Forsterite Phase Research Articles

Sintering and microstructure of spinel–forsterite bodies

Spinel–forsterite phase mixtures formed by the addition of different proportions of alumina to forsterite grog's mixes, containing 10–15% alumina, showed a good compromise of high density, cold crushing strength and microstructure after firing at 1450 and 1500 °C. Forsterite was first prepared from Egyptian talc and calcined magnesite at 1400 °C/2 h. Forsterite shows good density but the microstructure contains cracks along the grain boundaries affecting the mechanical properties. The sintering and mechanical properties are improved through the addition of alumina. Alumina reacts with equimolar MgO from forsterite to form MgAl 2O 4 spinel on sintering at 1450 °C/2 h. Forsterite occurs in either patches or euhedral rhombic grains. Spinel exists in different crystal habits; either in spots within forsterite patches, prismatic euhedral crystals or on rims of forsterite grains. Considerable amount of low melting enstatite phase was formed on 20% alumina addition as a result of the decrease in the MgO/SiO 2 of forsterite after the formation of spinel. Enstatite improves the sintering through enhancing the formation of glassy phase.

Development of Forsterite Refractories from Indian Olivine

Large amount of different magnesium silicate raw materials are available in the region of Karnataka and Tamilnadu, India, which did not get a wide commercial acceptance for the production of refractories even today. One such material is olivine, a by-product from the magnesite mines, which can be used for the manufacturing of forsterite refractories, a substitute for magnesite refractories in many applications. After chemical analysis of for sterite, compositional adjustment was done by adding dead burned magnesite to maximize the forsterite phase and to convert the impurities to high temperature compounds. Uniaxially pressed products of such compositions were sintered in the temperature range of 1200°—1600°C and characterized for various refractory properties.

フォルステライトの生成に及ぼす粉砕雰囲気の影響

Grinding a powder mixture of talc and MgCO3 was conducted under dry (in air) and wet (with water) conditions to investigate the effects of these conditions on the formation of forsterite in sintered bodies from the ground products.Dry grinding caused size reduction in the initial stage, followed by the aggregation of fine particles due to mechanical activation. This operation produced homogeneous mixture with structural change into amorphous state. The homogeneous mixture leads to acceleration of solid state reaction on its firing. On the contrary, wet grinding reduced the particle size in the mixture, but the crystal structure was maintained even in a prolonged grinding period. This is the main reason that different phases of forsterite are formed in the sintered bodies. Consequently, dry grinding is more advantageous than wet grinding in terms of forsterite formation in a body sintered at relative low temperatures.

フォルステライト前駆体の熱的挙動における表面近傍局所構造変化のＸＡＮＥＳ分光法による研究

Forsterite (Mg2SiO4) precursor was synthesized from heterogeneous alkoxide solution containing MgO powder (mean particle size: 10nm). In order to examine thermal behavior of the precursor, DTA was carried out. The precursor was calcined at temperatures before and after each exothermic and endothermic reaction. The calcined powders were examined by XRD analysis and FT-IR spectroscopy to clarify the reactions. It was found that C2H5OH adsorbed on the surface of the powder evaporates at 82°C, Mg(OH)2 decomposes at 390°C and that forsterite phase is formed at 796°C. The changes in near-surface crystal structure involving these reactions were evaluated by Si and Mg K-edge XANES spectroscopy. The forsterite precursor was found to have a metastable structure before crystallizing as forsterite crystal, resulting from the dehydration around 400°C. The phase was identified as amorphous and the electronic structure was close to Mg2Si3O8.

The effect of mixed modifiers on nuclear waste glass processing, leaching, and Raman spectra

Borosilicate glasses with different waste loadings were prepared by ambient melting, quenching, and annealing. Some melt compositions partially crystallize to durable phases of zircon and forsterite. The coexisting liquid quenches to glass and endures the leach tests. The waste loading dependent leach rate trends of these glasses are reminiscent of the mixed alkali effect. Raman spectra suggest initial increase in durability with increases in the depolymerization of silicate species. Fluorine and hydroxyl ions also contribute to depolymerization. Tetraborate and metaborate rings are identified in the Raman spectra. The durability is enhanced when tetraborate bands are more intense than the metaborate bands.

