Enstatite Content Research Articles

Innovative pathways to high-performance glass ceramics: Harnessing nature's treasures with chromium and zirconium nucleation catalysts

This study explores the novel production of high-performance glass ceramics (GCs) by harnessing readily available and cost-effective raw materials schist, dolomite, and magnesite for the first time. The nominal composition for the base glass, G(30), was designed based on the eutectic composition of the diopside-anorthite ratio with 30% enstatite content within the quaternary CaO-MgO-Al2O3-SiO2 system. To enhance nucleation, chromium (Cr2O3) and zirconium (ZrO2) oxide were separately introduced into the base glass batch in two ratios (0.5% and 1%), producing four additional batches-Cr(0.5), Cr(1), Zr(0.5), and Zr(1), orderly. All these batches were melted at temperatures ranging from 1450 °C to 1550 °C then cast into glass discs and rods. Double-stage heat treatment schedule, 750 °C /2 h followed by 950 °C and 1050 °C/1 h separately, was employed to induce the transformation of prepared glass into GCs. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques were applied to orderly investigate the developed crystalline phases and the prevailing micro-structures of the prepared GCs. The glass ceramic with the highest Cr2O3 ratio, Cy(1) which was treated at 1050 °C/1 h, displayed the most adequate microstructure and minimal pore formation in comparison with other GCs. As well, the favorable physicochemical properties, Vickers microhardness (6.43 GPa), low thermal expansion coefficient (0.171 ×10 −6 °C−1) at 29–100 °C, and the exceptional resistance for acid and alkali corrosion (95%), nominate such GS sample, Cy(1), for several architectural applications.

Preparation and characterization of enstatite-leucite glass-ceramics for dental restoration

For fabrication of novel glass-ceramic materials for use as dental restoration, seven glass compositions of different calculated percentages of enstatite and leucite, ranging from 10% to 70% enstatite, at 10% intervals, were prepared. Batches were melted in electric furnace at 1600°C-1670°C for 3-4 hours, casted, annealed, then subjected to heat treatments. Phase analysis using x-ray diffraction and microstructural characterization via scanning electron microscope were obtained. In addition, density, Vickers microhardness, flexural strength, and chemical solubility measurement were performed according to dental ceramics standards.Phase analysis revealed the characteristic peaks of leucite, forsterite, enstatite, and quartz that were confirmed through the microstructural investigations. Superior microhardness (9.081 GPa), flexural strength (122.075 MPa) and chemical durability (0.026% weight loss) were documented for 70% enstatite and 30% leucite glass-ceramic sample, suggesting its use in many dental applications. It was concluded that density, mechanical properties and chemical resistance increase with increasing enstatite content.

Cryptic mineral variation and PGE distribution in the Merensky Reef and its discontinuous mela-gabbronoritic hanging wall (“Brown Sugar Norite” - BSN) at Two Rivers Platinum Mine, Eastern Bushveld Complex, South Africa

Two Rivers Platinum Mine, situated on the southern sector of the eastern limb of the Bushveld Complex, extracts Platinum Group Elements (PGE) from the UG-2 chromitite layer. A trial operation of the Merensky Reef (MR) exposed unusual occurrences of mela-gabbronoritic lenses, ‘brown sugar norite’ (BSN in mine terminology), within the pyroxenite of the hanging wall. The BSN is a fine-grained mela-gabbronorite, which appears to only occur where the upper chromitite stringer of the Merensky Reef unit is present, and forms laterally inconsistent, flat lenses. Textural features, mineral chemistry of the various rock types, Sr analyses of plagioclase separates and whole rock geochemistry indicate multiple magma injections, fractionation and disequilibrium crystallization of the magma within the Merensky interval. Whole rock MgO and enstatite content of orthopyroxene are higher in BSN compared with the surrounding Merensky Reef pyroxenite, thus supporting the notion of the BSN having formed from a more primitive magma. Both the pyroxenite and BSN plagioclase have 87Sr/86Sr ratios representative of Critical Zone magma; that of the BSN has a lower initial 87Sr/86Sr isotope ratio relative to that of the pyroxenite, suggesting a more primitive magma source. It is suggested that the unusual, discontinuous BSN lenses within pyroxenite may have formed by magmatic erosion of a new magma injection, which affected the crystal mush of the earlier deposited BSN layer. This resulted in isolated lenses of relict, but more primitive orthopyroxene of the BSN in a residual plagioclase-rich melt from an earlier pyroxenitic MR magma. In both rock types (MR pyroxenite and BSN), the PGE occur as discrete platinum-group minerals (PGM), and in solid solution in sulphides. At Two Rivers Platinum Mine, the presence of PGE in sulphides has been identified by EMPA and LA-ICP-MS. The PGE occur mineralogically either in solid solution or as PGM inclusions of different size. Pentlandite, chalcopyrite, pyrrhotite and rare pyrite occur disseminated as individual grains with associated PGM throughout the Merensky pyroxenite interval, but more enriched near the bottom and top chromitite stringers. PGM encountered are moncheite, platarsite and irarsite, relatively enriched in Pt, but poor in Pd and Rh. Our results show that most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE. Textural features of the encountered PGM grains suggest their redistribution through late stage magmatic (probably hydrothermal) processes. The BSN contains relatively little if any base metal sulphides and PGM, indicating its formation from magma depleted in PGE.

