Stable Phase Equilibrium of the Aqueous Ternary System Li~+//Cl~-,borate- H_2O at 348 K

正The Sichuan Basin,with an area of about 20×104 km2,is situated at 102.5°~110°E and 27.67°~32.67°N.The underground brine resources in Sichuan basin are a type of comprehensive liquid mineral resource.Pingluo     

Phase Equilibria of the Aqueous Systems Containing Lithium and Carbonate Ions

正1 Introduction Alkaline lakes are widely distributed in the area of the Qinghai-Tibet Plateau.Most of the salt lakes are famous for their high concentration of lithium,potassium,magnesium,boron(Ma,2000).In recent years,as a new energy material,lithium and its compounds are widely used in the new area,such as aerospace industry,nuclear     

Phase Equilibria in the Ternary Systems KBr–K2B4O7–H2O and KCl–K2B4O7–H2O at 373 K

According to the compositions of the underground gasfield brines in the west of Sichuan Basin,the phase equilibria in the ternary systems KBr-K2B4O7-H2O and KCl-K2B4O7-H2O at 373 K were studied using the isothermal dissolution equilibrium method.The solubilities of salts and the densities of saturated solutions in these ternary systems were determined.Using the experimental data,phase diagrams and density-composition diagrams were constructed.The two phase diagrams were simple co-saturation type,each having an invariant point,two univariant curves and two crystallization regions.The equilibrium solid phases in the ternary system KBr-K2B4O7-H2O are potassium bromide （KBr） and potassium tetraborate tetrahydrate （K2B4O7·4H2O）,and those in the ternary system KCl-K2B4O7-H2O are potassium chloride （KCl） and potassium tetraborate tetrahydrate （K2B4O7·4H2O）.Comparisons of the phase diagrams of the two systems at different temperatures show that there is no change in the crystallization phases,but there are changes in the size of the crystallization regions.As temperature increases,the solubility of K2B4O7·4H2O increases rapidly,so the crystallization field of K2B4O7·4H2O becomes smaller.     

Predictive phase equilibria for the aqueous ternary system(Li_2SO_4 + K_2SO_4 + H_2O) from 273.15 to 373.15 K

正1 Introduction Many variable temperature chemical models were developed to predict mineral solubility in the natural waters(Na+,K+,Ca2+,Mg2+//Cl-,SO42-–H2O)in the temperature range below 298.15 K(to near 213.15 K)and(Na+,K+,     

交互四元体系Li~+,Mg~(2+)//SO_4~(2-),B_4O_7~(2-)-H_2O 288K时相平衡研究

采用等温溶解平衡法研究了288K时Li+,Mg2+//SO42-,B4O72--H2O四元体系的固液相平衡关系,测定了该四元体系在288K时平衡液相的溶解度和密度。依据实验测定的平衡溶解度数据及对应的平衡固相,绘制了该四元体系的平衡相图及密度组成图。研究结果表明:交互四元体系Li+,Mg2+//SO42-,B4O27--H2O288K时平衡相图中有2个共饱点,5条单变量曲线,4个结晶区对应的平衡固相分别为Li2B4O7.3H2O,Li2SO4.H2O,MgB4O7.9H2O和MgSO4.7H2O。     

交互四元体系Li+,Mg2+//SO2-4,B4O2-7-H2O 288 K时相平衡研究

采用等温溶解平衡法研究了288K时Li+, Mg2+//SO2-4, B4O2-7- H2O四元体系的固液相平衡关系,测定了该四元体系在288K时平衡液相的溶解度和密度.依据实验测定的平衡溶解度数据及对应的平衡固相,绘制了该四元体系的平衡相图及密度组成图.研究结果表明:交互四元体系Li+, Mg2+//SO2-4, B4O2-7- H2O 288K时平衡相图中有2个共饱点,5条单变量曲线,4个结晶区对应的平衡固相分别为Li2B4O7·3H2O,Li2SO4·H2O,MgB4O7·9H2O和MgSO4·7H2O.     

