Pyroxene Structure Research Articles

The first-principle study on structural, mechanical, electronic and optical properties of half-metallic CaMSi2O6 (M= Co, Fe, Mn) clinopyroxenes

Ca-based pyroxene structures have gained significant attention due to the higher abundances of calcium in the earth's crust and relatively easy synthesis process. Here, we have analyzed the structural, mechanical, elastic, thermal, and optoelectronic properties of CaMSi2O6 (M = Co, Fe, Mn) pyroxene structures in the monoclinic phase using the first-principles density functional theory (DFT) approach using CASTEP code. The lattice constants of the simulated structures in ferromagnetic (FM) orientations using GGA-WC and GGA-PBSOL were consistent with the available experimental data. Using the Born stability criteria, these structures are considered mechanically stable. These are elastically anisotropic and brittle compounds. The melting temperatures are in the order of 103 K which symbolizes these as potential candidates for high-temperature applications. The band structure alongside the electronic density of states at the Fermi level reveals half metallicity of these structures. CaMnSi2O6 structure has a maximum half-metallic gap of around 4.60 eV. The remaining two structures have an indirect band gap in the visible photon range given as 3.14 eV, and 2.65 eV for CaCoSi2O6, and CaMnSi2O6, respectively. Observing optical and electronic properties shows that the compound holds a promising future to be utilized in the optoelectronic and plasmonic fields.

Morphologies and mechanical properties of basalt fibre processed at elevated temperature

In this study, the morphologies and properties of basalt fibre (BF) treated at different temperatures in air atmosphere were studied. The results showed that the heat treatment caused the formation of crystals on fibre surface, which resulted in the alteration of BF from the paramagnetic material to the ferromagnetic one. The tensile strength of BF was reduced significantly upon thermal treatment, and such reduction occurred in three temperature ranges: i) Decomposition of organic sizing on fibre surface, which led to the exposure of more defect structures (100–400 °C), and the structural changes of BF may also occur; ii) Simultaneous relaxation of excessive enthalpy and structural anisotropy, which were responsible for the phase separation in the amorphous matrix in the fibre (500–600 °C); iii) Structural modification due to the formation of multifarious crystals in the fibre (700–1000 °C). The crystallization in BF began with spontaneous spinel phase formation, which was favored by the initial phase separation and the oxidation of Fe2+. The spinel crystals became nucleation sites for the formation of pyroxene structure, and this process was accompanied by the enrichment of Ca, Mg and Fe on fibre surface. Therefore, the inhibition of crystallization for BF was important to maintain the mechanical performance of filament under the continuous thermal conditions for practical applications.

A novel Eu2+/Tb3+ co‐doped phosphor with pyroxene structure applied for cryogenic thermometric sensing

AbstractPyroxene‐type phosphors were widely developed due to the advantages of high chemical stability, luminous efficiency, and low production cost. In this contribution, a series of Eu2+/Tb3+ co‐doped Ca0.75Sr0.2Mg1.05Si2O6 (CSMS) phosphors with pyroxene structure were successfully synthesized by the solid‐state method. Under the 340 nm excitation, the emission peaks of the phosphor show a redshift with the increase of Eu2+ concentration. The emitting color of Eu2+/Tb3+ co‐doped samples shows a redshift attributed to the energy transfer from Eu2+ to Tb3+. Simultaneously, acquired thermometer exposes superbly temperature‐sensitive properties (Sa and Sr having maximum values 4.7% K−1 and 0.6% K−1, respectively) over the cryogenic temperature range (77–280 K). Furthermore, it has good stability and precision at cryogenic temperatures, indicating that CSMS:0.03Eu2+/0.03Tb3+ phosphor is a very promising fluorescent material suitable for cryogenic temperature sensing.