High-Temperature Elastic Properties of Ceramics in the System MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Measured by Impulse Excitation

Youngs moduli of talc-based ceramics from the system MgO-Al2O3-SiO2 are measured for temperatures up to 1000 °C via impulse excitation. It is shown that, after pressing at 50 MPa and firing at 1280 °C, MgO-rich compositions exhibit higher porosity and lower Youngs moduli (approximately 2030 % lower than predicted via micromechanical relations). The Young moduli of materials with less MgO decrease with temperature, but those of MgO-rich ceramics increase with temperature and exhibit a large hysteresis between heating and cooling. Lower absolute values are mainly due to increased porosity, but the reason for the modulus increase with temperature and the hysteresis is the higher enstatite content in the MgO-rich compositions. For a special composition the Youngs moduli are more or less temperature-independent and without significant hysteresis effects, probably due to the low content of enstatite and the high content of sapphirine.

Mixing of crystal mushes and melts in the genesis of the Bogács Ignimbrite suite, northern Hungary: An integrated geochemical investigation of mineral phases and glasses

The 16.4Ma old Bogács Ignimbrite, located south of the Bükk Mountains, northern Hungary, was formed during a silicic ignimbrite flare-up in the Pannonian Basin that occurred from 20Ma to 13Ma. It comprises two main units, a lower, variably welded pumiceous and an upper, scoriaceous pyroclastic flow unit. Bulk chemistry of the juvenile clasts indicates a gradual change of geochemical character with an upward decreasing SiO2 content through the stratigraphic section. A detailed in-situ major and trace element investigation of the main mineral phases and glasses combined with petrogenetic model calculations reveals complex magma reservoir processes. Based on the major and trace element variability, six juvenile clast types were distinguished and each contain fresh glass fractions with distinct compositions. The mineral assemblage consists of plagioclase, orthopyroxene, biotite with minor and variable amounts of quartz, amphibole, ilmenite, zircon and allanite. The anorthite content of the plagioclases varies from 20 to 90mol%, whilst the Enstatite content of orthopyroxenes covers also a wide range from 40 to 90mol%. This large compositional variation can be detected even in single crystals. This extreme geochemical variability can be explained by mixing of crystal mush bodies evolved from both basaltic and more silicic magmas. The calcic plagioclases (An=80–90mol%) and magnesian orthopyroxenes (En=70–90mol%) clearly indicate the role of primitive mafic magmas in the growth of the silicic magma reservoir, even though no basaltic volcanic activity was associated with the Miocene silicic volcanism in the Pannonian basin. The prolonged crystallization in the mushy sills resulted in compositionally different residual melt fractions that moved upwards and accumulated in separated melt pods at the roof of the magma reservoir. Intermittent intrusions of mafic and intermediate magmas into this silicic magma system could have resulted in thorough stirring of the crystal mush bodies and the melt pods, leading to eruptive products having compositionally heterogeneous glass and mineral assemblage.

