Forsterite Crystals Research Articles

High temperature cyclic loading experiments on synthetic forsterite single crystals. Mechanical data.

Cyclic loading deformation experiments on [110]c oriented synthetic forsterite single crystals have been performed at about 1640K and ambient pressure, in order to confirm cyclic loading effects on deformation. Loading pattern was set as a systematic alternation of lower loading steps and higher loading steps. According to the present mechanical data, it seems that the response of strain rate to an additional stress has a hysteresis in the early period during which dislocation multiplication controls the deformation. With increasing the number of cyclic loading, dislocation glide comes to dominate the deformation, and then the hysteresis disappears.

Oxygen self-diffusion along high diffusivity paths in forsterite.

Diffusion profiles of 18O tracer in single crystals of forsterite following anneals at 1100° and 1200°C have been determined by a depth profiling technique using secondary ion mass spectrometry. The diffusion penetration profiles showed error function forms with developed tails. The profiles were analyzed in terms of diffusion in the forsterite lattice together with a contribution from dislocation. The present results of lattice diffusion lie in the range of the extrapolation of lines on Arrhenius plots reported previously for intracrystalline diffusion in forsterite. The dislocation diffusion coefficients are about 104 times faster than the lattice diffusion at the same temperature. Diffusion along the dislocations takes place within a pipe with radius of about 0.1 nm surrounding the dislocation line. Based on the above results, O diffusion along high diffusivity paths, such as dislocations and grain boundaries, play an important role on the diffusional creep in olivine under laboratory condition.

Pressure induced effects in forsixrite monitored by ESR

Summary Chromium doped single crystals of Forsterite where investigated directly under the influence of uniaxial pressure in situ by ESR. The two Cr3+ centres M1 and M2 exhibit distinct response to the action of pressure on the crystal as a whole. The pressure effects are manifested in remarkable changes of the line width of the fine structure transitions. Uniaxial pressure in the MPa range induces nearly symmetrical distribution of distortions of the coordination polyhedra of the Cr3+ centres accompanied by reorientation effects.

Thermooptical and thermophysical characteristics of forsterite crystals

An experimental investigation was made of the thermooptical and thermophysical characteristics of Mg2SiO4:Cr laser crystals. The thermal refraction coefficient was small (∂n/∂T = 2.8 × 10 − 6 K − 1) and the quantum efficiency of the luminescence was fairly high (η1 = 0.7). The familiar laser calorimetric method was modified to determine the various contributions to the efficiency of optical heating η of the investigated crystals.

Structure and Properties of Hot‐Pressed Clay Mineral/Acrifavine Complex Disk

Saponite Na(a‐b)+[(Mg6‐bAlb) (Si8‐aAla)O20(OH)4]−, a kind of clay mineral, was intercalated with acriflavine to form their complex (SAC). The resulting SAC powder was hot‐pressed into small disks (II mm in diameter, ∼10 mm long) under ∼20 MPa and temperatures up to 1300°C. Saponite powder (Sap) and saponite/carbon black mixture (SKM) reference samples were also hot‐pressed according to the same procedure. H2, CO, and CO2 gases evolved more abundantly, and at lower temperature, from the SAC than from the two reference samples. Fine carbon particles dispersed homogeneously throughout the SAC after the sample was heated. Enstatite and forsterite crystals formed in the SAC at higher temperatures and grew less favorably than in the reference samples. The electrical resistivity of the SAC disk sharply decreased with increased heat treatment temperature. At 1300°C, the SAC disk with only 3 wt% of carbon content exhibited a resistivity of 2 × 10−1Ω/cm, about 1/20 that of the SKM with a 5.5 wt% carbon after heating to 1300°C. The compressive strength of SAC after heating to 1300°C was near that of Sap and twice that of SKM.