Synthesis of Cu-containing Diopside through a One-Step Crystallization

The incorporation of copper into pyroxene structure was investigated through the melt quenching technique and one-step crystallization procedure. Two series of glasses have been studied, one set with Ca=Mg and another set with Ca>Mg in diopside formula Cux(Ca Mg)2-xSi2O6. The glasses were nucleated by TiO2, Cr2O3, or CaF2 additions as nucleating agents to variably control the phases produced. X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the obtained samples. The heat treatment studied at 700, 800, 900 and 1000 °C for 2 h produced green and dark green glasses based on Cu-containing diopside. Various crystalline wollastonite, cuprite, tenorite, cristobalite, quartz, and fluorite phases were developed with different ratios combined with diopside formation depending on the heat treatment and nucleating agents used. As the heat treatment increased in temperature, the crystallized fraction increased with the development of nano-aggregates and the observed reticulated textures confirmed a radical change in the euhedral crystals. This emphasizes that the Cu-containing diopside can be created by a facile one step process. These compositions may find some applications in biological and optical fields.

Mantle metasomatic influence on water contents in continental lithosphere: New constraints from garnet pyroxenite xenoliths (France & Cameroon volcanic provinces)

Quantifying water contents in the lithospheric mantle is key to our understanding of global geodynamics, mantle composition, and related physical properties. Most mantle lithologies (peridotite) contain little water (~50 ppm), but petrological heterogeneities such as pyroxenites are more hydrous (~300 ppm) relative to the mantle rocks. Pyroxenites also melt at lower temperatures than peridotites and are thus important to magma genesis. Thus, quantifying pyroxenite water contents provides new information on the distribution of water in the mantle. Here, we present phase-specific FTIR measurements of the water contents in pyroxenite mantle xenoliths from two continental lithospheric domains that experienced intense metasomatism: the French Massif Central (FMC, France) and the Adamawa Volcanic Plateau (AVP, Cameroon). The AVP garnet pyroxenites are more hydrated ([H2O]Clinopyroxene = 386–685 ppm; [H2O]Orthopyroxene = 124–155 ppm; [H2O]Garnet < 0.5 ppm) than FMC ones ([H2O]Clinopyroxene = 112–465 ppm; [H2O]Orthopyroxene = 61–104 ppm; [H2O]Garnet < 0.5 ppm). These water concentrations are homogenous at the grain and correlate with equilibrated major element concentrations, indicating that they are representative of lithospheric water, although the FMC pyroxenites were dehydrated during metasomatism by a carbonatitic fluid (based on the correlation between LaN/SmN and Ti/Eu ratios); the water contents of AVP pyroxenites were likely not affected by metasomatism. FMC pyroxenites show peculiar FTIR spectra that may reflect the preferential dehydration of specific sites in the pyroxene structure. In both regions, metasomatism modified the light rare Earth element contents (e.g., Ce) of the pyroxenites, resulting in highly variable H2O/Ce ratios. Therefore, we conclude that the utility of the H2O/Ce ratio to identify the involvement of pyroxenites in magmas genesis is limited.

Single-phase synthesis, average, electronic, and local structure and cathode properties of pyroxene type LiFeSi2O6

Single-phase LiFeSi2O6 with pyroxene structure was firstly synthesized via a hydrothermal route under specific conditions with optimal water amount. Using both synchrotron X-ray and neutron diffraction techniques, crystal structure refinement with the C2/c space group resulted in well converged anisotropic displacement parameters. The electron density distributions were calculated by the maximum-entropy method (MEM) and showed the difference in the Si-O bond before and after charging. A carbon composite with LiFeSi2O6 exhibited strongly enhanced electrochemical properties as the cathode for Li-ion batteries due to the improved electronic conductivity. The X-ray absorption near edge structure (XANES) spectra revealed the redox of iron was not involved in the charge and discharge processes, while other mechanisms for charge and discharge were predicted to be related to Si or O. The local structure was refined by pair distribution function (PDF) analysis of the synchrotron X-ray and neutron total scatterings, which indicated Li-ion diffusion as a one-dimensional path.

Structural, mechanical, electronic and optical properties of half-metallic XFeSi2O6 (X = Li, Na, K) clinopyroxenes: A first-principles investigation

Pyroxene structures are ferromagnetic silicates that are found in abundance in the Earth’s crust and upper mantle, which crystallizes in orthorhombic and monoclinic systems. Here, we present a study using first-principles density functional theory approach in analyzing the structural, physical, optical and magnetic properties of XFeSi2O6 (X = Li, Na, K) pyroxenes in a monoclinic phase. The lattice parameters and lattice angles of the simulated structures in a ferromagnetic structural ordering were consistent with the available experimental data. The optical properties suggested that the structures have a high absorption of UV photons, high reflectivity for visible photons, and calculated loss function spectra show the presence of plasmon peaks near 15 eV–17 eV. The NaFeSi2O6 structure was found to be ductile and the KFeSi2O6 structure demonstrated an auxetic nature. All three compounds were found as ferromagnetic half-metals with a magnetic moment of 5.0μB. These properties suggest potential applications of such pyroxenes in optoelectronics and plasmonics.