A condensation model for the formation of chondrules in enstatite chondrites

Abstract— It is proposed that the chondrules in enstatite chondrites formed near the Sun from rain‐like supercooled liquid silicate droplets and condensed Fe‐Ni alloys in thermodynamic equilibrium with a slowly cooling nebula. FeO formed and dissolved in the droplets in an initial stage when the nucleation of iron was blocked, and was later mostly reduced to unalloyed Fe. At high temperatures, the silicate droplets contained high concentrations of the less volatile components CaO and Al2O3. At somewhat lower temperatures the equilibrium MgO content of the droplets was relatively high. As cooling progressed, some droplets gravitated toward the Sun, and moved in other directions, depleting the region in CaO, Al2O3, and MgO and accounting for the relatively low observed CaO/SiO2, Al2O3/SiO2, and MgO/SiO2 ratios in enstatite chondrites. At approximately 1400 K, the remaining supercooled silicate droplets crystallized to form MgSiO3 (enstatite) with small amounts of olivine and a high‐SiO2 liquid phase which became the mesostases. The high enstatite content is the result of the supercooled chondrules crystallizing at a relatively low temperature and relatively high total pressure. Finally, FeS formed at temperatures below 680 K by reaction of the condensed Fe with H2S. All calculations were performed with the evaluated optimized thermodynamic databases of the FactSage thermodynamic computer system. The thermodynamic properties of compounds and solutions in these databases were optimized completely independently of any meteoritic data. Agreement of the model with observed bulk and phase compositions of enstatite chondrules is very good and is generally within experimental error limits for all components and phases.

Fe and Mg local environment in the synthetic enstatite-ferrosilite join: an experimental and theoretical XANES and XRD study

Synthetic orthopyroxenes on the join enstatite-ferrosilite (Mg 2 Si 2 O 6 -Fe 2 Si 2 O 6 ) have been studied by XANES spectroscopy with the aim to interpret the variation found in the spectra due to the Mg-Fe substitution in the orthopyroxene structure. The experimental Fe K -edge and Mg K -edge spectra show variation of the peak intensities as a function of the chemical composition along the enstatite-ferrosilite (En-Fs) join. At the Fe K -edge, the intensity ratio between the two most intense peaks varies linearly with the enstatite content (En mole %). Theoretical spectra have been calculated both at the Fe and Mg K -edge to interpret the experimental data. The spectra, obtained by locating Fe (or Mg) in M1 and M2 sites and weighting these contributions according to occupancy data obtained by Rietveld structure refinement, reproduce very well the experimental spectra, and allow to discriminate the contributions to the total spectrum due to Fe (Mg) located in each site.

Fabrication of dense forsterite-enstatite polycrystals for experimental studies

A practical and reliable method for the preparation of fine-grained forsterite and forsterite–enstatite polycrystals, in the system MgO–SiO2, is described. The method employs state-of-the-art techniques from materials science, namely sol-gel synthesis, powder processing and sintering. The resulting dense polycrystalline materials are homogeneously fine-grained (1–2 μm) and uniformly foam-textured. Moreover, they are highly reproducible in composition and microstructure. The enstatite content can be controlled to within ∼1%, in the range 0–50%, and densities up to 98% of theoretical density can be achieved. The method thus yields polycrystalline forsterite–enstatite material suitable for a range of geochemical, petrological and geophysical studies.

Effect of varying enstatite content on the deformation behavior of fine‐grained synthetic peridotite under wet conditions

The effect of varying enstatite content on the deformation behavior of synthetic, fine‐grained (1 to 2 μm) forsterite‐enstatite rock with ∼0.5 wt% added water was investigated at temperatures of 900° to 1000°C, strain rates between 10−7 and 10−5 s−1, and a confining pressure of ∼600 MPa. The samples exhibited approached steady state flow at stresses ≤60 MPa. The results show that, at constant strain rate, increasing enstatite content is associated with a sharp decrease in flow strength in the range 0–2.5% vol% enstatite, with little further change up to 20 vol%. The observed power law n value of ∼1.7 and microstructures are similar to those obtained in previous work on material with 2.5 wt% enstatite and are consistent with a water‐enhanced grain boundary sliding (GBS) dominated deformation mechanism. Significantly, a negative correlation was found between grain size and enstatite content, indicating that enstatite content played a role in controlling the grain size of the starting materials. Moreover, a high correlation between measured flow strength and grain size was found, consistent with a grain size exponent of −3 in a conventional grain size sensitive flow equation. A water‐enhanced deformation mechanism involving GBS accommodated probably by grain boundary diffusion and/or dislocation activity is therefore implied, with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control. Other effects of second‐phase content, such as weakening caused by interphase boundary diffusion and/or migration processes, seem to be unimportant under the conditions investigated.

