Structural characterisation of the high-pressure phase CaAl4Si2O11

Single crystals of CaAl₄Si₂O₁₁ were synthesised at 1,500?°C and 14 GPa in a multi-anvil press, and the structure of the phase determined by single-crystal X-ray diffraction at room conditions. The structure-type is that of the “hexagonal barium ferrites”. The space group of the average structure is P6 ₃ /mmc and the cell parameters are a?=?5.4223(4) Å, c?=?12.7041(6) Å, V?=?323.28(5) ų, with Z?=?2, and its density is 3.905?g?cm^?3, which is reasonable for a high-pressure alumino-silicate phase. The 22 oxygen and two calcium atoms within the unit-cell form an approximate hexagonal-close-packed array. Ten of the twelve octahedral interstices within this array that have only oxygen atoms for apices are filled with Si and/or Al. M1 octahedra share edges to form a spinel-like sheet of octahedra. The average bond length ?=?1.833 Å suggests mixed occupancy by Si and Al. The M1 octahedral sheets are linked by shared corners to pairs of face-sharing M2 octahedra containing Al, with ?= 1.918 Å. The remaining two cations of the unit-cell contents statistically occupy four tetrahedrally-coordinated interstices, which occur as face-sharing pairs. The average bond length for these sites (1.742 Å) suggests that they are occupied by Al, although Si occupancy cannot be excluded by the data. It is proposed that only one interstice of each pair is locally occupied, with the possibility of some short-range ordering of such occupancies. Complete long-range order leading to the acentric space group P6 ₃ mc is excluded by the data, as is the possibility of the average structure being comprised of merohedral (0?0?0?1) twins of P6 ₃ mc symmetry.     

The heat capacity of two natural chlorite group minerals derived from differential scanning calorimetry

The heat capacity of natural chamosite (X_Fe=0.889) and clinochlore (X_Fe=0.116) were measured by differential scanning calorimetry (DSC). The samples were characterised by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy. DSC measurements between 143 and 623?K were made following the procedure of Bosenick et?al. (1996). The fitted data for natural chamosite (CA) in J?mol^?1?K^?1 give: C _p,CA = 1224.3–10.685?×?10³?×?T ^??0.5???6.4389?× 10⁶?×T ^??2?+?8.0279?×?10⁸?×?T ^??3 and for the natural clinochlore (CE): C _p,CE = 1200.5–10.908?×?10³?×T ^??0.5?? 5.6941?×?10⁶?×?T ^??2?+?7.1166?×?10⁸?×?T ^??3. The corrected C _p-polynomial for pure end-member chamosite (Fe₅Al)[Si₃AlO₁₀](OH)₈ is C _p,CAcor = 1248.3–11.116?× 10³?×?T ^??0.5???5.1623?×?10⁶?×?T ^??2?+?7.1867?×?10⁸×T ^??3 and the corrected C _p-polynomial for pure end-member clinochlore (Mg₅Al)[Si₃AlO₁₀](OH)₈ is C _p,CEcor = 1191.3–10.665?×?10³?×?T ^??0.5???6.5136?×?10⁶?×?T ^??2?+ 7.7206?×?10⁸?×?T ^??3. The corrected C _p-polynomial for clinochlore is in excellent agreement with that in the internally consistent data sets of Berman (1988) and Holland and Powell (1998). The derived C _p-polynomial for chamosite (C _p,CAcor) leads to a 4.4% higher heat capacity, at 300?K, compared to that estimated by Holland and Powell (1998) based on a summation method. The corrected C _p-polynomial (C _p,CAcor) is, however, in excellent agreement with the computed C _p-polynomial given by Saccocia and Seyfried (1993), thus supporting the reliability of Berman and Brown's (1985) estimation method of heat capacities.     