Disappearance of the Liquid-Liquid Miscibility Gap in the System CaO-MgO-SiO2 at High Pressure

The system CaO--MgO-SiO2 has been extensively studied in the past because it includes the phases forsterite, enstatite, and diopside which are major constituents of peridotitic and basaltic rocks. The presence of a liquid-liquid miscibility gap in the silica-rich portion of this ternary has been known since Greig's study (1927). Experimental work has been performed recently along the CaO-SiO2 (Tewhey and Hess, 1979; Hageman et al., 1986) and the MgO~iO2 binaries (Hageman and Oonk, 1986) in order to constrain this miscibility gap. All these investigations have been performed at atmospheric pressure and no attempt has been made to explore the effect of pressure on the miscibility gap in this system, The present study reports preliminary results related to the disappearance of the liquid-liquid miscibility gaps in the CaO-SiO2 and MgO-SiO2 binaries at high pressure.

Melting of forsterite, Mg2SiO4, from 9.7 to 16.5 GPa

A multianvil apparatus has been used to determine the pressure‐temperature melting curve of forsterite from 9.7 to 16.5 GPa. At 10.1 GPa a singular point occurs that marks the change from congruent melting at lower pressures to incongruent melting (forsterite = periclase + liquid) at higher pressures. The melting curve also passes through two invariant points. At one (15.6 GPa, 2310°C), the phases forsterite, periclase, anhydrous B, and liquid coexist and the melting reaction changes from forsterite = periclase + liquid at lower pressures to forsterite = anhydrous B + liquid at higher pressures. At the other (16.7 GPa, 2315°C), the phases forsterite, modified spinel, anhydrous B, and liquid coexist, and the melting reaction changes from forsterite = anhydrous B + liquid at lower pressures to modified spinel = anhydrous B + liquid at higher pressures. The Simon equation, P(GPa) = 2.44[(T(°K)/2171)11.4 − 1], fits both our melting curve data and the lower pressure data of Davis and England (1964). At low pressures, the melting curve of forsterite lies at higher temperatures than that of enstatite, but the two curves cross at 13.3 GPa because of the lower dT/dP slope of the forsterite melting curve. This causes the forsterite‐enstatite eutectic to shift toward forsterite as pressure increases, but our data are consistent with earlier findings that the shift is not sufficient to support an origin for the mantle by eutectic‐like melting at high pressures.

Mineral and melt physics

The previous four years have been a period of enormous discovery, growth and diversification in the mineral physics community. To underscore this point, several highlights of the U.S. research effort can be listed from a wide range of fields. Relatively young experimental methods have quickly matured and come to produce exceedingly important results; a prime example of this is the measurement of the elastic moduli of the β and γ high‐pressure phases of forsterite by Brillouin spectroscopy [Weidner et al., 1984; Sawamoto et al., 1984]. The technical capabilities of more established techniques have been considerably extended, with a spectacular example being provided by the attainment of a statically‐ maintained pressure of 550 GPa (5.5 Mbar) using a diamond anvil cell [Xu et. al, 1986]; this pressure far exceeds that in the center of the Earth! Moreover, existing static and dynamic high pressure technologies have been utilized in new and creative ways. A wide variety of spectroscopic methods are being used not only to measure the properties of materials, but also to precisely define the temperature and pressure conditions which are achieved within the small sample volume of a diamond anvil cell. The determinations of the melting points of Fe to 43 GPa [Boehler, 1986] and Mg.9Fe.1SiO3‐perovskite to 60 GPa [Jeanloz and Heinz, 1984] are but two examples of how the diamond cell is being used to perform quantitative phase equilibrium measurements at ultrahigh pressures. Additional constraints on the phase relations of high‐pressure silicates are being provided by calorimetric measurements of thermochemical properties [e.g., Akaogi et al., 1984]. Complementary to these efforts are dynamic measurements of the sound velocity [Brown and McQueen, 1986] and temperature [Lyzenga et al., 1983] during shock wave experiments. Both of these variables are sensitive to phase transitions and have been used to constrain the phase diagrams of Fe and SiO2, respectively, at pressures ranging to hundreds of GPa. Another new application of shock wave methods has been in the measurement of the pressure‐density equation‐of‐state of silicate melts [Rigden et al., 1984], which gives experimental support to the interesting possibility of melts sinking, rather than rising, at depth in the mantle. Parallel to these experimental advances have been a series of theoretical efforts to model the equation‐of‐ state and stability of high‐pressure phases. First‐principles calculations were recently performed to investigate the high‐pressure equation‐of‐state of oxides [Hemley et al., 1985; Bukowinski, 1985], as well as structurally complex high‐pressure silicates such as perovskites [Wolf and Bukowinski, 1986], and are in broad agreement with available experimental data on crystal structures and physical properties.