Production of Potash and N-Mg Compound Fertilizer via Mineral Shoenite from Kunteyi Salt Lake: Phase Diagrams of Quaternary System(NH_4)_2SO_4-Mg SO_4-K_2SO_4-H_2O in the Isothermal Evaporation and Crystallization Process

Based on the requirement of the new technology for producing potassium sulfate and N-Mg compound fertilizer,boussingaultite,by the reaction of the mineral shoenite from Kunteyi Salt Lake,Qinghai province,and the industrial byproduct ammonium sulfate,the solubilities of the quaternary system (NH_4)_2SO_4-Mg SO_4-K_2SO_4-H_2O at 25.0oC in the isothermal evaporation and crystallization process were measured using the isothermal evaporation method,and the corresponding phase diagrams were plotted.According to the diagram,this system contains six saturation points and six solid phase fields of crystallization,which correspond to (K_(1-m),(NH_4)_m)_2SO_4,(NH_4)_2SO_4·Mg SO_4·6H_2O,K_2SO_4·Mg SO_4·6H_2O,Mg SO_4·6H_2O,(K_(1-n),(NH_4)_n)_2SO_4·Mg SO_4·6H_2O and Mg SO_4·7H_2O,respectively.By analyzing and calculating the isothermal evaporation and dissolution phase diagram of this quaternary system at 25.0oC,K_2SO_4 and (NH_4)_2SO_4·Mg SO_4·6H_2O can be separated via K_2SO_4·Mg SO_4·6H_2O and (NH_4)_2SO_4 as raw materials.Theoretical calculations about the proposed process were carried out and verified by experiment,which indicated that the yield of potassium sulfate was improved and the magnesium resources were fully utilized.     

The reaction MgCr2O4 + SiO2 = Cr2O3 + MgSiO3 and the free energy of formation of magnesiochromite (MgCr2O4)

Low-temperature heat capacity measurements for MgCr₂O₄ have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K⁻¹ mol⁻¹) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which magnetic ordering of the Cr³⁺ ions in MgCr₂O₄ occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr₂O₄ has been missed. We have determined the position of the near-univariant reaction MgCr₂O₄+SiO₂=MgSiO₃+Cr₂O₃. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H₂O, BaO-B₂O₃ or K₂O-B₂O₃) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr₂O₃, MgSiO₃ and SiO₂, give the entropy and enthalpy of formation of MgCr₂O₄ spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Δ _f H ^∘ ₂₉₈=−1759.2±1.5 kJ mol⁻¹ and S ^∘ ₂₉₈=122.1±1.0 J K⁻¹ mol⁻¹ for MgCr₂O₄. This entropy is some 16 J K⁻¹ mol⁻¹ more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr₂O₄ consistent with an effective spin quantum number (S') for Cr³⁺ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching. Received: 9 May 1997 / Accepted: 28 July 1997     

Behavior and effects of phosphorus in the system Na2O−K2O−Al2O3−SiO2−P2O5−H2O at 200 MPa(H2O)

The addition of phosphorus to H₂O-saturated and initially subaluminous haplogranitic (Qz–Ab–Or) compositions at 200 MPa(H₂O) promotes expansion of the liquidus field of quartz, a marked decrease of the solidus temperature, increased solubility limits of H₂O in melt at low phosphorus concentrations, and fractionation of melt out of the haplogranite plane (projected along an Or₂₈ isopleth) toward a peralkaline, silica-poor but quartz-saturated minimum composition. The partition coefficient for P₂O₅ between aqueous vapor and melt with an ASI (aluminum saturation index, mol Al/[mol Na+K])=1 is negligible (0.06), and consequently so are the effects of phosphorus on other melt-vapor relations involving major components. Phosphorus becomes more soluble in vapor, however, as the concentration of a NaPO₃ component increases via the fractionation of melt by crystallization of quartz and feldspar. The experimental results here corroborate existing concepts regarding the interaction of phosphorus with alkali aluminosilicate melt: phosphorus has an affinity for alkalis and Al, but not Si. Phosphorus is incorporated into alkali feldspars by the exchange component AlPSi_-2. For subaluminous compositions (ASI=1), the distribution coefficient of phosphorus between alkali feldspar and melt, D[P]Af/m, is 0.3. This value increases to D[P]Af/m=1.0 at a melt ASI value of 1.3. The increase in D[P]Af/m with ASI is expected from the fact that excess Al promotes the AlPSi_-2 exchange. With this experimental data, the P₂O₅ content of feldspars and whole rocks can reveal important facets of crystallization and phosphorus geochemistry in subaluminous to peraluminous granitic systems.     