A Raman spectroscopic study of shock-wave densification of anorthite (CaAl2Si2O8) glass

Structural modifications induced by shock-wave compression up to 40 GPa in anorthite glass are investigated by Raman spectroscopy. In the first investigation, densification increases with increasing shock pressure. A maximum densification of 2.2% is obtained for a shock pressure of 24 GPa. This densification is attributed to a decrease of the average ring size, favoring three-membered rings. The densification is much lower than in silica glass subject to shock at similar pressures (11%), because the T-O-T bond angle decrease is impeded in anorthite glass. For higher shock pressures, the decrease of the recovered densification is attributed to partial annealing of the samples due to high after-shock residual temperatures. The study of the annealing process of the most densified glass by in-situ high temperature Raman spectroscopy confirms that relaxation of the three-membered rings occurs above about 900 K. Received: 21 July 1998 / Revised and accepted: 27 January 1999     

Heat capacity of ferrosilite, Fe2Si2O6

The heat capacity of synthetic ferrosilite, Fe₂Si₂O₆, was measured between 2 and 820 K. The physical properties measurement system (PPMS, Quantum Design^®) was used in the low-temperature region between 2 and 303 K. In the temperature region between 340 and 820 K measurements were performed using differential scanning calorimetry (DSC). The C _p data show two transitions, a sharp λ-type at 38.7 K and a small shoulder near 9 K. The λ-type transition can be related to collinear antiferromagnetic ordering of the Fe²⁺ spin moments and the shoulder at 10 K to a change from a collinear to a canted-spin structure or to a Schottky anomaly related to an electronic transition. The C _p data in the temperature region between 145 and 830 K are described by the polynomial $C_{p} {\left[ {\hbox{J\,mol}^{{ - 1}}\,{\hbox{K}}^{{ - 1}} } \right]} = 371.75 - 3219.2T^{{ - 1/2}} - 15.199 \times 10^{5} T^{{ - 2}} + 2.070 \times 10^{7} T^{{ - 3}} $ The heat content [H ₂₉₈ – H ₀] and the standard molar entropy [S ₂₉₈ – S ₀] are 28.6 ± 0.1 kJ mol^?1 and 186.5 ± 0.5 J mol^?1 K^?1, respectively. The vibrational part of the heat capacitiy was calculated using an elastic Debye temperature of 541 K. The results of the calculations are in good agreement with the maximum theoretical magnetic entropy of 26.8 J mol^?1 K^?1 as calculated from the relationship 2*Rln5.     

CH4-H2O体系流体包裹体均一过程激光拉曼光谱定量分析

对南黄海盆地二叠纪地层中某石英脉中的CH4-H2O体系流体包裹体均一过程进行了激光拉曼光谱定量分析。利用甲烷与水的拉曼峰面积比值计算不同温度下流体包裹体中水溶液相中甲烷的浓度,除了在100℃附近出现最小值,随温度增加甲烷浓度呈指数增大。包裹体在214℃完全均一,均一时甲烷的浓度为0·1347mol/L。同时利用甲烷的拉曼特征对流体包裹体均一过程的内压变化作了分析。压力变化可以分为三个区间:19~100℃,随温度升高压力增大;100~150℃压力随温度升高减小;150℃之后压力迅速增大。均一温度下的内压为21·92MPa。流体包裹体内压的变化主要是由甲烷溶解行为和封闭体系的热力学特征决定的。实验表明激光拉曼光技术可以作为定量分析含甲烷流体包裹体的一种有效方法。     

原位低温拉曼光谱技术在人工合成CaCl_2-H_2O和MgCl_2-H_2O体系流体包裹体分析中的应用Ⅰ:低温拉曼光谱研究

利用原位低温拉曼光谱分析技术,对在设定的实验条件下合成的CaCl2-H2O体系和MgCl2-H2O体系的流体包裹体进行分析研究,结果表明低温下CaCl2水溶液和MgCl2水溶液形成的盐水合物具有各自不同的特征拉曼光谱,通过测定特征光谱,能够简易直接的鉴定这些物质,进而确定盐水体系包裹体的成分.因此,原位低温拉曼光谱技术,能够有效地测定Ca2+和Mg2+这两种盐水体系包裹体流体中常见,但显微测温过程难以观察和判别的二价阳离子.实践表明,原位低温拉曼光谱分析技术是对传统的流体包裹体显微测温技术的一个十分有效的补充,在包裹体研究领域会得到越来越广泛的应用.     

