下地幔CaSiO3钙钛矿的稳定性及其对LLSVPs的启示

LLSVPs的成因是当前固体地球科学研究的前沿热点,对这一成因的认识仍然存在很大的争议。认识LLSVPs的成因,对了解全球地幔对流性质、下地幔热结构以及化学物质的运输和演化等具有重要的科学意义。CaSiO₃钙钛矿（Ca-Pv）作为下地幔最丰富的矿物之一,也是近年来学术界关注的热点。Ca-Pv在下地幔条件下的晶体结构仍然是目前研究的主要问题之一,Ca-Pv能否与其结构和化学式都相近的含铁布里奇曼石形成固溶体?本文通过第一性原理地球化学计算,研究了Ca-Pv及固溶体的基态稳定性,弹性性质和地震波波速特性。研究结果表明,在20～120 GPa压力下,Ca-Pv最稳定的基态结构为四方结构。基态下Ca-Pv的弹性波速值低于Mg-Pv约5%。与非互溶体系相比,x（Fe）为0%～10%的固溶体体系,引起了-2%～0%的剪切波波速负异常;-1.2%～0%的纵波波速负异常;-0.45～0.15%的体波速异常。因此,推测固溶体Mg_1-x-yCa_xFe_ySiO₃可能作为下地幔LLSVPs的候选矿物组...     

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).     

聚合氯化铝铁中Al(Ⅲ)对Fe(Ⅲ)稳定性的保护作用

对以NaOH为碱化剂合成的聚合氯化铝铁(PAFC)及其合成前体聚合氯化铝(PAC)、聚合氯化铁(PFC)的低温干燥胶体进行了表征。红外光谱证实PAFC中存在Fe-O-HAl基团;X射线衍射结果证实了PAFC中有着与PAC、PFC不同形态的Al(Ⅲ)、Fe(Ⅲ)羟基配合物,这些Fe-Al羟合异核多核共聚物是长程无序的;扫描电镜能谱分析表明:短程有序的Al-OH-Fe羟合物在Fe(Ⅲ)、Al(Ⅲ)继续水解的过程中与它们一起随机排列,形成长程无序的Fe-Al羟合异核多核共聚物无定形胶体。Al(Ⅲ)对Fe(Ⅲ)的保护作用包括三个方面:Al(Ⅲ)对Fe(Ⅲ)的水解有催化作用,使Fe(Ⅲ)形成更多细微的结晶中心而不是相互聚集生成较大的结晶;大量Al(Ⅲ)的存在使Fe(Ⅲ)羟合物之间的碰撞受到制约,相当于降低了Fe(Ⅲ)的有效浓度,延缓了Fe(Ⅲ)的迅速水解;大量短程有序的Al-OH-Fe羟合物的存在破坏了Al13相及-βFeOOH相的生成环境,有效地保护了Fe(Ⅲ)胶团并使它们在水溶液中的稳定时间大大增加。     

基于FLAC~(3D)的边坡稳定性分析及治理模拟

在现场考察和工程地质勘察的基础上,详细分析了贵匀高速公路百鸟坡隧道左线进口边坡的工程地质条件及其变形特征,认为在暴雨条件下,坡体的覆盖土层与部分强风化层沿强弱风化基岩接触面附近产生滑动而发生坍塌,将隧道左洞掩埋;该边坡破坏机制属牵引式破坏;根据规范及有关经验对边坡力学参数取值,用FLAC~(3D)有限差分技术,建立边坡地质力学模型,计算暴雨条件下边坡的稳定性。结果表明:在暴雨条件下边坡处于不稳定状态,需进行治理。本文通过设计抗滑桩、钢管桩及挡土墙等措施对边坡进行支护并验证该边坡处于稳定状态,为贵匀公路的顺利完成提供了有力保障。     

西南某电站右岸开挖边坡稳定性的FLAC~(3D)分析

西南某大型水电站引水发电洞进水口边坡顶部在开挖过程中出现带状裂缝 ,引起了多方的极大关注。在对该边坡系统的地质调查基础上 ,确立了其变形破坏模式 ;通过FLAC3D模拟开挖过程 ,客观地验证了实际开挖过程中边坡的变形及其顶部平台出现的裂缝 ;并预测了在采取加固措施后边坡内部变形趋势。经计算 ,开挖面临空方向最大位移量达到 5 .4cm。此后 ,经过有效的锚固处理 ,边坡经过变形和应力的调整 ,总体上趋于稳定 ,为确保进水口隧洞的顺利开挖提供了科学依据     