Crystal Growth of Mg2SiO4 and MgSiO3 Single Crystals by the Flux Method

Single crystals of Mg2SiO4 and MgSiO3 have been synthesized by the evaporation‐aided slow‐cooling flux method. Acidic oxides such as V2O5, MoO3, and WO3 were used as the flux, and alkali metallic oxides or calcium oxides were added to the MoO3 and WO3 as flux modifiers. The starting mixture was first held at 1400°C and then slow cooled to 1000°C at a rate of 10 C/h. The temperature difference between the upper and lower limits of the melt was ∼25°C. Using an Li2O─MoO3 system flux with an Li2O:MoO3 ratio of 1:2 produces a polycrystalline clinoenstatite with a 9‐mm crystal size; a 1:1 ratio, however, results in forsterite crystals of 90‐μm size. The metallic oxides (Na2O and K2O) as flux modifiers produce forsterite with a crystal size of 65 and 40 μm, respectively. Similar results are observed when a Li2O─WO3 flux is used. Enstatite (10‐mm size) and forsterite (1.6‐mm size) are produced when the Li2O:WO3 ratios are 1:2 and 1:1, respectively. A V2O5 or CaO·WO3 flux produces no forsterite. The V2O5 flux results in tridymite as a primary phase, whereas calcium and magnesium tungstate were grown from a CaO·WO3 flux. The forsterite single crystals are tabular in form, generally bounded by the planes (001), (100), (010), (110), and (021), and are transparent. The enstatite consists of an aggregation of individual single crystals. The refractive indexes of the forsterite are α= 1.636 ± 0.001, β= 1.653 ± 0.001, and γ= 1.671 ± 0.001 (20°C).

A new approach to laser heating in high pressure mineral physics

Single crystals of olivine, forsterite, and enstatite were heated to over 2000 K between 100 and 250 kbar with a 120 W CO2‐laser in a diamond anvil cell using a hydrostatic, inert gas pressure medium. The conversion to some of their high pressure polymorphs, β‐phase, γ‐phase, perovskite, majorite, ilmenite, and clinoenstatite yielded single crystals large enough to obtain high quality Raman spectra, thus providing a quick, economic, and precise method to measure phase diagrams without pressure quenching.The phase boundaries between olivine or forsterite and β‐phase and between γ‐phase and perovskite were accurately bracketed. Our preliminary results on the latter transition are within the P‐T uncertainties of previous measurements using multi‐anvil devices [Ito and Takahashi, 1989], constraining the temperature at 670 km depth to 1900±100 K which is about 300 K higher than using their best estimate of the phase boundary.The unquenchable phase of MgSiO3‐clinopyroxene was synthesized at ∼170 kbar; Raman spectra show that this phase is of C2/c symmetry. Its stability field is between 50 and ∼165 kbar at ambient temperature.MgSiO3 perovskite was synthesized above 230 kbar and ∼2000 K. Raman spectra were measured from 75 to 470 kbar. Above 370 kbar, two modes disappear and two others change slope with pressure, suggesting a change in symmetry from Pbnm at room temperature.

Effect of gamma irradiation on the texture, acidity and catalytic activity of silica-aluminium and silica-magnesia catalysts

The textural properties of non-irradiated and γ-irradiated silica-aluminium and silica-magnesia catalysts were determined from nitrogen adsorption. The acidities of these catalysts were measured by the chemisorption of pyridine. Cracking of cumene and dehydration of isopropanol were investigated on non-irradiated and γ-irradiated catalysts. Irradiation with γ-rays enhanced sintering, i.e. decreased the surface area and increased pore size. For silica-magnesia, γ-irradiation enhanced crystallization of Forsterite and Enstatite. The surface acidity decreased upon γ-irradiation and consequently the activity of the catalysts towards cracking of cumene and dehydration of isopropanol was reduced. The former reaction is structure-sensitive, while the latter is structure-insensitive.