Phase transition of synthetic zinc ferrite spinel (ZnFe2O4) at high pressure, from synchrotron X-ray powder diffraction

 Synthetic Zn-ferrite (ideally ZnFe₂O₄; mineral name: franklinite) was studied up to 37 GPa, by X-ray powder diffraction at ESRF (Grenoble, France), on the ID9 beamline; high pressure was achieved by means of a DAC. The P-V equation of state of franklinite was investigated using the Birch-Murnaghan function, and the elastic properties thus inferred [K₀ = 166.4(±3.0) GPa K₀ ^′ = 9.3(±0.6) K₀ ^″ = −0.22 GPa⁻¹] are compared with earlier determinations for MgAl-spinel and magnetite. The structural behaviour of Zn-ferrite as a function of pressure was studied by Rietveld refinements, and interpreted in the light of a phase transition from spinel to either CaTi₂O₄- or MnFe₂O₄-like structure; this transformation occurs above 24 GPa. Received: 15 March 1999 / Accepted: 22 April 2000     

Phase transitions in langbeinites I: Crystal chemistry and structures of K-double sulfates of the langbeinite type M 2 + + K2(SO4)3, M + +=Mg,Ni, Co,Zn, Ca

The crystal structures of the langbeinite type M ₂ ^{+ +} K₂(SO₄)₃ with M ^{+ +}=Mg, Ni, Co, Zn, Ca in their cubic phase (P 2 ₁ 3) and Ca₂K₂(SO₄)₃ in its orthorhombic phase (P 2 ₁ 2 ₁ 2 ₁) are determined. Whereas the SO₄-tetrahedra in these compounds are almost undistorted, the two symmetry-independent coordination polyhedra of M ^{+ +} are highly distorted octahedra with trigonal site symmetry in P 2 ₁ 3. The deformation of the oxygen octahedra and the off-centering of M ^{+ +} along the trigonal axis show systematic dependences on the ionic radii and the electronegativities of the M ^{+ +}-ions. The correlations are remarkably different for the two symmetry-independent M ^{+ +}-ions indicating different M ^{+ +} — O bonding. The octahedral deformations show also linear correlations with the phase transition temperatures (P 2 ₁ 3 — P 2 ₁ 2 ₁ 2 ₁) of the different compounds. This observation leads to a new model for the phase transition mechanism which is based on thermal instabilities of the M ^{+ +} — O and K — O polyhedral distortions. The cubic high temperature phase is characterized by high symmetric oxygen coordinations around M ^{+ +} which distort with decreasing temperature. At T _c the trigonal site symmetry is broken in such a way that the K — O coordination becomes denser at the expense of a wider and less symmetric M ^{+ +} — O coordination.     

Polarized single crystal absorption spectroscopy of the Pnam-P2 1/a transition of Ilvaite Ca(Fe2+, Fe3+)Fe2+Si2O8(OH) as measured between 300 K and 450 K

The temperature dependence of the absorption spectra of ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P2₁/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and 450 K in a frequency range between 600 and 23 000 cm⁻¹. No Drude —absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling with the Fe²⁺-Fe³⁺ ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH) _x .     