澜沧江某电站左岸地下洞室群围岩稳定性的FLAC~(3D)分析

在岩体结构模型概化的基础上,采用FLAC3D数值分析方法,系统研究了澜沧江某大型水电站大跨度、高边墙地下洞室群开挖完成后围岩的二次应力场、变形场和塑性破坏区的变化特征。总结了地下洞室群围岩应力、变形和破坏区的分布特征和变化规律,为洞室群稳定性评价和工程施工设计提供了基础资料和参考依据。     

红河石Ca18(□,Ca)2Fe2+Al4(Fe3+,Mg,Al)8(□,B)4BSi18O69(O,OH)9——符山石族新矿物

作为符山石族矿物的新成员,红河石(Hongheite,IMA 2017-027新矿物),Ca18(,Ca)2Fe2+Al4(Fe3+,Mg,Al)8(,B)4BSi18O69(O,OH)9发现于个旧世界级Sn-多金属矿田东北缘、与马拉格Sn矿床毗邻的北沙冲花岗岩(77.43Ma)内矽卡岩中。红河石常呈横径达4~25mm的放射状针-柱状集合体产出。当位于晶洞中时,红河石则呈发育良好的自形柱状晶体(0.5~4.0mm长,0.3~1.0mm宽)产出。与红河石共生的矿物见有赛黄晶、萤石、斧石-(Fe)、硅硼钙石、枪晶石、硼锡钙石、石英和羟鱼眼石-(K)等。红河石为墨绿色,条痕浅灰绿色,玻璃光泽,性脆,断口不规则。主要的晶面是:{100}、{110}、{101}和{001}。红河石的显微硬度:988.3N/mm2,相当于摩氏硬度6~7。其实测密度与计算密度分别是3.446g/cm3和3.423g/cm3。红河石一轴正晶,No=1.720(2),Ne=1.725(2);多色性弱。红河石的化学成分:SiO235.85%;TiO20.01%;Al2O311.00%;Fe2O37.92%;FeO2.14%;CaO 33.57%;MnO 0.42%;MgO 3.48%;B2O32.82%;Cr2O30.01%;Na2O 0.01%;F 0.40%(F≡O-0.17);Cl 0.14%(Cl≡O-0.03);H2O 0.75%,总量98.32%。依据晶体结构精测和Si在单位分子式中的原子数(即Si=18 apfu),计算和书写的红河石简化晶体化学式:Ca18(,Ca)2Fe2+Al4(Fe3+,Mg,Al)8(,B)4BSi18O69(O,OH)9。其三条最强粉晶线[d(?)(I/I0)(hkl)]为:2.9289(47)(004),2.7661(100)(342)和2.6079(68)(243)。红河石属四方晶系,空间群为P4/nnc,晶胞参数:a=15.667(3)?,c=11.725(1)?,V=2878(1)?3,Z=2。红河石晶体结构精测的R因子为0.063。红河石殊异于为已知的符山石族矿物种,在于其X(4)位以空位()为主、Y(3)位以Fe3+居优和T(2)位被B所占。顺便对符山石族矿物晶体-化学式的计算与书写予以讨论并提出建议。     

山东金伯利岩蒙山矿(mathiasite)研究的新进展──上地幔含大阳离子Cr、Ti、Fe氧化物矿物研究之三

本文回顾了国内外金伯利岩及其它地幔岩中发现的K、Ba、Ca、REE的Ti-Cr－Fe－Mg氧化物矿物的研究现状，包括沂蒙矿系列、蒙山矿系列及柱红石系列。在此基础上概括性地总结了笔者近一、二年对山东蒙阴金伯利岩中蒙山矿矿物所进行的微区矿物学方向研究取得的最新进展，包括：①揭示了K、Ba、Ca、REE等半径较大的元素在蒙山矿中的不均匀分布特点；②首次在金伯利岩中发现了蒙山矿系列矿物的Ca端元——Ca蒙山矿及K－Ba－Ca三个端元蒙山矿在同一矿物颗粒中的共存现象；③进行了高分辨透射电镜工作，拍摄了晶格条纹象，从而完善了蒙山矿的晶体结构资料；④确定了蒙山矿后期熔融交代形成的多种矿物相。这些研究成果，证实了地幔交代作用的存在和复杂性，应引起矿物岩石学界的充分重视。     