Клинопироксеновые твердые растворы в сечении CaMgSi2O6 – Ca0,5AlSi2O6 при высоких Р-Т-параметрах

Clinopyroxenes (Cpx) are one of the main rock-forming minerals, but stoichiometry of their compositions was called into question. In particular, an idea of hypothetical calcium molecule Eskola (CaEs, Ca0,5AlSi2O6) existence was expressed. This minal has structure vacancy and silica excess. Numerous experimental investigations in CMAS-system (CaO-MgO-Al2O3-SiO2) have showed that the question of non-stoichiometric Cpx existence remains open. This paper presents the results of an experimental study of the diopside Di (CaMgSi2O6) – calcium molecule Eskola CaEs (Ca0,5AlSi2O6) cross-section in the CMAS-system. The experiments were carried out in the following pressure and temperature range: P=10-4 – 3,0 GPa; T=966 – 15250C. Experiments at atmospheric pressure (10-4 GPa) were performed on a vertical shaft electric resistance furnace; high-pressure ones were performed on a "piston-cylinder" type apparatus. Samples obtained were analyzed using electron microprobe (EMP), scanning electron microscope (SEM) and Raman spectrometer. Depending on the P-T conditions, the samples contain the following phases: anorthite An, garnet Grt, diopside Di, clinopyroxene Cpx, quartz Qtz (tridymite Tr – for experiments at atmospheric pressure), and glass L. The data array on the composition of clinopyroxenes crystallized in this cross-section with diopside in various associations is generalized and supplemented. Clinopyroxenes were found to form quaternary solid solutions of diopside Di (CaMgSi2O6) – enstatite En (Mg2Si2O6) – calcium molecule Tschermak CaTs (CaAl2SiO6) – calcium molecule Eskola CaEs (Ca0,5AlSi2O6). The CaTs and CaEs minals contents are positively correlated with the amount of aluminum in clinopyroxene, and this relationship is particularly pronounced for CaTs. It is confirmed that clinopyroxenes in this cross-section can contain an excess of silica at both atmospheric and high pressures. Apparently, the cation vacancy that exists in pyroxene structure can participate in ordering processes. As a result the pyroxenes of another structure (not diopside – C2/c-symmetry) can be crystallized from total compositions in the Di-CaEs cross-section. Additional research is needed to support this hypothesis. Besides, at present investigation it was not possible to establish an unambiguous relationship between the Cpx composition and P-T-parameters, since it is also associated with both the mixture initial composition and the mineral association. Further experiments are required to justify any geothermobarometric dependence.

Crystallization characteristics and properties of lithium germanosilicate glass-ceramics doped with some rare earth oxides

Lithium disilicate glass-ceramic, modified by GeO2/SiO2 replacement to give 33.60Li2O-5.00GeO2-61.40SiO2 (mol%) composition, was investigated by the conventional melt technique. The effect of adding 0.1mol of some rare earth oxides like Y2O3, La2O3 or CeO2 on the crystallization behavior, chemical durability, Vicker's microhardness, and density of the obtained glass-ceramics were studied. The effect of In2O3 was also investigated. The glass transition temperature (Tg) was reduced slightly by the replacement of GeO2/SiO2, while it markedly increased when GeO2-containing glass was doped with different rare earth or In2O3 oxides. Different forms of disilicate phases like Li2Si2O5, Y2Si2O7 and La2Si2O7 in addition to two forms of Li2(Ge,Si)2O5 and Li2(Ge,Si)O3 solid solution were developed. LiInSi2O6 of pyroxene structure, Li6Ge8O19 and CeO2 phases were also detected. The density values of the glass-ceramics vary from 2.38 to 3.14g/cm3 and the hardness was in the range of 4150–5585MPa. The chemical durability of the modified glass-ceramics was greatly improved. The modification of glass-ceramics with GeO2 or other dopant oxides led to producing of materials with more dense and ultra-fine microstructures. Such materials are promising for superior applications as catalysis, electrolyte in fuel cells, biomedical, as well as dental restorative applications.

