Diopside Crystals Research Articles

Compositional effects on the growth of diopside crystals in the simulated high‐level waste glass

Compositional effects on the growth of diopside crystals in the simulated high‐level waste glass

Are Clay Minerals Systematically the Products of Aqueous Alteration in Cosmic Bodies?

The formation of chondrite materials represents one of the earliest mineralogical processes in the solar system. Phyllosilicates are encountered at various stages of the chondrule formation, from the initial stages (IDP agglomerates) to the final steps (chondrule internal alteration). While typically linked to aqueous alteration, recent studies reveal that phyllosilicates could precipitate directly from residual fluids in post-magmatic or deuteric conditions and under a wide range of temperatures, pressures, water/rock ratios, and H2/H2O ratio conditions. This study re-examined the formation of hydrated phyllosilicates in chondrules and associated fine-grained rims (FGRs) using published petrographical, mineralogical, and chemical data on carbonaceous chondrites. Given that chondrules originate from the melting of interplanetary dust particles, the water liberated by the devolatilization of primary phyllosilicates, including clay minerals or ice melting, reduces the melting temperature and leads to water dissolution into the silicate melt. Anhydrous minerals (e.g., olivine and diopside) form first, while volatile and incompatible components are concentrated in the residual liquid, diffusing into the matrix and forming less porous FGRs. Serpentine and cronstedtite are the products of thermal metamorphic-like mineral reactions. The mesostasis in some lobated chondrules is composed of anhydrous and hydrous minerals, i.e., diopside and serpentine. The latter is probably not the alteration product of a glassy precursor but rather a symplectite component (concomitant crystallization of diopside and serpentine). If so, the symplectite has been formed at the end of the cooling process (eutectic-like petrographical features). Water trapped inside chondrule porosity can lead to the local replacement of olivine by serpentine without external water input (auto-alteration). In the absence of water, hydrated phyllosilicates do not crystallize, forming a different mineral assemblage.

Glass-ceramics prepared from desulfurized electrolytic manganese residue and the nucleation promotion effect of calcium fluoride in glass phase transition

Electrolytic manganese residue (EMR) is toxic industrial solid waste. Electrolytic manganese enterprises have established a desulfurization and sulfuric acid production process to address its storage risks leaving desulfurized EMR (DEMR). In this study, DEMR was used as a raw material to prepare glass-ceramics. A method of melting and one-stage heat treatment was used to realize controlled crystallization of diopside, with calcium fluoride sludge (FS) as a nucleating agent. The impact of CaF2 on the glass network structure, as well as on the microstructure and performance of the glass-ceramics, are discussed. CaF2 can greatly improve the nucleation ability of glass precursors. When the addition of FS is small, the phase transition of glass is dominated by the diffusion of atoms and growth of diopside, forming dendritic crystals. When introducing more FS, the glass-ceramics form smaller island-shaped crystals due to many nucleation sites. The prepared glass-ceramics show excellent mechanical properties and chemical stability. Its hardness exceeds 6.5 GPa, with bending strength over 110 MPa. The acid resistance of glass-ceramics is higher than 99.5%, and the toxic metals in the raw materials are stable solidified. This study perfects the technology of EMR desulfurization pretreatment, which provides an effective graded recycling approach for EMR.

Effect of TiO2 on the occurrence and distribution of chromium in stainless‐steel slag glass ceramics

AbstractStainless steel slag waste can be used to prepare value‐added glass ceramics, which can fix potentially toxic Cr from the slag within the crystalline phase. The occurrence and distribution of Cr during the preparation of glass ceramics has a great influence on the final Cr fixation effect. In this study, the effects of the TiO2 content on the occurrence and distribution of Cr during the nucleation and crystallization steps and on the final properties of the glass ceramics were systematically studied. In the nucleation stage, with increasing TiO2 content, the Cr distributed in the spinel containing chromium nuclei first increases and then decreases. In the crystallization stage, Diopside crystal phase nucleates and grows with spinel containing chromium nanocrystals as heterogeneous nuclei. X‐ray photoelectron spectroscopy analysis showed that the chromium distributed in the diopside crystals first increased and then slightly decreased as the TiO2 content increased. The optimal TiO2 content is 3.4 wt.%, which resulted in 97 wt.% of the total Cr being fixed in the diopside crystalline phase (with a very low Cr leaching concentration of 0.009 mg/L), and a high compressive strength of the final glass ceramic of 267.4 MPa, and a Vickers hardness of 1211.8 HV. The research results provide theoretical and technical support for strengthening Cr fixation to enable harmless and high‐value utilization of stainless steel slag for fabricating glass ceramics.