Tetrahedral compression in (Mg,Fe)SiO3 orthopyroxenes

Single-crystal structure determinations at pressure have shown that the structural response of synthetic (Mg_0.6Fe_0.4)SiO₃ orthopyroxene to compression is the same as that previously observed in MgSiO₃ orthoenstatite. At pressure below ～4?GPa there is no significant compression of the SiO₄ tetrahedra, while above ～4?GPa the tetrahedra decrease in volume as a result of Si?O bond shortening. A study of the compressional behaviour of synthetic FeSiO₃ orthoferrosilite also shows the same behaviour below 4?GPa, but studies at higher pressures are precluded due to the transformation of the sample to the higher density C2/c high-clinoferrosilite polymorph. A further single-crystal study to 6?GPa of a Ca²⁺-containing natural orthopyroxene shows that chemical substitution of, primarily, Al³⁺ and Ca²⁺ into the structure of orthopyroxene inhibits the initial rapid compression of the M2?O3 bonds observed in the synthetic samples, and no significant tetrahedral compression is observed in this sample. Raman data collected from synthetic MgSiO₃ orthoenstatite show that there is a change in the rate of change of frequency with pressure, δν/δP, between 3.5 and 6.0?GPa, but no changes in the number of observed bands. These observations indicate a non-symmetry-breaking change in the properties of the orthoenstatite, which is associated with the change in compression mechanism observed using X-ray diffraction techniques at this pressure.     

Phase Stability and Reactions of Subducting CaCO3 under Upper Mantle Conditions

CaCO₃ is an important component of marine sediments and one of the major deep-carbon carriers at subduction zones. Some subducted CaCO₃ can be dissolved in subduction fluids and recycled back to the surface via arc volcanoes degassing. At the same time, there still remain large amounts of CaCO₃ and its reaction products, which could be further transported into Earth’s deep interior. These internal processes link atmosphere, hydrosphere and biosphere with the deep...     

Optical Absorption Spectra of (Mg,Fe)SiO3 Silicate Perovskites

Electronic absorption spectra have been measured at room temperature and pressure for polycrystalline samples of (Mg, Fe)SiO₃ silicate perovskites synthesized by multi-anvil device. One strong near-infrared band at about 7000 cm^-1 and several weak bands in the visible region were found. The near-infrared band at 7000 cm^-1 is assigned to a spin-allowed transition of Fe²⁺ at the 8–12 coordinated site in perovskite. However, definite assignments of the weak bands in the visible region are difficult because of their low intensities and the scattering effect at the gain boundaries. Crystal field calculations for Fe²⁺ at different sites in perovskite have been carried out based on the crystal structure data. The results agree with the assignment of Fe²⁺ to the 8–12 coordinated site in perovskite. Crystal field stabilization energy of Fe²⁺ with coordination number of 8 in perovskite is 3332 cm^-1 which is small compared to the octahedral site of magnesiowüstite (4320 cm^-1), another important lower-mantle mineral.     

Thermochemistry of (Fe2+, Mg)SiO3 orthopyroxene

Enthalpies of solution in eutectic (Li, Na)₂B₂O₄ melts at 1023 K were measured for five synthetic orthopyroxenes on the join MgSiO₃-FeSiO₃. The pyroxenes were synthesized at 1120°C and 20 kbar and thus were presumed to be highly disordered. The measurements indicate a small positive enthalpy of mixing, with W_H = 950 cal/MSiO₃.Enthalpy of solution measurements were made on a natural, well-ordered orthopyroxene near the composition En_52.5Fs_47.5 and on this material after heat-treatment at 1150°C and 20 kbar. Irreversible expansion of the unit-cell constants of the natural pyroxene after heat-treatment at various temperatures was used to characterize the degree of M-site disorder. The observed enthalpy of solution decrement of 0.85 kcal/MSiO₃ between the natural En_52.5 and the same material heated at 1150° corresponds to about half of the maximum possible disordering, or ΔX_Fe^M1? 0.25, which leads to a ΔH of 7.5 kcal/M₂Si₂O₆, for the exchange reaction: Fe(M2) + Mg(Ml) = Fe(Ml) + Mg(M2) if M-site interaction energy terms are ignored. This ΔH is larger than inferred from any of the analyses of site-occupancy data except that of Besancon (1981), who gave a very similar value. The measured ΔH of disorder and the W_H of mixing together indicate a large ΔH as great as 3.2 kcal for the reciprocal reaction: Fe₂Si₂O₆ + Mg₂Si₂O₆ = Fe(M2)Mg(M1)Si₂O₆ + Fe(M1)Mg(M2)Si₂O₆ as anticipated by Sack (1980).As a consequence of the inferred magnitudes of ΔHof the exchange and reciprocal reactions, departures from ideality of Gibbs energy of mixing of orthopyroxene are very small at 700°–1000°C. Activities of MgSiO₃ and FeSiO₃ may be replaced by their mol fractions at all temperatures in most petrologic calculations.     