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。     

Heat capacity and phase equilibria of wadeite-type K2Si4O9

The low-temperature heat capacity (C _p) of Si-wadeite (K₂Si₄O₉) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured heat capacity data is 253.8 ± 0.6 J mol⁻¹ K⁻¹, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the system K₂O–Al₂O₃–SiO₂ are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order approximations were undertaken to calculate the phase diagram in the system K₂Si₄O₉ at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure phase relations of the K₂Si₄O₉ polymorphs. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Vibrational spectroscopy of aluminate spinels at 1 atm and of MgAl2O4 to over 200 kbar

Single-crystal Raman and infrared reflectivity data including high pressure results to over 200 kbar on a natural, probably fully ordered MgAl₂O₄ spinel reveal that many of the reported frequencies from spectra of synthetic spinels are affected by disorder at the cation sites. The spectra are interpreted in terms of factor group analysis and show that the high energy modes are due to the octahedral internal modes, in contrast to the behavior of silicate spinels, but in agreement with previous data based on isotopic and chemical cation substitutions and with new Raman data on gahnite (～ ZnAl₂O₄) and new IR reflectivity data on both gahnite and hercynite (～Fe_0.58Mg_0.42Al₂O₄). Therefore, aluminate spinels are inappropriate as elastic or thermodynamic analogs for silicate spinels. Fluorescence sideband spectra yield complementary information on the vibrational modes and provide valuable information on the acoustic modes at high pressure. The transverse acoustic modes are nearly pressure independent, which is similar to the behavior of the shear modes previously measured by ultrasonic techniques. The pressure derivative of all acoustic modes become negative above 110 kbar, indicating a lattice instability, in agreement with previous predictions. This lattice instability lies at approximately the same pressure as the disproportionation of spinel to MgO and Al₂O₃ reported in high temperature, high pressure work.     

The stability and compressibility of MgAl2O4 high-pressure polymorphs

High-pressure and high-temperature experiments using a laser-heated diamond anvil cell (LHDAC) and synchrotron X-ray diffraction have revealed a phase transition in MgAl₂O₄. CaTi₂O₄-type MgAl₂O₄ was found to be stable at pressures between 45 and at least 117 GPa. The transition pressure of CaTi₂O₄-type phase in MgAl₂O₄ is much lower than that in the natural N-type mid-oceanic ridge basalt composition. The Birch–Murnaghan equation of state for CaTi₂O₄-type MgAl₂O₄ was determined from the experimental unit cell parameters with K ₀=219(±6) GPa, K ₀′=4(constrained value), and V ₀=238.9(±9) ų. The observed compressibility was in agreement with the theoretical compressibility calculated in a previous study. ε-MgAl₂O₄ was observed at pressures between 40 and 45 GPa, which has not been reported in natural rock compositions. The gradient (dP/dT slope) of the transition from the ε-type to CaTi₂O₄-type MgAl₂O₄ had a positive value. These results should resolve the dispute regarding the stable high-pressure phase of MgAl₂O₄, which has been reported in earlier studies using both the multi-anvil press and the diamond anvil cell.     