Evolution of the Lower Critical Zone, central sector, eastern Bushveld Complex, and its chromite deposits

The Lower Critical Zone of the Eastern Bushveld Complex is a layered sequence of bronzitite layers interrupted by layers of chromitite and chromitic bronzitite and by two units in which olivine is an abundant cumulus mineral. Layering involves variations in both number and proportions of cumulus minerals and in mineral compositions.Bulk mineralogical and chemical compositions of successive units of the zone, as determined from modal analyses and electron microprobe analyses, indicate that the pile is the product of fractional crystallization, but the sequence of rock units and the upward changes in the enstatite content of the bronzite indicate a major reversal of normal fractionation, culminating in the appearance of the olivine-bearing units in the middle part of the zone. Of the various possible explanations of the reversal, resorption of olivine sinking from the top of the magma chamber (Macdonald, 1967) and slow decrease in total pressure (Osborn, 1980) appear most attractive, but slow influx and mixing of new parental magma cannot be excluded.The layering of the Lower Critical Zone reflects a system in which the composition of the liquid phase of the magma was on or close to the bronzite-chromite boundary on the liquidus during accumulation of most of the zone but was on or close to the bronzite-olivine boundary, the olivine-chromite boundary, or the bronzite-olivine-chromite triple junction at various stages of the development of the two olivine-bearing units. Slight changes in T, P, or X could therefore result in changes in the phases precipitating. It is suggested that changes in total pressure, causing shifts in phase boundaries on the liquidus, were the prime factor controlling the changes in mineral assemblages which are represented in the layering of the zone. One result was the formation of the remarkably regular and persistent chromitite layers for which the Lower Critical Zone is justly famous.Structural relations, variations in the enstatite content of bronzite, and the bulk mineralogical and chemical compositions of the Lower Critical Zone as a whole and its lowest unit in particular indicate that the zone continues the fractionation represented in the Lower Zone of the Olifants River trough and is not the product of a separate, later magma.

A lunar rock of deep crustal origin: sample 76535

Lunar sample 76535 is a coarse-grained troctolitic granulite exhibiting a texture indicative of long annealing times. It is composed of homogeneous crystals of plagioclase (58 per cent, An 96), olivine (37 per cent, Fo 88) and bronzite (4 per cent, En 86). Chromian spinel-bronzite-diopside (Wo 46En 50Fs 4) symplectic intergrowths commonly occur along olivine-plagioclase boundaries and as tiny inclusions within olivine grains. These symplectites apparently formed by a reaction of the type: OI + An + Chromite → Opx + Cpx + Al-Mg-chromite . The reaction is related to the experimentally determined reaction OI + An = Opx + Cpx + Sp of Kushiro and Yoder (1966). The enstatite content of the diopside coexisting with the bronzite indicates equilibration at about 1000°C. Thermodynamic calculations for 1000°C indicate that the symplectites formed at a minimum pressure of about 0.6 kb. Low alumina contents of the pyroxenes indicate equilibration near this minimum pressure. Clusters of the same assemblage found in the symplectic intergrowths, but containing accessory metal, troilite, Ca-phosphates, baddeleyite, plagioclase and/or K-feldspar occur sporadically throughout the rock. These apparent late stage products crystallized in the low temperature-high pressure region discussed above. Phase relations of co-existing metal phases indicate that the rock cooled at a few tens of degrees/my, corresponding to depths of 10–20 km below the lunar surface, in agreement with the above pressure estimate. We infer that 76535 represents an original cumulate deposited at a depth between about 10 and 30 km. The last liquid crystallized in the relatively high pressure-low temperature field opx + cpx + Al- Mg- chromite. Cooling was extremely slow and accompanied by extensive chemical and textural re-equilibration. Reaction to form the symplectites occurred during the late stages of re-equilibration.

Ultramafic and mafic protoclastic intrusions in the Dahomeyan gneiss of Ghana

SummaryCharacteristics of mineralogy and texture, together with systematic changes in the enstatite content of orthopyroxene and in the anorthite content of plagioclase, indicate that intrusions of pyroxenite, norite and gabbro, and diorite and leucocratic diorite in the Dahomeyan gneiss of Ghana are genetically related, and that they belong to a subalkaline comagmatic series derived from a parental mafic magma. Partial recrystallization (deuteric) of the igneous rocks during and after intrusion and protoclasis produced minerals characteristic of temperatures higher than those that existed during the medium-grade regional metamorphism of the Dahomeyan gneiss, thus indicating that emplacement of the igneous rocks occurred late during, or perhaps after, the metamorphism. Contact metamorphism appears to have resulted in higher than normal anorthite contents of the plagioclase in the surrounding Dahomeyan gneiss.