Crystal field and charge transfer spectrum of (Mg, Fe)SiO3 majorite

Majorite of bulk composition Mg_0.86Fe_0.15SiO₃ was synthesized at 19 GPa and 1900 °C at an oxygen fugacity close to the Re/ReO₂ buffer. Optical absorption spectra of polycrystalline samples were measured from 4000 to 25000cm^?1. The following features were observed: (1) Three bands at 4554, 6005 and 8093 cm^?1 due to the ⁵E_g → ⁵T_2g transition of Fe²⁺ in a distorted dodecahedral site. (2) A band at 9340 cm^?1 due to the transition ⁵T_2g → ⁵E_g of octahedral Fe²⁺. (3) A band at 22784 cm^?1 resulting from Fe³⁺, probably in an octahedral site (⁶A_1g → ⁴A_1g, ⁴E_g). (4) A very intense system of Fe²⁺ → Fe³⁺ intervalence charge transfer bands which can be modelled by two Gaussian components centered at 16542 and 20128 cm^?1. The existence of two components in the charge transfer spectrum could be related to the fact that the tetragonal majorite structure may contain Fe³⁺ in two different octahedral sites. The crystal field splitting Δ of Fe²⁺ in dodecahedral coordination is 5717 cm^?1. If a splitting of the ground state in the order of 1000 cm^?1 is assumed, this yields a crystal field stabilization energy (CFSE) of 3930 cm^?1, comparable to the CFSE of Fe²⁺ in pyrope-rich garnet. However, the splitting of ⁵T_2g is significantly higher than in pyrope. This would be consistent with Fe²⁺ preferentially occupying the more distorted one of the two dodecahedral sites in the majorite structure. For octahedral Fe²⁺, Δ= 9340 cm^?1 and CFSE=3736 cm^?1, assuming negligible splitting of the ground state.     

Crystal chemistry of Fe3+-bearing (Mg,Fe)SiO3 perovskite: a single-crystal X-ray diffraction study

Magnesium silicate perovskite is the predominant phase in the Earth’s lower mantle, and it is well known that incorporation of iron has a strong effect on its crystal structure and physical properties. To constrain the crystal chemistry of (Mg, Fe)SiO₃ perovskite more accurately, we synthesized single crystals of Mg_0.946(17)Fe_0.056(12)Si_0.997(16)O₃ perovskite at 26 GPa and 2,073 K using a multianvil press and investigated its crystal structure, oxidation state and iron-site occupancy using single-crystal X-ray diffraction and energy-domain Synchrotron Mössbauer Source spectroscopy. Single-crystal refinements indicate that all iron (Fe²⁺ and Fe³⁺) substitutes on the A-site only, where \( {\text{Fe}}^{ 3+ } /\Upsigma {\text{Fe}}\sim 20\,\% \) based on Mössbauer spectroscopy. Charge balance likely occurs through a small number of cation vacancies on either the A- or the B-site. The octahedral tilt angle (Φ) calculated for our sample from the refined atomic coordinates is 20.3°, which is 2° higher than the value calculated from the unit-cell parameters (a = 4.7877 Å, b = 4.9480 Å, c = 6.915 Å) which assumes undistorted octahedra. A compilation of all available single-crystal data (atomic coordinates) for (Mg, Fe)(Si, Al)O₃ perovskite from the literature shows a smooth increase of Φ with composition that is independent of the nature of cation substitution (e.g., \( {\text{Mg}}^{ 2+ } - {\text{Fe}}^{ 2+ } \) or \( {\text{Mg}}^{ 2+ } {\text{Si}}^{ 4+ } - {\text{Fe}}^{ 3+ } {\text{Al}}^{ 3+ } \) substitution mechanism), contrary to previous observations based on unit-cell parameter calculations.     

Single-crystal infrared reflectivity of pure Mg2SiO2 forsterite and (Mg0.86,Fe0.14)2SiO4 olivine

New polarized infrared reflectance spectra of pure synthetic forsterite and natural Fo₈₆-olivine have been recorded from 5000 to 100cm^-1. Out of the 35 expected infrared active modes, 33 have been observed (8 B_1u, 12 B_2u, 13 B_3u). The observed frequency shift from pure forsterite to Fo₈₆-olivine is consistent with the higher mass of the substituted iron. The substitution of only 14% of iron also reduces the overal far-infrared reflectivity of olivine as compared to pure forsterite. Several discrepancies associated with previous studies of forsterite are explained by our investigation. We suggest that some of the previous investigations were complicated by polarization mixing.     