Raman Micro-Spectroscopic Study of Sulfate Ion in the System Na2SO4 – H2O

This work aims to quantify sulfate ion concentrations in the system Na₂SO₄-H₂O using Raman micro-spectroscopy.Raman spectra of sodium sulfate solutions with known concentrations were collected at ambient temperature（293 K） and in the 500 cm^₁-4000 cm^-1 spectral region.The results indicate that the intensity of the SO₄^2- band increases with increasing concentrations of sulfate ion.A linear correlation was found between the concentration of SO₄^2-（c） and parameter I₁,which represents the ratio of the area of the SO₄^2- band to that of the O-H stretching band of water（A_s/A_w）:I₁=-0.00102+0.01538 c.Furthermore,we deconvoluted the O-H stretching band of water(2800 cm^-1-3800 cm^-1) at 3232 and 3430 cm^-1 into two sub-Gaussian bands,and then defined Raman intensity of the two sub-bands as A_Bi(3232 cm^-1) and A_B2(3430 cm^-1),defined the full width of half maximum（FWHM） of the two sub-bands as W_B1(3232 cm^-1) and W_B2(3430 cm^-1).A linear correlation between the concentration of SO₄^2-（c） and parameter I₂,which represents the ratio of Raman intensity of SO₄^2-（A_s）(in 981 cm^-1) to(A_B1+A_B2),was also established:I₂=-0.0111+0.3653 c.However,no correlations were found between concentration of SO₄^2-（c） and FWHM ratios,which includes the ratio of FWHM of SO₄^2-（W_s） to W_B1 W_B2 and W_B1+B2(the sum of W_B1 and W_B2),suggesting that FWHM is not suitable for quantitative studies of sulfate solutions with Raman spectroscopy.A comparison of Raman spectroscopic studies of mixed Na₂SO₄ and NaCI solutions with a constant SO₄^2- concentration and variable CI^- concentrations suggest that the I\ parameter is affected by CI^-,whereas the I₂ parameter was not.Therefore,even if the solution is not purely Na₂SO₄-H₂O,SO₄^2- concentrations can still be calculated from the Raman spectra if the H₂O band is deconvoluted into two sub-bands,making this method potentially applicable to analysis of natural fluid inclusions.     

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     

CH4-H2O体系流体包裹体拉曼光谱定量分析和计算方法

CH4—H2O体系流体包裹体研究对含油气盆地流体分析和成矿流体研究都有重要的意义。本文详细介绍了H2O、CH4和CH4—H2O体系的拉曼光谱特征及分子作用,分析了CH4—H2O体系热力学特征,同时对CH4—H2O体系流体包裹体拉曼光谱定量分析和计算的方法及步骤进行了叙述。利用人工合成流体包裹体建立甲烷浓度与拉曼特征峰面积比值之间的校正曲线是实现CH4—H2O体系流体包裹体定量分析的基础。盐度对包裹体定量分析的影响最为显著,在恒定甲烷浓度下,甲烷与水的拉曼峰面积比值随着盐度增加而减少。对于流体包裹体封闭体系,随温度升高,液相甲烷浓度增大。校正曲线必须包含对温度和盐度的校正。石英主矿物性质和方位对甲烷浓度定量分析的影响可以忽略。实验研究表明,原位拉曼光谱技术是准确获取流体包裹体中甲烷水合物生成条件的一种有效方法。因此,基于拉曼光谱分析和显微测温分析结果,采用热力学模型可以定量计算CH4—H2O体系流体包裹体的相关参数。     

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: I. Volume and heat capacity of mixing

The unite cell parameters and heat capacities of a series of synthetic clinopyroxenes on the join Ca-Tschermak (CaTs)−diopside (Di) were measured using X-ray powder diffraction and calorimetric methods, respectively. The volume of mixing at 298 K shows a negative asymmetric deviation from ideality. A two-parameter Margules fit to the data yields W _CaTs−Di ^V = −0.29 ± 0.11 cm³ mol⁻¹ and W _Di−CaTs ^V = −1.14 ± 0.14 cm³ mol⁻¹. Heat capacities were determined between 5 and 923 K by heat-pulse at 5−302 K and differential-scanning calorimetry at 143−923 K. The precision of the low and high temperature C _p data is better than ±1%. Polynomials of the form C _p = a + bT ^−1/2 + cT ⁻² + dT ⁻³ were fitted to the C _p data in the temperature range between 250 and 925 K. Thermal entropy values [S ₂₉₈ − S ₀] and [S ₉₀₀ −S ₀] as well as enthalpies [H ₂₉₈ − H ₀] and [H ₉₀₀ − H ₀] were calculated for all members of the solid solution series. No significant deviation from ideal mixing behavior was observed.     