(斜长石 + 黑云母 + 石英 ) NaCl(NaHCO3) H2O 矿物蚀变与金矿化反应研究 : 450～ 250 ℃和 50 MPa

采用（斜长石＋黑云母＋石英）这三种单矿物组合与1mol/L NaCl或0．5mol/L NaCl 0.5mol/L NaHCO3溶液在450－250℃和50MPa条件下反应7d。实验表明，反应后流体pH值发生了变化，NaCl介质向酸性变化，N aCl NaHCO3介质向中性转化。溶液中K,Ca,Mg,Fe和Au量也随之发生变化。矿物表面发生溶解和离子置换等反应。斜长石表面形成钠长石反应边，黑云母变色，石英重结晶，反应器皿金管中的金被溶解后在金管壁和黑云母表面重结晶，黑云母周边出现红色Fe2O3，在450℃的NaCl介质中，金含量可达1070μg/g，但随温度下降迅速减低，在NaCl NaHCO3介质中，金含量较低，显著，金的活化迁移和富集与Cl,pH,Fe^3 /Fe^2 密切相关，这中金起到示踪作用，显示出金在水／岩反应中的原电池效应。     

Temperature-dependent magnetic susceptibility behaviour of spinelloid and spinel solid solutions in the systems Fe2SiO4–Fe3O4 and (Fe,Mg)2SiO4–Fe3O4

The magnetic behaviour and Curie temperatures (T _C ) of spinelloids and spinels in the Fe₃O₄–Fe₂SiO₄ and Fe₃O₄–(Mg,Fe)₂SiO₄ systems have been determined from magnetic susceptibility (k) measurements in the temperature range –192 to 700 °C. Spinelloid II is ferrimagnetic at room temperature and the k measurements display a characteristic asymmetric hump before reaching a T _C at 190 °C. Spinelloid V from the Mg-free system is paramagnetic at room temperature and hysteresis loops at various low temperatures indicate a ferri- to superparamagnetic transition before reaching the T _{C .} The T _C shows a non-linear variation with composition between –50 and –183 °C with decreasing magnetite component (X _Fe3O₄). The substitution of Mg in spinelloid V further decreases T _C . Spinelloid III is paramagnetic over nearly the total temperature range. Ferrimagnetic models for spinelloid II and spinelloid V are proposed. The T _C of Fe₃O₄–Fe₂SiO₄ spinel solid solutions gradually decrease with increasing Si content. Spinel is ferrimagnetic at least to a composition of X _Fe3O₄=0.20, constraining a ferrimagnetic to antiferromagnetic transition to occur at a composition of X _Fe3O₄<0.20. A contribution of the studied ferrimagnetic phases for crustal anomalies on the Earth can be excluded because they lose their magnetization at relatively low temperatures. However, their relevance for magnetic anomalies on other planets (Mars?), where these high-pressure Fe-rich minerals could survive their exhumation or were formed by impacts, has to be considered.     

Isopiestic measurement of the osmotic and activity coefficients for the NaOH-NaAl(OH)4-H2O system at 313.2 K

By using a specially designed and constructed isopiestic apparatus, we measured the osmotic coefficients at 313.2 K for the NaOH-NaAl(OH)₄-H₂O system with the total alkali molality, m_NaOHT (m_NaOH + m_NaAl[OH]4), from 0.05 mol/kg H₂O to 12 mol/kg H₂O and α_K (m_NaOHT/m_NaAl(OH)4) from 1.64 to 5.53. The mean standard deviation of the measurements is 0.0038. Several sets of the Pitzer model parameters for NaOH-NaAl(OH)₄-H₂O system were then obtained by regressing the measured osmotic coefficients with the Pitzer model and the Pitzer model parameters for NaOH(aq). One set of the results is as follows: β⁽⁰⁾_NaOH: 0.08669, β⁽¹⁾_NaOH: 0.31446, β⁽²⁾_NaOH: −0.00007367, C^Φ_NaOH: 0.003180, β⁽⁰⁾_NaAl(OH)4: 0.03507, β⁽¹⁾_NaAl(OH)4: 0.02401, C^Φ_NaAl(OH)4: −0.001066, θ_{OH⁻Al(OH)4⁻}: 0.08177, Ψ_Na⁺OH⁻_Al(OH)4⁻: −0.01162. The mean standard difference between the calculated and the measured osmotic coefficients is 0.0088. With the obtained Pitzer model parameters, we calculated the values of K = (γ_{NaAl(OH)4,cal²} · m_{Al(OH)4⁻,exp})/(γ_NaOH,cal² · m_OH⁻,exp) for the gibbsite solubility. The results show that the obtained Pitzer model parameters are reliable, and the relative error of the calculated activity coefficients should be < 2.1%. We also compared the calculated gibbsite solubility data among several activity coefficients models over a range of m_NaOHT at various temperatures. The comparison indicates that our activity coefficients model may be approximately applied in the ranges of temperature from 298.2 to 323.2 K and m_NaOHT from 0 to 8 mol/kg H₂O. We also calculated the stoichiometric activity coefficients of NaOH and NaAl(OH)₄ and the activity of H₂O for the NaOH-NaAl(OH)₄-H₂O system, and these calculations establish their variations with m_NaOHT and α_K. These variations imply that the strengths of the repulsive interactions among various anions are in the following sequence: Al(OH)₄⁻-Al(OH)₄⁻ < Al(OH)₄⁻-OH⁻ < OH⁻-OH⁻, and the attractive interaction between Al(OH)₄⁻ and H₂O is weaker than that between OH⁻ and H₂O.     