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.     

Thermal expansion,Debye temperature and Grüneisen parameter of synthetic (Fe,Mg)SiO3 orthopyroxenes

Thermal expansion properties of synthetic orthopyroxenes (Fe_0.20Mg_0.80)SiO₃, (Fe_0.40Mg_0.60)SiO₃, (Fe_0.50Mg_0.50)SiO₃, (Fe_0.75Mg_0.25)SiO₃ and (Fe_0.83Mg_0.17)SiO₃ were systematically studied by means of single-crystal x-ray diffraction in the temperature range from 296 to 1300 K. The measurements of unit cell dimensions as a function of temperature reveal that the a and c dimensions and the unit cell volume V increase nonlinearly with a positive curvature with rising temperature, whereas the b dimension behaves differently, depending on the total Fe content. For Mg-rich orthopyroxenes (Fe/(Fe+Mg)<30%), the b dimension expands similarly as the a and c dimensions, but it exhibits a nonlinear increase with a negative curvature for orthopyroxenes with Fe/(Fe+Mg)>30%. Together with the high temperature neutron diffraction data on enstatite (MgSiO₃) (McMullan, Haga and Ghose, unpublished) and x-ray diffraction data on ferrosilite (FeSiO₃) (Sueno et al. 1976), the measured unit cell dimensions were analyzed in terms of the Grüneisen theory of thermal expansion. The linear thermal expansion coefficients α _a and α _c both increase as temperature is elevated, with α _c increasing faster, while α _b changes gradually from increasing for Mg-rich orthopyroxenes to decreasing for Fe-rich orthopyroxenes. The relative magnitudes of linear thermal expansion coefficients are always in the order α _b >α _c >α _a between 300 and 500 K, but at higher temperatures, the order changes to α _c >α _b >α _a for Mg-rich orthopyroxenes and α _c >α _a >α _b for Fe-rich ones. The linear thermal expansion behavior is interpreted on the basis of the structural mechanical model of Weidner and Vaughan (1982). The anomalous behavior of α _b is mainly attributed to the changes in the Fe₂₊ population at the M2 site and the relative stiffness of the M2(Fe₂₊)-O bonds compared to the M2(Mg₂₊)-O bonds. The volume thermal expansion coefficients are nonlinear functions of temperature and lie between 23 and 49×10^?6/K. The previously reported results of mean volume thermal expansion coefficients appear to represent the α _V values characteristic of higher temperatures compared to our results. The thermal Debye temperatures are composition-dependent, decreasing linearly from 812 (MgSiO₃) to 561 K (FeSiO₃), and are systematically higher than the corresponding acoustic Debye temperatures. The Grüneisen parameters range from 0.85 to 0.89 and do not seem to vary with composition. The linear compressibilities derived from thermal expansion and elastic moduli data agree very well. The pressure derivatives of the isothermal bulk modulus (dK₀/dP) are also composition-dependent and decrease from 11.2 (MgSiO₃) to 8.77 (FeSiO₃). Such large values indicate possible anomalous elastic behavior of orthopyroxenes at high pressures in the Earth's upper mantle.     

Back transformation and oxidation of (Mg,Fe)2SiO4 spinels at high temperatures

Based on the in situ and temperature-quench X-ray measurements, the back transformation in the (Mg, Fe)₂SiO₄-spinels has been characterized in terms of the transformation temperature (T _r ),mechanism and kinetics of the transformation, and of the end product(s), with specific emphasis on the effect of oxygen on this transformation. The in situ measurements were conducted to 900° C in vacuum (10^-4 to 10^-5 torr) and to 600° C in air using synchrotron radiation (SR) at Stanford Synchrotron Radiation Laboratory (SSRL). In the quench-type measurements, samples were heated in air to 1100° C, quenched and examined at ambient conditions using the conventional X-ray diffraction facilities. Important results are (1) in vacuum, all the spinels convert back into the olivine phase, with their T _r decreasing with increasing iron content; (2) the spinel olivine back transformation is a nucleation and growth type of transformation and can be described quantitatively using the Avrami equation; (3) in air, the (Mg, Fe)₂SiO₄-spinels with 0.2 mole fraction Fe or more are all oxidized, and the composition and phase of the end products depend upon the temperature and the starting composition; and (4) the oxidation of the iron-rich (Mg, Fe)₂SiO₄-spinels in air occurs at 350–400° C, which is significantly lower than its T _r ( 300° C) in vacuum.