Heat capacity and entropy of rutile (TiO<Subscript>2</Subscript>) and nepheline (NaAlSiO<Subscript>4</Subscript>)

 From heat capacities measured adiabatically at low temperatures, the standard entropies at 298.15 K of synthetic rutile (TiO₂) and nepheline (NaAlSiO₄) have been determined to be 50.0 ± 0.1 and 122.8 ± 0.3 J mol⁻¹ K, respectively. These values agree with previous measurements and in particular confirm the higher entropy of nepheline with respect to that of the less dense NaAlSiO₄ polymorph carnegieite. Received: 23 July 2001 / Accepted: 12 October 2001     

N2-CH4（CO2）混合气体在线标样制备及其拉曼定量因子测定

利用混合气体的标准样品对激光拉曼探针进行标定,可以快速准确地对包裹体中的无机及有机气相组分进行定量分析。而常用的商用钢瓶装混合气体标样,存在费用高、气体组成单一固定等缺点。本文设计了一套在线标样制备装置,提出一种在线配置不同浓度和压力条件下混合气体标样的方法。利用高纯度(纯度99.999%)的N2、CH4以及CO2钢瓶气,经过在线混合增压,在5 MPa和10 MPa条件下制备了N2摩尔分数为30%、50%和70%的N2-CH4以及N2-CO2混合气体在线标样。该方法制备的标样与70%N2+30%CO2的商用钢瓶气标样对比表明,CO2与N2的拉曼相对峰高以及相对峰面积值的误差在4%以内,具有较高的准确度和重现性。通过不同压力和浓度条件下CH4以及CO2的拉曼相对定量因子测定表明,气体的相对定量因子在5~10 MPa压力条件下与压力及组成无关。地质样品应用结果表明,本方法可以方便、灵活、准确地按任意比例将两瓶及两瓶以上纯气体钢瓶样品进行混合及增压,为激光拉曼标定、气体组成原位测量等提供了一种新的技术思路。     

High-precision analysis of carbon isotopic composition for individual CO2 inclusions via Raman spectroscopy to reveal the multiple-stages evolution of CO2- bearing fluids and melts

The carbon isotopic composition of CO₂ inclusions trapped in minerals reflects the origin and evolution of CO₂-bearing fluids and melts, and records the multiple-stages carbon geodynamic cycle, as CO₂ took part in various geological processes widely. However, the practical method for determination isotope composition of individual CO₂ inclusion is still lacking. Developing a microanalytical technique with spatial resolution in micrometers to precisely determinate the δ¹³C value of individual CO₂ inclusion, will make it possible to analyze a tiny portion of a zoning mineral crystal, distinguish the differences in micro-scale, and possible to find many useful information that could not be obtained with the bulk extraction and analysis techniques. In this study, we systematically collected Raman spectra of CO₂ standards with different δ¹³C values (?34.9 ‰ to 3.58 ‰) at 32.0 °C and from ～7.0 MPa to 120.0 MPa, and developed a new procedure to precisely determinate the δ¹³C value of individual CO₂ inclusion. We investigated the relationship among the Raman peak intensity ratio, δ¹³C value, and CO₂ density, and established a calibration model with high accuracy (0.5 ‰?1.5 ‰), sufficient for geological application to distinguish different source of CO₂ with varying δ¹³CO₂. As a demonstration, we measured the δ¹³C values and the density of CO₂ inclusions in the growth zones of alkali basalt-hosted corundum megacrysts from Changle, Shandong Province. We found the significant differences of density and δ¹³C between the CO₂ inclusions in the core of corundum and those inclusions in the outer growth zones, the δ¹³C value decreases from core to rim with decreasing density: δ¹³C values are from ?7.5 ‰ to ?9.2 ‰ for the inclusions in the core, indicating the corundum core was crystallized from mantle-derived magmas; from ?13.5 ‰ to ?18.5 ‰ for CO₂ inclusions in zone 1 and from ?16.5 ‰ to –22.0 ‰ for inclusions in zone 2, indicating the outer zones of corundum grew in a low δ¹³C value environment, resulted from an infilling of low δ¹³C value fluid and/or degassing of the ascending basaltic magma.     