Solid-state reactions of talc toward NI(II) at high temperatures

Solid mixtures within the system talc- x NiCl 2·6H 2O in molar ratio 1 : x ( x = 0.1. 0.2. 0.4. 0.6, 0.8 and 1.0) were prepared and fired at temperatures of 1000 and 1200°C. Investigation of the rapidly cooled coloured reaction products to room temperature was worked out using X-ray diffraction and ligand-field absorption spectrum techniques. The measured diffractograms showed lines related to forsterite phase and enstatite modifications (proto, ortho and clino). In addition, unreacted NiO and traces of Ni 2SiO 4 phases were detected in reaction products obtained at 1000°C. whereas cristoballite appeared in abundance in products obtained at 1200°C. Spectra results showed, on the other hand. absorption bands characteristic of an octahedrally coordinated Ni 2+ in enstatite and forsterite lattices, in agreement with X-ray diffraction data. In general, crystallinity and concentration of some resulting phases were observed to increase on increasing firing temperature as well as Ni concentration.

Electron beam-induced phase decompositiqn of cordierite and enstatite

Abstract Electron irradiation induces phase decomposition of single-crystal enstatite (MgSiO3) to polycrystalline forsterite (Mg2SiO4) and vitreous silica and phase decomposition of cordierite (2MgO.2Al2O3.5SiO2=Mg2Al4Si5O18) to polycrystalline spinel (MgAl2O4) and vitreous silica. The decomposition processes are observed to occur in two steps and involve an intermediate amorphous phase which nucleates from the free surface before final recrystallization. Ion-beam thinning of these silicates accelerates the decomposition.

Effect of Minor Additions on the Synthesis and Properties of Forsterite

The effect of minor additions of barium oxide and titania on the formation of forsterite from pure raw materials and commercial grade raw materials was studied at 1450°C. It was observed that TiO2 was more effective than BaO in the formation of forsterite phase. The addition of 2–3 wt% of each additive produced an optimum value of density and dielectric constant of the forsterite ceramics.

Post‐oxide phases of forsterite and enstatite

Both forsterite (Mg2SiO4) and enstatite (MgSiO3) enter a post‐oxide phase characterized by the orthorhombic perovskite structure when subjected to high pressure and temperature in the diamond‐anvil press coupled with laser heating. The lattice parameters for the perovskite phase of MgSiO3 are ao = 4.790 ± 0.002, bo = 4.943 ± 0.002, and co = 6.897 ± 0.003 Å with Z = 4. The calculated density of MgSiO3 (perovskite) is thus 4.083 g/cm³, or 2.8% denser than its isochemical mixed oxides with rocksalt and rutile structures. The density of a mixture of MgSiO3 (perovskite) plus MgO (periclase) is 1.9% greater than that of the mixed oxides with the forsterite stoichiometry.

Point defects and non-stoichiometry in forsterite

Existing experimental evidence suggests that in pure Mg-forsterite with a fixed Mg/Si ratio, vacancies and interstitials on the Mg sublattice are the dominant intrinsic point defects, while O vacancies and conduction-band electrons are the primary oxygen-deficient defects. Orthopyroxene appears to dissolve readily in pure Mg-forsterite at high temperatures, the change in Mg/Si ratio being accommodated by Mg vacancies and Si interstitials; as a consequence, upper-mantle forsterite may be substantially non-stoichiometric. It is postulated that the amount of Fe that can be oxidized in the forsterite phase is directly related to the excess Si present, either incorporated in the structure, or available from another phase richer in SiO 2 such as pyroxene.