Volcano–Plutonic Complex of the Tumrok Range (Eastern Kamchatka): An Example of the Ural-Alaskan Type Intrusion and Related Volcanic Series

Zoned plutons, composed of dunites, pyroxenites, and gabbroic rocks, have been referred to as the Ural-Alaskan type complexes (UA-complexes) and occur in numerous paleo-arc settings worldwide. Many of these complexes are source rocks for economic placers of platinum-group metals. Thus, it is important to understand how UA-complexes form and the origin and behavior of platinum-group elements (PGEs). It is widely assumed that the UA-complexes result from differentiation of supra-subduction high-Ca high-Mg sub-alkaline magmas. However, there is a lack of direct evidence for the existence and differentiation of such magmas, mainly because cases of UA-complexes being spatially and temporally linked to co-genetic volcanics are unknown. We studied an UA-complex from the Tumrok range (Eastern Kamchatka) where a dunite-clinopyroxenite-gabbro assemblage is spatially and temporary related to high-Ca volcanics (i.e., picrites and basalts). Based on the mineral and chemical composition of the rocks, mineral chemistry, and composition of melt inclusions hosted within rock-forming minerals, we conclude that the intrusive assemblage and the volcanics are co-genetic and share the same parental magma of ankaramitic composition. Furthermore, the compositions of the plutonic rocks are typical of UA-complexes worldwide. Finally, the rocks studied exhibit a full differentiation sequence from olivine-only liquidus in picrites and dunites to eutectic crystallization of diopside or hornblende, plagioclase, and K-Na feldspar in plagio-wehrlites and gabbroic rocks. All these results make the considered volcano–plutonic complex a promising case for petrological studies and modelling of UA-complex formation.

Effect of Al2O3 on migration and distribution of chromium and chromium fixation effect in stainless-steel slag glass-ceramics

Stainless-steel slag was used to prepare glass-ceramics, which could solidify chromium (Cr) in the slag. The migration and distribution state of chromium in the slag during the preparation of the glass-ceramics have a great influence on the chromium fixation ability of the glass-ceramics. In this paper, the effects of Al2O3 content in the glass-ceramics on the migration and distribution of chromium during the nucleation and crystallization steps, and on the chromium fixation effect of the glass-ceramics were systematically studied. The results show that in the nucleation stage, with the increase of Al2O3 content, the number of chromium spinel (Cr-spinel) nanocrystals formed in the glass first increases and then decreases, and Cr in the glass phase gradually migrates to Cr-spinel nanocrystals. TEM and XPS analyses show that during crystallization, part of Cr in Cr-spinel diffuses into diopside lattice, and the other part of Cr still exists in Cr-spinel wrapped by diopside. Cr in the glass phase also diffuses and migrates into the diopside lattice with the formation of diopside crystals. The optimal Al2O3 content is 11.8wt.%, which results in 97.94wt.% of the total Cr being fixed in the diopside crystalline phase. The ability of chromium fixation is very high giving a very low Cr leaching concentration of 0.004mg/l. The research results provide theoretical and technical support for increased chromium fixation and realizing harmless and high-value utilization of stainless-steel slag.

Synthesis of schorl doped iron ore tailings glass-ceramics with superior performance

The diopside-based glass-ceramics were prepared by using iron ore tailings as main raw materials and schorl as additive. The addition of schorl was found to effectively reduce the crystallization activation energy and to promote the crystallization of diopside. The diopside and schorl were evenly distributed and sufficiently interlocked together, which improved the flexural strength and bulk density of glass-ceramics. The far-infrared emissivity of glass-ceramics increased with the addition of schorl due to intrinsic far-infrared characteristics of schorl, while the replacement of Mg and Si ions in diopside by Fe ions also contributed to the enhancing far-infrared emissivity due to lattice distortion and symmetry reduction of diopside crystal. At 10 wt.% schorl addition, the glass-ceramics was at its best, with flexural strength of 107.03 MPa, bulk density of 2.66 g/cm3, and far-infrared emissivity of 0.928. This work lays foundation to manufacture of high value glass-ceramics with superior properties utilizing waste iron ore tailings and schorl.

Ryabchikovite, CuMg(Si2O6), a new pyroxene group mineral, and some genetic features of natural anhydrous copper silicates

Abstract Ryabchikovite, ideally CuMg(Si2O6), a new pyroxene-group mineral (IMA No. 2021-011) was discovered in exhalations of the active Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. The associated minerals are diopside, hematite, cuprospinel, fluorophlogopite, anhydrite, johillerite, tilasite, and aphthitalite-group sulfates. Ryabchikovite forms thin (up to 25 μm), light brown to reddish-brown epitactic crusts on short-prismatic brownish-gray crystals of diopside (up to 0.5 mm). The new mineral is optically biaxial (+), α = 1.685(5), β = 1.690(5), γ = 1.703(4), and 2V (meas) = 60(15)°. The average chemical composition (wt%, electron microprobe data) is: MgO 18.05, CaO 0.77, CuO 26.46, ZnO 2.23, Al2O3 0.93, Fe2O3 1.89, SiO2 50.10, total 100.43. The empirical formula calculated based on 6 O atoms per formulas unit is (Mg1.05Cu0.78Zn0.06Fe0.063+Ca0.03)(Si1.96Al0.04O6). Electron backscattered diffraction and powder X-ray diffraction show that ryabchikovite is a Cu,Mg-ordered analog of clinoenstatite. Ryabchikovite adopts the space group P21/с and has the following unit-cell parameters: a = 9.731(9), b = 8.929(8), c = 5.221(4) Å, β = 110.00(6)°, V = 426.3(7) Å3, and Z = 4. Ryabchikovite is named in honor of the outstanding Russian geochemist and petrologist Igor Dmitrievich Ryabchikov (1937–2017). Our studies reveal that copper analogs of rock-forming minerals could be found in fumarolic systems. Their crystallization does not require high temperatures or/and pressures (below 500 °C/Pa).