High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates: systematics in enthalpies of formation of A2+B4+O3 perovskites

Phase transitions in MgGeO₃ and ZnGeO₃ were examined up to 26 GPa and 2,073 K to determine ilmenite–perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite–perovskite boundaries have negative slopes and are expressed as P(GPa)=38.4–0.0082T(K) and P(GPa)=27.4−0.0032T(K), respectively, for MgGeO₃ and ZnGeO₃. Enthalpies of SrGeO₃ polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO₃ pseudowollasonite–walstromite and walstromite–perovskite transitions at 298 K were determined to be 6.0±8.6 and 48.9±5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO₃, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (ΔH _f°) of SrGeO₃ perovskite from the constituent oxides at 298 K was determined to be −73.6±5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite–perovskite transition boundaries in MgGeO₃ and ZnGeO₃ and the boundary of formation of SrSiO₃ perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The ΔH _f° of MgGeO₃, ZnGeO₃ and SrSiO₃ perovskites from constituent oxides were 10.2±4.5, 33.8±7.2 and −3.0±2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A²⁺B⁴⁺O₃ perovskites stable at both atmospheric and high pressures to explore the relationship between ΔH _f° and ionic radii of eightfold coordinated A²⁺ (R _A) and sixfold coordinated B⁴⁺ (R _B) cations. The results show that enthalpy of formation of A²⁺B⁴⁺O₃ perovskite increases with decreasing R _A and R _B. The relationship between the enthalpy of formation and tolerance factor ( R _o: O²⁻ radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A²⁺ and B⁴⁺ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A²⁺ and B⁴⁺ radii indicates a systematic change with equienthalpy curves. These relationships of ΔH _f° with R _A and R _B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.     

邦铺钼(铜)矿床二长花岗斑岩、黑云二长花岗岩锆石Hf同位素和Ce4+/Ce3+比值

文章对邦铺钼(铜)矿区产出的中新世二长花岗斑岩[(16.23±0.19)Ma]及古新世黑云二长花岗岩[(62.1±1.9)Ma]的锆石进行了微量元素和Hf同位素组成的研究.锆石Hf原位分析表明,邦铺钼(铜)矿区含矿二长花岗斑岩、成矿前黑云二长花岗岩的176Hf/177Hf比值分别为0.282 818～0.282 904...     

Determination of the mechanism of cation ordering in magnesioferrite (MgFe2O4) from the time- and temperature-dependence of magnetic susceptibility