Condensation Experiments of Mg-Silicate Minerals

AbstractCondensation experiments were performed in the simple but most fundamental system Mg-Si-O-H with forsterite vaporization source. At temperatures above about 1000°C, euhedral crystals of forsterite (Mg2SiO4) of a few μm were formed. These crystals are similar to olivines in Allende matrix. At temperatures below about 1000°C, whiskers of forsterite and enstatite (MgSiO3) were formed by vapor-liquid-solid growth mechanism. These whiskers are different from enstatite whiskers in interplanetary dust, which were probably formed at small super coolings.

Chemical resistance of the moulding of saponite/acriflavine complex after carbonization

Powders of saponite/acriflavine complex (SAC) saponite/carbon black mixture (SKM) and saponite itself (Sap) were pressed into small discs (20 mm diameter × 3 mm), followed by heat treatment at 1073 to 1473 K under nitrogen or argon. Their chemical resistance to 10% NaOH, conc. HCl, conc. HNO3 and conc. H2SO4 solutions at room temperature was examined on the basis of weight loss. The resistance of the three samples to NaOH solution was found to increase with increasing heat-treatment temperature, and both SAC and Sap after heating to 1473 K exhibited only a slight weight loss even after soaking for 20 d. On the whole, SAC had the highest resistance among three. The three samples were more resistant to acid solutions than NaOH, the resistance of both SKM and Sap increasing with increasing heat-treatment temperature. The amorphous component and forsterite crystal in the samples after heating to high temperature were preferentially attacked by NaOH and acid solutions, respectively. These results are discussed on the basis of X-ray diffraction, SEM observation, carbon content, etc.

Aluminum partitioning between olivine and ultrabasic silicate liquid to 6 GPa

Trace element analyses of 1-atm and high-pressure experiments show that in komatiite and peridotite, the olivine (OL)/liquid (L) distribution coefficient for Al2O3 (\(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \)) increases with pressure and temperature. Olivine in equilibrium with liquid accepts as much as 0.2 wt% Al2O3 in solution at 6 GPa. Convergence to equilibrium compositions at this high level is shown by cation diffusion of Al into synthetic forsterite crystals of low-Al contents in the presence of melt. Convergence to low-Al equilibrium compositions at lower P and T is shown by diffusion of Al out of synthetic forsterite with high initial Al content. Isobaric and isothermal experimental data subsets reveal that temperature and pressure variations both have real effects on \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \). Variation in silicate melt composition has no detectable effect on \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \) within the limited range of experimentally investigated mixtures. Least-squares regression for 24 experiments, using komatiite and peridotite, performed at 1 atm to 6 GPa and 1300 to 1960°C, gives the best fit equation: \({\text{ln D}}_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} }^{{\text{OL}} - {\text{L}}} = {\text{0}}{\text{.082 }}P\left( {{\text{GPa}}} \right) - {\text{8290 1/}}T{\text{(K), }}\left[ {{\text{R = 0}}{\text{.98}}} \right]\) Increase in \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \) with increasingly higher-pressure melting is consistent with incorporation of a spinel-like component of low molar volume into olivine, although other substitutions possibly involving more complex coupling cannot be ruled out. High P-T ultrabasic melting residues, if pristine, may be recognized by the high \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \) calculated from microprobe analyses of Al2O3 concentrations in residual olivines and estimated Al2O3 concentration in the last liquid removed. In general the low levels of Al in natural olivine from mantle xenoliths suggest that pristine residues are rarely recovered.

Activation energy for crystal growth using isothermal and continuous heating processes

The kinetics of crystallization of plasma-sprayed amorphous cordierite and forsterite have been studied using both the isothermal Johnson-Mehl-Avrami and the continuous-heating Kissinger approaches. The activation energies for crystal growth obtained from the two processes are essentially equivalent: 1600±400 and 1600±300 kJ mol−1 for cordierite and 340±88 and 410±45 kJ mol−1 for forsterite, using isothermal and continuous heating, respectively. A correlation between the isothermal and non-isothermal crystallization processes is established.