P21/c-C2/c phase transition and mixing properties of the (Li,Na)FeGe2O6 solid solution: A calorimetric and thermodynamic study

Abstract A calorimetric and thermodynamic study of the (Li1-xNax)FeGe2O6 solid solution with the pyroxene structure was undertaken. The molar heat capacity at constant pressure (Cp,m) for compositions with x = (0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) was measured using a Physical Properties Measurement System at temperatures from 2 K to 300 K and by differential scanning calorimetry between 282 and 870 K (these measurements were performed upon heating as well as upon cooling). Magnetic transitions below 20 K and structural P21/c ↔ C2/c phase transitions at higher temperatures were observed that decrease in temperature with increasing Na/(Na + Li). The P21/c ↔ C2/c transition showed a considerable temperature hysteresis that is largest for LiFeGe2O6 (∼30 K) and decreases with increasing Na/(Na + Li) in the pyroxene. The molar enthalpy and entropy changes associated with the P21/c ↔ C2/c phase transitions, ΔHtr,m and ΔStr,m, were determined. They can be described by quadratic and linear functions across the solid solution. A linear correlation between ΔHtr,m and the square of the volume strain at the transition temperature was established. The P21/c ↔ C2/c phase transition of LiFeGe2O6 is compared to that of α-β quartz, whose Cp,m was also measured. The molar enthalpy of order, ΔHord,m, was calculated from the calorimetric data. It has a maximum of ΔHord,m = (−11.6 ± 1.5)kJ·mol−1 for the P21/c ↔ C2/c phase transition of LiFeGe2O6, which is in reasonable agreement with a maximum ΔHord,m = −9.9 kJ·mol−1 calculated using density functional theory. ΔHord,m vs. T behavior was described using a tricritical Landau model with an a parameter of a = (41.8 ± 1.0)J·K−1·mol−1. All properties associated with the P21/c ↔ C2/c phase transition in LiFeGe2O6 can be calculated using this value. The Cp,m behavior at T 0.4 have the P21/c ↔ C2/c phase transition below 298.15 K, this results in a discontinuity in Scal,m298.15 behavior as function of composition at Na/(Na + Li) = 0.5. DQF was applied to model this behavior and the corresponding parameters were retrieved. The entropy of mixing behavior of the (Li,Na)FeGe2O6 solid solution at 298.15 K is characterized by positive excess molar magnetic entropies of mixing onto which entropy changes due to the structural P21/c ↔ C2/c phase transitions are superimposed in the Na-rich part of the binary system. Excess volumes of mixing for the (Li,Na)FeGe2O6 solid solution were calculated from published data and DQF parameters were derived that allow volume as function of temperature and composition to be calculated. The molar third law entropies at T = 298.15 K are Som = (188.3 ± 2)J·K−1·mol−1 and Som = (203.1 ± 2)J·K−1·mol−1 and the molar enthalpies of formation from the elements at T = 298.15 K are ΔfHom = (−1817.2 ± 9)kJ·mol−1 and ΔfHom = (−1834.0 ± 9)kJ·mol−1 for the end members LiFeGe2O6 and NaFeGe2O6, respectively. ΔfHom was calculated using density functional theory (DFT). Values for ΔfHom, Som, and molar volume, for the coefficients of Cp,m polynomials, for thermal expansion and for the bulk modulus of the end members were compiled and can be used, in combination with the derived volumetric and entropic mixing properties, for phase diagram calculations involving the (Li,Na)FeGe2O6 solid solution.