A thermodynamic analysis of the system LiAlSiO4-NaAlSiO4-Al2O3-SiO2-H2O based on new heat capacity, thermal expansion, and compressibility data for selected phases

Heat capacity, thermal expansion, and compressibility data have been obtained for a number of selected phases of the system NaAlSiO₄-LiAlSiO₄-Al₂O₃-SiO₂-H₂O. All C _p measurements have been executed by DSC in the temperature range 133–823 K. The data for T ≥ 223 K have been fitted to the function C _p (T) = a + cT ⁻² + dT ^−0.5 + fT ⁻³, the fit parameters being The thermal expansion data (up to 525 °C) have been fitted to the function V ⁰(T) = V ⁰(T) [1 + v ₁ (T−T ⁰) + v ₂ (T−T ⁰)²], with T ⁰ = 298.15 K. The room-temperature compressibility data (up to 6 GPa) have been smoothed by the Murnaghan equation of state. The resulting parameters are These data, along with other phase property and reaction reversal data from the literature, have been simultaneously processed by the Bayes method to derive an internally consistent thermodynamic dataset (see Tables 6 and 7) for the NaAlSiO₄-LiAlSiO₄-Al₂O₃-SiO₂-H₂O quinary. Phase diagrams generated from this dataset are compatible with cookeite-, ephesite-, and paragonite-bearing assemblages observed in metabauxites and common metasediments. Phase diagrams obtained from the same database are also in agreement with the cookeite-free, petalite-, spodumene-, eucryptite-, and bikitaite-bearing assemblages known to develop in the subsolidus phase of recrystallization of␣lithium-bearing pegmatites. It is gratifying to note that the cookeite phase relations predicted earlier by Vidal and Goffé (1991) in the context of the system Li₂O-Al₂O₃-SiO₂-H₂O agree with our results in a general way. Received: 19 May 1998 / Accepted: 25 June 1998     

Heat capacity and phase equilibria of hollandite polymorph of KAlSi3O8

The low-temperature heat capacity (C _p ) of KAlSi₃O₈ with a hollandite structure was measured over the range of 5–303 K with a physical properties measurement system. The standard entropy of KAlSi₃O₈ hollandite is 166.2±0.2 J mol⁻¹ K⁻¹, including an 18.7 J mol⁻¹ K⁻¹ contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K₂Si₄O₉ with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K₂O–Al₂O₃–SiO₂. The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K₂O–Al₂O₃–SiO₂ confirm a substantial stability field for kyanite–stishovite/coesite–Si-wadeite intervening between KAlSi₃O₈ hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al₂SiO₅) = corundum (Al₂O₃) + stishovite (SiO₂), which is located at 13–14 GPa for 1,100–1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlSi₃O₈·H₂O) were modified to better fit global best-fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi₃O₈ hollandite + H₂O, which is located at 8.3–10.0 GPa for the temperature range 800–1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi₃O₈ hollandite + corundum + H₂O is placed at 10.0–10.6 GPa for the temperature range 900–1,500 K, in reasonable agreement with some but not all experiments on this reaction.     

Raman spectra of hydrous γ -Mg2SiO4 at various pressures and temperatures

 One well-defined OH Raman band at 3651 ± 1 cm⁻¹ and one weak feature near 3700 ± 5 cm⁻¹ are recognized for the hydrous γ-phase of Mg₂SiO₄. Like the hydrous β-phase, the H₂O content in the γ-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous γ-phase were investigated up to about 200 kbar at room temperature and in the range 81–873 K at atmospheric pressure. Unlike the anhydrous γ-phase, which remains intact up to at least 873 K, the hydrous γ-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous γ-phase remains intact up to at least 800 K, Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands. Received: 27 April 2001 / Accepted: 12 September 2001