Planar arrays of synthetic fluid inclusions in spontaneously nucleated forsterite crystals grown at constant pressure and temperature in a hydrostatic environment

Planar arrays of H 2O-CO 2-NaCl fluid inclusions have been observed in synthetically produced forsterite crystals which were nucleated and grown at constant P and T in a hydrostatic environment. Planar inclusion arrays are only observed in two runs performed with fluid compositions of 0.31 and 0.26 mol% CO 2 (moles CO 2/(total moles fluid)) and 25 and 40 wt% NaCl relative to H 2O, respectively. The origin of these inclusion arrays is probably related to the decoration of either 1. (1) irregular growth fronts formed during host growth or 2. (2) fractures resulting from stress buildup caused by lattice misfits formed during rapid host growth (as suggested by Roedder, 1984). The distinction between these two mechanisms determines whether these inclusions are of primary or pseudosecondary origin. Regardless of the mechanism, these experiments demonstrate that curved planar surfaces can be generated in a crystal growing under constant hydrostatic pressure-temperature conditions.

Transformation mechanism of olivine to modified spinel in Mg2SiO4 under stress.

Mechanism of the transformation of forsterite to modified spinel structure has been studied using multi-anvil apparatus. In order to examine the effect of stress on the transformation mechanism, forsterite single crystals were placed in two different media of powdered forsterite and NaCl, where the latter medium was found to provide one order of magnitude smaller differential stress in the sample. Optical and transmission electron microscope observations for the run products at 15.5GPa, 1000°C clearly show evidence which supports the in coherent nucleation and growth mechanism for the olivine-modified spinel transformation in these experimental conditions. The present results, combined with those of earlier works, suggest that the other possible mechanism, a martensitic transformation, may be realized only under substantially large differential stress of over 2GPa for the olivine-modified spinel transformation in Mg2SiO4.

Q−1 of forsterite single crystals

Internal friction ( Q −1) experiments have been performed on synthetic forsterite single crystals in order to investigate the role of temperature, frequency and dislocation density on the Q −1 factor. Data have been obtained for both deformed and undeformed specimens over a frequency range from 10 to 10 −4 Hz at 15, 1000, 1200 and 1400°C. Measurements were done using a torsional pendulum operating in forced oscillations on specimens with dimensions of 20 mm × 5 mm × 1 mm. Samples were maintained in vacuum. Stress amplitudes were 0.1 MPa and strains 10 −6-10 −5. Pre-deformation experiments were creep tests performed at 1600°C and 20 MPa. The amount of strain was 10 −2. Dislocation microstructures were investigated by transmission electron microscopy. At room temperature, undeformed samples show low attenuation with no frequency dependence. At high temperature (1400°C), Q −1 increases up to 5 × 10 −2 for very low frequencies (10 −4 Hz). Deformed specimens show the same trend but there is a more pronounced increase in Q −1 with both frequency and temperature ( Q −1 = 2 × 10 −1, 10 −4 Hz at 1400°C). This high-temperature attenuation is the result of dislocation processes which are probably non-linear in the strain range explored by the present experiments ( ϵ > 10 −6). Non-linearity appears to be the best explanation for the disagreement between high Q −1 data reported in laboratory and low Q 1 data reported for the upper mantle.

Igneous History of the Aubrite Parent Asteroid: Evidence from the Norton County Enstatite Achondrite

Abstract— We studied numerous specimens of the Norton County enstatite achondrite (aubrite) by optical microscopy, electron microprobe, and neutron‐activation analysis. Our main conclusions are the following: 1. Norton County is a fragmental impact breccia, consisting of a clastic matrix made mostly of crushed enstatite, into which are embedded a variety of mineral and lithic clasts of both igneous and impact melt origin. 2. The Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, not grains formed by condensation from the solar nebula. 3. The Mg‐silicate‐rich aubrite parent body experienced extensive melting and igneous differentiation, causing formation of diverse lithologies, some of which have not been described previously. These lithologies include dunites (represented by forsterite crystals), plutonic orthopyroxenites (represented by most enstatite crystals in the matrix), plutonic pyroxenites (the pyroxenitic clasts), and plagioclase‐silica rocks (like the feldspathic clasts). Presence of impact melt breccias (the microporphyritic clasts and the diopside‐plagioclase‐silica clast) of still different compositions further attest to the lithologic diversity of the aubrite parent body.