Melting, crystallization, and the glass transition: Toward a unified description for silicate phase transitions

The melting mechanism of Na 2 SiO 3 , a crystal with pyroxene structure, includes three distinct reactions. All are driven by heating with each reaction commencing at a different temperature. The first two reactions proceed within the crystal at temperatures well below the melting point and are expressed by distinctive crystallographic, calorimetric, and Raman spectroscopic changes to the crystal. With the reactions identified and explained for Na 2 SiO 3 (c) and the melting mechanism elucidated, the Na 2 SiO 3 system becomes the “Rosetta Stone” by which to decipher the melting mechanisms of all pyroxenes and other silicate minerals. The first reaction produces itinerant Na + within the crystal. Itinerancy results from dissociation of some NBO-Na bonds due to heating, with dissociation commencing at ~770 K. The reaction proceeds according to: Si-O-Na → Si-O – + Na + . The Si-O − moiety remains attached to its SiO 3 chain and it is charged because one of its NBO atoms has no associated Na ion. The second reaction is characterized by the appearance of a Q 3 band in Raman spectra of the crystal at temperatures >770 K. It is produced via a polymerization reaction involving the Si-O − species, a product of the first reaction, and a Q 2 species of an adjacent SiO 3 chain according to: 1 Na 2 -Q 2 + 1 ( Na 1 -Q 2 ) − → 2 Na 1 -Q 3 + Na + + O 2 − . Na atoms are included with each Q species to preserve mass balances and (Na 1 -Q 2 ) − is equivalent to the Si-O − species. The produced Q 3 species form cross-chain linkages that affect the crystallographic properties of the crystal. They are responsible for the cessation of thermal expansion of the Na 2 SiO 3 unit cell in the a – b axial plane at T >770 K, and the near-constancy of a and b unit-cell parameters between ~770 and ~1300 K. The presence of Q 3 species in Raman spectra and the inhibited expansion in the a – b axial plane provide exceedingly strong evidence for this reaction. The third reaction commences at ~1200 K where Q 1 Raman band first appears. It can be produced only through depolymerization of Q 2 chains according to: 2 Na 2 -Q 2 + 2 Na + + O 2 − → 2 Na 3 -Q 1 where Na + and O 2− are itinerant species produced by the second reaction. With conversion of Q 2 to Q 1 species, SiO 3 chains are ruptured, long-range order is lost, and melt is produced at 1362 K. The last two reactions proceed by nucleophilic substitution where Si centers are attacked to form fivefold-coordinated activated complexes. Si-O − acts as nucleophile in the second reaction (producing Q 3 species), and O 2− acts as nucleophile in the third reaction (producing Q 1 ). Taken in reverse, these mechanisms describe the formation of nuclei in crystallizing melts and in addition provide insight into the elusive changes that occur at the glass transition. Elucidation of the melting mechanism could thus provides a unified framework within which melting, crystallization, and the glass transition can be understood.

The modular structure of pyroxenes

The structures of pyroxenes can be described as cell-twins based on a common module (a layer) consisting of half the unit cell of the P 2 1 / c polymorph. Atomic models of the different polymorphs are computed and the close similarity between the models and the corresponding experimental structures is shown. The cell-twin operations building the structures of pyroxenes are partial operations belonging to a special case of space groupoid which was called “twinned space group” in the pioneering studies by Ito started in 1935. The groupoid analysis of clino-, proto- and orthoenstatite is presented and the special place occupied by pyroxenes in the category of polytypes is discussed.

Crystallization of pyroxene phases and physico-chemical properties of glass-ceramics based on Li2O–Cr2O3–SiO2 eutectic glass system

The crystallization characteristics, crystalline phase assemblages and solid solution phases developed due to thermally crystallized glasses based on the Li2SiO3–Li2Si2O5–LiCrSi2O6 (1028 ± 3 °C) eutectic glass system by replacing some trivalent oxides instead of Cr2O3 were investigated. The microhardness and chemical durability of the glass-ceramics were also determined. Lithium meta and disilicate (Li2SiO3 and Li2Si2O5), lithium gallium silicate (LiGaSiO4), and varieties of pyroxene phases, including Cr-pyroxene phase, i.e. lithium-kosmochlor (LiCrSi2O6), lithium aluminum silicate (LiAlSi2O6), lithium indium silicate (LiInSi2O6) and pyroxene solid solution of Li-aegerine type [Li (Fe0.5, Cr0.5) Si2O6] were the main crystalline phases formed in the crystallized glasses. There is no evidence for the formation of solid solution or liquid immiscibility gaps between LiAlSi2O6 or LiInSi2O6 phases and LiCrSi2O6 phase. However, LiCrSi2O6 and LiFeSi2O6 components were accommodated in the pyroxene structure under favorable conditions of crystallization to form monomineralic pyroxene solid solution phase of the probably formula [Li (Fe0.5, Cr0.5) Si2O6]. The type and compatibility of the crystallized phases are discussed in relation to the compositional variation of the glasses and heat-treatment applied. The microhardness values of the crystalline materials ranged between 5282 and 6419 MPa while, the results showed that the chemical stability of the glass-ceramics was better in alkaline than in acidic media.