The kinetics of non-convergent cation ordering in MgFe₂O₄ have been studied by measuring the Curie temperature (T _c) of synthetic samples as a function of isothermal annealing time. The starting material was a synthetic sample of near-stoichiometric MgFe₂O₄, synthesised from the oxides in air and quenched from 900 °C in water. Ordering experiments were performed using small chips of this material and annealing them at temperatures between 450 °C and 600 °C. The chips were periodically removed from the furnace, and their Curie temperatures were determined from measurements of alternating-field magnetic susceptibility (χ) as a function of temperature (T) to 400 °C. The Curie temperature of MgFe₂O₄ is very sensitive to the intracrystalline distribution of Fe³⁺ and Mg cations between tetrahedral and octahedral sites of the spinel crystal structure, and hence provides a very sensitive probe of the cation ordering process. The χ-T curve for the starting material displays a single sharp magnetic transition at a temperature of 303 °C. During isothermal annealing, the χ-T curve develops two distinct magnetic transitions; the first at a temperature corresponding to T _c for the disordered starting material and the second at a higher temperature corresponding to T _c for the equilibrium ordered phase. The size of the magnetic signal from the ordered phase increases smoothly as a function of time, until equilibrium is approached and the shape of the χ-T curve corresponds to a single sharp magnetic transition for the homogeneous ordered phase. These observations demonstrate that cation ordering in MgFe₂O₄ proceeds via a heterogeneous mechanism, involving the nucleation and growth of fine-scale domains of the ordered phase within a matrix of disordered material. Disordering experiments were performed by taking material equilibrated at 558 °C and annealing it at 695 °C. The mechanism of isothermal disordering is shown to involve nucleation and growth of disordered domains within an ordered matrix, combined with continuous disordering of the ordered matrix. This mixed mechanism of disordering may provide an explanation for the difference between the rates of ordering and disordering observed in MgFe₂O₄ using X-ray diffraction. The origin of the heterogeneous ordering/disordering mechanism is discussed in terms of the Ginzburg-Landau rate law. It is argued that heterogeneous mechanisms are likely to occur in kinetic experiments performed far from equilibrium, whereas a homogeneous mechanism may operate under slow equilibrium cooling. The implications of these observations for geospeedometry are discussed. Received: 12 May 1998 / Accepted: 25 June 1998     

Structure of cobalt doped K2Cd2(SO4)3 langbeinite at three temperatures above the P213-P212121 phase transition,and a new trigger mechanism for the ferroelastic transformation

X-ray structure determinations of Langbeinite type K₂(Cd_1-xCo_x)₂(SO₄)₃, x≅0.02 at three temperatures (440, 540 and 640 K) above the P2 ₁3-P2 ₁2₁2₁ transition temperature (434 K) reveal that the M ²⁺ (M ²⁺=Cd) ion is displaced from the centre of the octahedron at all temperatures in the cubic phase. Simultaneously the distortion of the oxygen framework decreases with increasing temperature. The structural phase transition occurs when the bond lengths of the six bonds in each of the M ²⁺ octahedra are all equal, and it is proposed that this equalisation of bond lengths acts as the trigger for the phase transition. The structural deformation of the oxygen sublattice is such that rather regular octahedra around Cd occur at very high temperatures with Cd displaced from the centre. With decreasing temperature the octahedra distort under conservation of the triad, such that the differences between the various bond lengths Cd-O decrease. The phase transition occurs when all bond lengths around the Cd position become equal. The behaviour of the oxygen framework and the offcentring of the Cd/Co atom combine to produce an increasing distortion with increasing temperature as viewed by the central atom. Thus the interpretation of Optical Spectra, in which an increase in line splitting with temperature was observed, as being due to the off-centring of the Co, is confirmed.     

An experimental study of glaucophanic amphiboles in the system Na2O-MgO-Al2O3-SiO2-SiF4 (NMASF): some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures

The phase relations of glaucophanic amphiboles have been studied at 18–31 kbar/680–950°C in the synthetic system Na₂O–MgO–Al₂O₃–SiO₂–SiF₄ (NMASF) using the bulk composition of fluor-glaucophane, Na₂Mg₃Al₂Si₈O₂₂F₂. Previous experimental studies of glaucophane in the water-bearing system (NMASH) have been hampered by problems of fine grain size (electron microprobe analyses with low oxide totals and contamination by other phases), and consequently good compositional data are lacking. Fluor-amphiboles, on the other hand, generally have much higher thermal stabilities than their hydrous counterparts. By using the fluorine-analogue system NMASF, amphibole crystals sufficiently coarse for electron microprobe analysis have been obtained. Furthermore, NMASH amphibole phase relations are directly analogous to those of the NMASF system because SiF₄ fills the role of H₂O as the fluid species. High-pressure NMASF amphibole parageneses are comparable to those obtained for NMASH amphiboles under similar pressure-temperature conditions, except that the NMASF solidus was not encountered. In the pressure-temperature range of the NMASF experiments, fluor-glaucophane is unstable relative to glaucophanenyböite-Mg-magnesio-katophorite amphiboles. Variations in synthetic fluor-amphibole composition with P and T are discussed in terms of changes in the thermodynamic activities of the principal amphibole end-members, such as glaucophane (a_Gp) and nyböite (a_Ny) using an ideal-mixing-on-sites model. The most glaucophanic amphiboles analysed have a_Gp=0.50–0.60 and coexist with jadeite and coesite at 30 kbar/800°C. Amphiboles become increasingly nyböitic with decreasing pressure through the NaAlSi_-1 exchange, which is the principal variation observed. The most nyböitic amphiboles have a_Ny =0.65–0.70 and coexist with fluor-sodium-phlogopite and quartz at 21–24 kbar/800–850°C. At 800°C amphiboles are essentially glaucophane-nyböite solid solutions. At 850°C there is some minor displacement along MgMgSi_-1, but Mg-magnesio-katophorite activities are very low (<0.06). Activities of the eight other NMASF amphibole end-members are <0.001, except for eckermannite activity which varies from 0.01–0.11. Our results indicate that: (a) synthetic amphiboles mimic the essential stoichiometries observed in blueschist amphiboles; (b) synthetic studies should be relevant to petrologically important high-pressure parageneses and reactions involving glaucophanicamphiboles, sodic pyroxenes, albite and talc; (c) the high-pressure stability limit of fluorglaucophane lies at pressures higher than those reached in this study (31 kbar); (d) in natural systems an approach to glaucophane stoichiometry should be favoured by high water activities as well as high pressures.Abbreviations and formulae used in this paper Glaucophane (Gp) oNa₂(Mg₃Al₂)Si₈O₂₂(OH,F)₂ - Nyböite (Ny) NaNa₂(Mg₃Al₂)Si₇AlO₂₂(OH,F)₂ - Eckermannite (Ek) NaNa₂(Mg₄Al)Si₈O₂₂(OH,F)₂ - Magnesio-cummingtonite (MC) oMg₂(Mg₅)Si₈O₂₂(OH,F)₂ - Sodium-magnesio-cummingtonite (SMC) NaNaMg(Mg₅)Si₈O₂₂(OH,F)₂ - Sodium-anthophyllite (SAn) NaMg₂(Mg₅)Si₇AlO₂₂(OH,F)₂ - Gedrite (Gd) oMg₂(Mg₃Al₂)Si₆Al₂O₂₂(OH,F)₂ - Sodium-gedrite (SGd) NaMg₂(Mg₄Al)Si₆Al₂O₂₂(OH,F)₂ - Mg-magnesio-aluminotaramite (MAT) NaNaMg(Mg₃Al₂)Si₆Al₂O₂₂(OH,F)₂ - Mg-magnesio-katophorite (MKt) NaNaMg(Mg₄Al)Si₇AlO₂₂(OH,F)₂ - Mg-magnesio-barroisite (MBa) oNaMg(Mg₄Al)Si₇AlO₂₂(OH,F)₂ - Jadeite (Jd) NaAlSi₂O₆ - Enstatite (En) Mg₂Si₂O₆ - Forsterite (Fo) Mg₂SiO₄ - Nepheline (Ne) NaAlSiO₄ - Albite (Ab) NaAlSi₃O₈ - Quartz/Coesite (Qz/Co) SiO₂ - Sodium-phlogopite (Sphl) NaMg₃Si₃AlO₁₀(OH,F)₂ - Talc (Tc) oMg₃Si₄O₁₀(OH,F)₂ - o vacant A-site in amphiboles and interlayer site in talc. Octahedral cations in amphiboles are bracketted