil experiments have been carried out from 15 to 25 GPa, and 1,350 to 2,500°C. Both forward and reversal approaches to phase equilibria were adopted in these experiments. We find that kyanite breaks down to stishovite + corundum at pressures above ∼15 GPa, and stishovite + corundum should be the stable phase assemblage at the pressure–temperature conditions of the transition zone and the uppermost part of the lower mantle of the Earth, in agreement with previous multi-anvil experimental studies and ab initio calculation results, but in disagreement with some of the diamond-anvil cell experimental studies in the literature. The Al₂O₃ solubility in nominally dry stishovite has been tightly bracketed by forward and reversal experiments; it is slightly but consistently reduced by pressure increase. Its response to temperature increase, however, is more complicated: increases at low temperatures, maximizes at around 2,000°C, and perhaps decreases at higher temperatures. Consequently, the Al₂O₃ solubility in dry stishovite at conditions of high temperature–high pressure is very limited.     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln=Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1,100 K) and SmCaAl[Al₂O₇] (up to T=1,024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data was obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶ K⁻¹+3.6(7)×10⁻⁹ΔT K⁻² and α₃=15.0(1)×10⁻⁶ K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶ K⁻¹+2.0(2)×10⁻⁹ΔT K⁻² and α₃=8.5(2)×10⁻⁶ K⁻¹+2.0(3)×10⁻⁹ΔT K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)× 10⁻⁶ K⁻¹+1.7(2)×10⁻⁹ΔT K⁻² and α₃=9.344(5)×10⁻⁶ K⁻¹. The expansion-mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ’proper’ layer structure, the aluminates show increased framework structure behaviour. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)-(Ln ³⁺+Al³⁺) in the melilite-type structure. Electronic Supplementary Material Supplementary material is available for this article at     

Electrical conductivity measurements on olivines and pyroxenes under defined thermodynamic activities as a function of temperature and pressure

Electrical conductivities of Ni₂SiO₄, Fe₂SiO₄, and MgSiO₃ were measured on synthetic powders in the temperature range 340° to 1,100° C and at pressures up to 20 kbars. For ternary compounds such as olivines and pyroxenes the control of two further variables, like the chemical activities of two components are needed, besides temperature and pressure. The activities of the corresponding binary oxides were controlled by equilibrating the samples with their neighbour-phases. Control of the oxygen partial pressure was achieved by buffer techniques. From the slopes of the lg σ vs. 1/T lines the activation energies were calculated for 10 kbar: 0.56 eV and 2.7 eV for Ni₂SiO₄ in equilibrium with SiO₂ and Ni/NiO-buffer for the temperature range 500°–800°C and 800°–1,000°C resp. 0.52 eV for Fe₂SiO₄ in equilibrium with SiO₂ and metallic iron, and 0.38 eV in equilibrium with SiO₂ and magnetite; 1.11 eV for MgSiO₃ in equilibrium with SiO₂, and 1.25 eV in equilibrium with Mg₂SiO₄.     

Thermal degradation of rye and maize straw: Lipid pattern changes as a function of temperature

Future climatic conditions may coincide with an increased potential for wildfires in grassland and forest ecosystems, whereby charred biomass would be incorporated into soils. Molecular changes in biomass upon charring have been frequently analysed with a focus on black carbon. Aliphatic and aromatic hydrocarbons, known to be liberated during incomplete combustion of biomass have been preferentially analysed in soot particles, whereas determinations of these compounds in charred biomass residues are scarce. We discuss the influence of increasing charring temperature on the aliphatic and aromatic hydrocarbon composition of crop grass combustion residues. Straw from rye, representing C₃ grasses and maize, representing C₄ grasses, was charred in the presence of limited oxygen at 300, 400 and 500 °C. Typical n-alkane distribution patterns with a strong predominance of long chain odd-numbered n-alkanes maximising at C₃₁ were observed in raw straw. Upon combustion at 300 °C aliphatic hydrocarbons in char were dominated by sterenes, whereas at 400 °C sterenes disappeared and medium chain length n-alkanes, maximising around n-C₂₀, with a balanced odd/even distribution were present. At a charring temperature of 500 °C n-alkane chain length shifted to short chain homologues, maximising at C₁₈ with a pronounced predominance of even homologues. Even numbered, short chain n-alkanes in soils may thus serve as a marker for residues of charred biomass. Aromatic hydrocarbons indicate an onset of aromatization of biomass already at 300 °C, followed by severe aromatization upon incomplete combustion at 400–500 °C. The diagnostic composition of aliphatic and aromatic hydrocarbons from charred biomass affords potential for identifying residues from burned vegetation in recent and fossil soils and sediments.     