Vapor-phase recrystallization (Redeposition) processes in periclase-carbon products

We studied the mechanism of vapor-phase recrystallization of periclase and forsterite in the periclase-carbon products during rapid heating in air up to 1710–1720°. The formation of secondary crystals of periclase and forsterite occurs from the gaseous phase simultaneously with their crystallization in the form of filamentary and dendritic crystals in the developed cavity and with recrystallization of isometric periclase.

Single-crystal absorption and reflection infrared spectroscopy of forsterite and fayalite

Polarized single-crystal absorption and reflection spectra of fundamental modes in both the mid- and far-infrared are presented for microscopic crystals of forsterite and fayalite. All modes predicted by symmetry were observed for forsterite, but two B3u modes were not observed for fayalite. Consideration of previously determined frequency shifts for isotopically and chemically substituted olivines, along with symmetry analysis, produced a complete set of band assignments satisfying all constraints for forsterite. A plausible assingment was derived for fayalite by analogy. The frequency shifts from forsterite to fayalite are consistently small for bands assigned to SiO4 stretching and bending, moderate for rotations, and large for translations of M-site ions, suggesting that in olivine, SiO4 groups vibrate separately from the lattice. Allocating the bending and external modes among multiple continua in Kieffer's (1979c) model considerably improves prediction of quasiharmonic heat capacityCv and entropy for forsterite (∼1% discrepancy from 200–1000 K). The experimental entropy of fayalite is closely accounted for (1.8 to 0.1%) by summing lattice, electronic (from Burns' (1985) optical band assignment), and constant magnetic contributions above 200 K.Smagnetic determined from the difference of the experimental and model lattice entropies shows inflection points at the two magnetic transition temperatures (23 and 66 K) and indicates that complete spin disorder is not achieved below 680 K.

Creep Behavior of Single Crystals of Vanadium‐Doped Forsterite

Vanadium‐doped forsterite single crystals have been crept at atmospheric pressure, at temperatures ranging from 1571 to 1923 K under controlled oxygen partial pressure, po2. In the absence of an oxide buffer, the creep strength, the activation energy (550 ± 100 kJ/mol), the stress exponent (3.5 ± 0.5), and the po2 exponent (1/8 to 1/4) for this material are similar to those for unbuffered iron‐bearing olivine. At po2= 10‐11 MPa and T= 1773 K, the strength of vanadium‐doped forsterite is slightly larger than that of undoped forsterite.

High-temperature deformation of forsterite single crystals doped with vanadium

Creep experiments have been performed on samples from a single crystal of vanadium-doped forsterite under controlled\(p_{{\text{O}}_2 } \) conditions to investigate the effects of the addition of substitutional defects in the tetrahedral lattice sites. The addition of vanadium causes marked changes in the flow behavior of the forsterite, with a net increase in the creep rate at high\(p_{{\text{O}}_2 } \) and a new\(p_{{\text{O}}_2 } \) -dependent flow regime at low\(p_{{\text{O}}_2 } \) conditions. These observations can be interpreted as resulting from changes in the majority defect species that maintain the charge neutrality within the crystal. A climb-controlled dislocation creep model for the high-temperature deformation of vanadium-doped forsterite is proposed in which either (i) movement of uncharged jogs is rate-limited by the diffusion of silicon via a vacancy mechanism or (ii) movement of positively charged jogs is rate-limited by diffusion of oxygen via a vacancy mechanism.