Pyroxene-type compounds NaM3+Ge2O6, with M = Ga, Mn, Sc and In.

The four title compounds, namely sodium gallium germanate, NaGaGe2O6, sodium manganese vanadate germanate, NaMnV0.1Ge1.9O6, sodium scandium germanate, NaScGe2O6, and sodium indium germanate, NaInGe2O6, adopt the high-temperature structure of the pyroxene-type chain germanates, with monoclinic symmetry and space group C2/c. The lattice parameters, the individual and average bond lengths involving M1, and the distortion parameters scale well with the ionic radius of the M1 cation. NaGaGe2O6 has more distorted M1 sites and more extended tetrahedral chains than NaInGe2O6, in which a high degree of kinking is required to maintain the connection between the octahedral and tetrahedral building units of the pyroxene structure. An exceptional case is NaMnGe2O6, in which the strong Jahn-Teller effect of Mn(3+) results in more distorted octahedral sites than expected according to linear extrapolation from the other NaM(3+)Ge2O6 pyroxenes. In contrast with the literature, minor incorporations of V(5+) in the tetrahedral site and a corresponding reduction of Mn(3+) to Mn(2+) in the octahedral sites in the present sample lower the Jahn-Teller distortion and stabilize the Mn-bearing pyroxene, even allowing its synthesis at ambient pressure.

Structural investigation of glasses along the MgSiO3–CaSiO3 join: Diffraction studies

The structure of glasses along the MgSiO3–CaSiO3 join has been investigated by X-ray and neutron diffraction measurements. Structure models were constructed by fitting the experimental data using the Reverse Monte Carlo method (RMC). The structural data indicate a random mixing between MgSiO3 and CaSiO3 glasses, in accordance with their melt properties. Though important disordering is observed, the structure evolves continuously along the join. The Ca environment is essentially similar for all compositions, with an average of 6 to 7-coordinated sites. The Mg environment tends to have higher coordinated sites as the MgO content decreases. There is a continuous mixing of Ca–Mg pairs with a non-random distribution emphasized by the distinct cation–cation distances. Changes were observed in the topology of the silicate network. The proportion of non-bridging oxygens decreases, the number of free-oxygens increases and the ring size distribution is shifted to high-membered rings in the Mg-rich glasses. These structural investigations indicate important differences with the crystalline pyroxene structures.

Crystal and Magnetic Structures and Physical Properties of a New Pyroxene NaMnGe2O6 Synthesized under High Pressure

A new pyroxene compound, NaMnGe(2)O(6), has been synthesized at 3 GPa and 800 °C and fully characterized by X-ray single-crystal diffraction, neutron powder diffraction, and measurements of magnetization and specific heat. NaMnGe(2)O(6) crystallizes into a monoclinic C2/c structure with unit-cell parameters a = 9.859(2) Å, b = 8.7507(18) Å, c = 5.5724(11) Å, and β = 105.64(3)° at 153 K. A cooperative Jahn-Teller distortion is formed by an ordering of the longest Mn-O bonds between two neighboring octahedra along the chain direction. This feature distinguishes NaMnGe(2)O(6) from other pyroxene compounds without Jahn-Teller active cations and suggests that the Jahn-Teller distortion competes with the intrinsic local distortion in the pyroxene structure. No orbital order-disorder transition has been found up to 750 K. Like other alkali-metal pyroxenes with S > (1)/(2), NaMnGe(2)O(6) (S = 2) was found to undergo a long-range antiferromagnetic (AF) ordering at T(N) = 7 K due to intrachain and interchain exchange interactions. Due to the peculiar structural features and the corresponding magnetic coupling, the weak AF spin ordering gives way to a ferromagnetic-like state at a sufficiently high magnetic field. Specific-heat measurements demonstrated that a large portion of the magnetic entropy, >60%, has been removed above T(N) as a result of strong spin correlations within the quasi-one-dimensional Mn(3+)-spin chains. The Reitveld refinement of neutron powder diffraction data gives a commensurate magnetic structure defined by k = [0 0 0.5] with Mn moments aligned mainly along the c-axis with a small component along both a- and b-axes.