A Single Step Acid Assisted Microwave Digestion Method for the Complete Dissolution of Bauxite and Quantitation of its Composition (Al2O3, Fe2O3, SiO2, TiO2, Cr2O3, MgO,MnO and V2O5) by ICP-AES

An acid assisted microwave-based method for the complete dissolution of bauxite using mixture of H₂SO₄, H₃PO₄ and HF acids in a single step was developed for the determination of various analytes (Al₂O₃, Fe₂O₃, SiO₂, TiO₂, Cr₂O₃, MgO, MnO and V₂O₅) using ICP-AES. The method was validated with respect to ruggedness, linearity, trueness, precision, limit of detection (LOD), limit of quantification (LOQ), working range and measurement uncertainties by analysing a bauxite reference material (Alcan BXT-12) and four certified reference materials (IPT-131, BXBA-4, NIST SRM 600, NIST SRM 697). The expanded uncertainties obtained for Al₂O₃ (40.0%), Fe₂O₃ (17.0%), SiO₂ (20.3%), TiO₂ (1.31%), Cr₂O₃ (0.024%), MgO (0.05), MnO (0.013), and V₂O₅ (0.60%), were 0.80, 0.40, 0.50, 0.033, 0.0008, 0.002, 0.0007 and 0.002 respectively, which are fit for the intended use to characterise bauxite. The developed method was also evaluated through participation in an interlaboratory comparison exercise organised by the Jawaharlal Nehru Aluminium Research Development and Design Centre (JNARDDC), Nagpur, India, using bauxite sample (BXT-JNA), with satisfactory z-scores achieved.     

Experimentally derived thermochemical data for pargasite and reinvestigation of its stability with quartz in the system Na2O–CaO–>MgO–Al2O3–SiO2–H2O

 The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C in the system Na₂O– CaO–MgO–Al₂O₃–SiO₂–H₂O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the following three equilibria: (a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An₅₀)+1.5 enstatite+H₂O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An₅₀)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An₅₀). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature, and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic amphiboles, which favor a single-coupled Na^A-Al^T1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is estimated as for ΔG ⁰ _f ,Pg=−12022.11±5.2 kJ mole^-1, and S ⁰ _Pg=591.7 ±7.9 JK^-1 mole^-1. Received: 31 July 1995/Accepted: 3 June 1996     

The high-pressure and temperature equation of state of a majorite solid solution in the system of Mg4Si4O12-Mg3Al2Si3O12

The high-pressure and temperature equation of state of majorite solid solution, Mj_0.8Py_0.2, was determined up to 23 GPa and 773 K with energy-dispersive synchrotron X-ray diffraction at high pressure and high temperature using the single- and double-stage configurations of the multianvil apparatuses, MAX80 and 90. The X-ray diffraction data of the majorite sample were analyzed using the WPPD (whole-powder-pattern decomposition) method to obtain the lattice parameters. A least-squares fitting using the third-order Birch-Murnaghan equation of state yields the isothermal bulk modulus, K _T0  = 156 GPa, its pressure derivative, K′ = 4.4(±0.3), and temperature derivative (∂K _T /∂T) _P = −1.9(±0.3)× 10⁻² GPa/K, assuming that the thermal expansion coefficient is similar to that of pyrope-almandine solid solution. Received: 5 October 1998 / Revised, accepted: 24 June 1999