Melting Relations in the System MgO-Al2O3-SiO2 at 15 Kb

Crystal-liquid equilibria have been determined in a portion of the system MgO-Al 2 O 3 -SiO 2 at 15 kb. The isobaric binary eutectic between enstatite and forsterite has been established at a composition of En 85 Fo 15 (weight percent) at 1710°C. The pressure at which enstatite changes from incongruent to congruent melting, as pressure is increased, is estimated to be approximately 4 kb. Six isobaric invariant points within the ternary system have been established which involve the phase relations between a liquid and the crystalline phases forsterite, spinel, aluminous enstatite, sapphirine, quartz, sillimanite, and corundum. Enstatite was found to contain up to 18.9 wt percent Al 2 O 3 in the assemblage quartz + sapphirine + enstatite at 1420°C. The AlAl/MgSi substitution in enstatite seems to be highly dependent upon temperature. Due to a reduction in the size of the primary phase field of forsterite with increasing pressure, partial fusion of a simplified upper-mantle material consisting of enstatite, forsterite, and spinel would produce an initial liquid richer in MgO and poorer in SiO 2 and Al 2 O 3 as pressure is increased to 15 kb. Due to a reaction relation between aluminous enstatite, forsterite, and liquid, silica-enriched liquids can be produced from silica-undersaturated compositions by fractional crystallization at high pressures. Similarly, aluminous enstatite can melt at pressures greater than approximately 4 kb, in an anhydrous environment, to produce silica-enriched liquids. The effect of alumina solid-solution and the effect of water on the melting of enstatite at high pressures are complementary effects which allow the incongruent melting of enstatite at greater depths than previously expected and support the possibility of generating silica-enriched magmas by partial fusion at high pressures as well as the production of such magmas by high-pressure fractional crystallization.

The distribution of some elements between the metal and silicate phases obtained in a smelting reduction process of dunite from Almklovdalen, West Norway

A dunite was smelted at 2000°C and reduced by the addition of coke and anhydrous magnesite. The dunite and the smelting products which consist of an almost pure forsterite phase and a metal phase (75% Fe) were analysed. The partition ratios between metal phase and silicate phase are given for Sc, Si, Mn, Cr, Fe, Co, Ni, Ag, Au and Ir. The bulk composition after the reduction process and the percentage of each element entering the metal phase have been determined for the same elements. The iron metal-phase obtained in this process contains 13.4% Si and 4.7% Ni. Chromium, nickel and cobalt are concentrated in the iron metal-phase by factors above 200 relative to the forsterite phase. The noble metals Ag, Au and Ir are concentrated in the iron metal-phase by factors above 500 relative to the forsterite phase.

Equation of state of forsterite

Shock wave data for pure forsterite with initial bulk densities of 2.6 and 3.1 g/cm3 are obtained to 0.370 Mb by impacting series of specimens with tungsten alloy plates that are launched at speeds of up to 2.3 km/sec with a high-performance propellant gun. The onset of a shock-induced phase change, probably corresponding to the forsterite-‘post spinel’ phase change is observed at 0.280±0.025 Mb. Because of the low shock temperatures, the transition is believed to be limited by the reaction rate and this pressure value should be taken only as an upper limit. Adiabats derived from the Hugoniot data for the forsterite phase are fit to the two-parameter finite strain Birch-Murnaghan equation and to two simple ionic equations of state. The Birch-Murnaghan form of the equation of state gives a zero-pressure bulk modulus (1.29 Mb) that agrees more closely with the ultrasonic data than the modulus obtained from the ionic equations of state. An unusual relaxation effect, in which the elastic shock precursor velocity varies from 5.8 to 9.5 km/sec, is also observed. The characteristic time of the relaxation process appears to be less than 1 μsec.

Shock-wave Data for Several Minerals and their Implication to Mineral Phases in the Lower Mantle

FROM studies of the density and elastic properties of the mantle, Birch1 showed that the upper mantle is characterized by a zero-pressure density of about 3.3 g cm−3 and (K/ρ)p = 0 about 35 km2 s−2, where K is the bulk modulus. The corresponding values for the lower mantle, however, are about 4 g cm−3 and 55 km2 s−2. Comparing these values with laboratory measurements for a number of materials showed that, while a number of common rock-forming minerals (olivine, pyroxene, garnet, jadeite) have the qualifications of the upper mantle material, none of the familiar silicates have similar properties to the lower mantle. The observation that a few oxides (periclase, corundum, rutile) have densities close to 4 g cm−3 and (K/ρ)p=0 close to 50 km2 s−2 inspired Birch1 to speculate that one of the possible phase changes in the transition zone is for silicates to break down into closely packed oxides. The popularity, and hence the importance, of this speculation is clearly indicated by the work of Al'tshuler et al.2, Ahrens and Syono3 and Anderson4. Al'tshuler et al. attempted to construct the equations of state of the high-pressure phases of forsterite and enstatite from the Hugoniot data for quartz and periclase; the constructed curves were compared with the P-ρ curve for the lower mantle. Anderson, on the other hand, calculated on thermodynamic grounds the temperature and pressure range for the breakdown of spinel polymorph of forsterite to form a mixture of closely packed oxides. The result was correlated with the density increase corresponding to one of the steep rises in the seismic velocity profile in layer c. In the light of shock-wave data, the nature of the phase change is examined here.