Amphibole lamellae: Evidence for water incorporation in the structure of pyroxene

Amphibole lamellae: Evidence for water incorporation in the structure of pyroxene

Non-isothermal crystallization kinetics of some glass-ceramics with pyroxene structure

The kinetics of non-isothermal crystallization of two glass-ceramics obtained from a basalt rock, namely one without nucleating agents (BsM) and the other with the addition of 10% TiO2 and 2% ZrO2 (BsM1) was investigated using differential thermal analysis (DTA). The pyroxene structure of the two glass-ceramics was confirmed by X-ray diffraction (XRD). The activation energy as a function of the crystallized fraction was evaluated by Kissinger–Akahira–Sunose (KAS), Starink, Tang and Vyazovkin methods. The crystallization process of the sample BsM is accompanied by a single exothermic effect while the crystallization of sample BsM1 is accompanied by two exothermic effects on the DTA curve. The activation energies for sample BsM are very close to those associated to the Peak-1 of sample BsM1, varying between 325 and 345kJmol−1 on the entire range of crystallized fraction, α. The activation energies associated to the Peak-2 of the sample BsM1 are about 100kJmol−1 higher on the entire range of α compared with the activation energies of the sample BsM and those associated to the Peak-1 of sample BsM1.

Determination of the crystal structure of diopside (Ca0.8Fe0.2)(Mg0.8Fe0.2)Si2O6 from the Gazakh Trough (Azerbaijan)

The crystal structure of a clinopyroxene is determined by X-ray diffraction. The unit cell parameters are as follows: a = 9.732(7) A, b = 8.896(6) A, c = 5.280(4) A, and β = 106.160(11)°; V = 439.1(5) A3; space group C2/c; and Z = 4. The structure is determined by the direct method and refined in the anisotropic approximation by the full-matrix least-squares method (R[F 2 > 2σ(F 2)] = 0.027, wR(F 2) = 0.072). According to the results of the X-ray structure analysis, the crystal chemical formula of the compound is (Ca0.8Fe0.2)(Mg0.8Fe0.2)Si2O6. In the structure, columns of (Mg, Fe)-octahedra are connected on two sides with Si2O6 chains. The resulting structural units have the [(Mg,Fe)2(Si2O6)2]4− composition. These units form a heterogeneous framework, which is typical of all pyroxene structures. The [(Mg,Fe)2(Si2O6)2]2− composition of the framework corresponds to the ratio [(Mg,Fe) + Si]: O = 3: 6 (1: 2). The (Ca, Fe) atoms are located in the holes of the framework.

Magnetic petrology of high Fe‐Ti eclogites from the CCSD main hole: Implications for subduction‐zone magnetism

The high Fe‐Ti eclogites with exsolved lamellar in 530–600 m depths from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu Ultra‐High Pressure (UHP) metamorphic belt, eastern China, record an anomalously high susceptibility (κ), natural remanent magnetization (NRM) and Köenigsberger ratio Q (NRM/Ji, Ji is induced magnetization). This provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks. In this paper, we have measured systematically magnetic properties and mineral assemblage and structure for three special samples (No. 83, No.86 and No.89). Results show that these Fe‐Ti‐rich eclogites are the result of the fine‐grained (titano)‐magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo‐ilmenite. We found that the dominant Fe‐bearing oxide minerals in samples studied are ilmenite, hematite (Hem + Ilm up to 25%), little (titano)magnetite and pyrite. The ferromagnetic susceptibility is mainly controlled by fine grained (titano)magnetite and NRM is closely related to the exsolved lamellar structure. We hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno‐hematite during cooling and decompression processes. These Fe‐Ti‐rich eclogites might be one of the sources of high‐magnetic anomalies observed in the Sulu subduction zone, eastern China.