Reconstruction of magma chamber processes preserved in olivine-phlogopite micro-ijolites from the Oldoinyo Lengai, Tanzania

A detailed petrographic and mineralogical investigation of olivine-phlogopite micro-ijolite xenoliths from Oldoinyo Lengai, Tanzania indicates a complex evolutional history. These xenoliths consist of diverse textural subdomains characterized by minerals ranging from early-formed olivine, through diopside-hosted perovskite and phlogopite, to evolved aegirine-augite and titanite. Thermometry and mineral compositions in the subdomains suggest crystallization temperatures from 1070–970 °C to 850–700 °C at plutonic pressures and SiO2-activities controlled by perovskite-titanite equilibria. Double coronas are a characteristic textural feature of the olivine-phlogopite micro-ijolite, consisting of olivine cores surrounded by an inner clinopyroxene corona and an outer phlogopite corona. These double coronas might have formed during early magma chamber processes, including magma movement to a subsequent chamber resulting in dissolution of olivine with subsequent crystallization and accumulation of diopside and phlogopite. Diopside−aegirine-augite compositional zonation indicates several magma injections followed by cooling periods, during the formation of micro-ijolite groundmass. Mg# (80–83) and Ca (0.1–0.3 in wt%) contents of olivine together with the presence of primary melt inclusions in clinopyroxene, phlogopite, and nepheline indicate a magmatic origin from a possible parental olivine-nephelinite melt. There is evidence for subsolidus, or near-solidus, re-equilibration processes as indicated by the reaction of olivine with titanite forming symplectitic textures of ilmenite and diopside with minor zirconolite. Ti-exchange between phlogopite phenocrysts and other Ti-bearing minerals (perovskite, titanite, magnetite) resulted in ∼750 °C equilibrium temperatures for phlogopite, which are much lower than mafic magmatic (>900 °C) conditions. Calculated subsolidus temperatures suggest crystallization of olivine-phlogopite micro-ijolites over a 10–20 km depth interval.

Kinetic controls on Sc distribution in diopside and geochemical behavior of Sc in magmatic systems

Scandium is important in modern technology and is regarded as a strategic metal in many countries. It is highly dispersed in Earth’s crust and rarely forms independent minerals. Clinopyroxene is the most important Sc-bearing mineral in some world-class deposits hosted in mafic–ultramafic intrusions, which are also the major source of laterite-hosted Sc deposits. However, the factors controlling Sc distribution in minerals have been little explored, impeding the understanding of the geochemical behavior of Sc and why it is common in some clinopyroxene grains. The newly discovered Mouding Sc deposit in SW China is hosted in a zoned intrusion composed, from core to rim, of monzogabbro, syenogabbro, gabbro, magnetite clinopyroxenite, and clinopyroxenite. Clinopyroxene in the intrusion is diopsidic in composition with high Sc contents (80–105 ppm). In-situ trace element mappings of diopside crystals reveal homogeneous, zoned, swallow-tailed, and hourglass internal Sc distribution patterns. These patterns can be produced through kinetically controlled incorporation of Sc on different crystal faces. The preferential substitution of Sc can take place on the {1 0 0}, {1 1 0} and {0 1 0} prism faces because of the high flexibility of the octahedral M1 protosites. The fast growth of diopside, which facilitates kinetically controlled crystallization, is dominated by textural coarsening and promoted by the hydrous parental magmas with low viscosities and active convection. The active flow and efficient interstitial communication of the magma can direct compatible elements from the magma into clinopyroxene, thus favoring formation of Sc-rich grains. Our study provides a feasible way to study intra-grain variations of Sc in minerals and emphasizes that kinetic effects may play a critical role in Sc distribution and enrichment in hydrous magmatic Sc deposits. We also show that disequilibrium crystallization may be more pervasive than previously thought, and the hourglass zoning of clinopyroxene can provide valuable information on this process.

Origin and evolution of nephrites, diopsidites and giant diopside crystals from the contact zones of the Pounamu Ultramafics, Westland, New Zealand

ABSTRACT Diopside-rich rocks (diopsidites) are interlaminated with nephrite in boulders derived from metasomatic contacts developed between Pounamu Ultramafic meta-serpentinite and country rock Alpine Schist, Westland, New Zealand. Petrographic textures indicate that parental tremolite rock, formed by metasomatic diffusion during metamorphism, has been intensely deformed and recrystallised to alternating semi-nephrite and nephrite domains during development of a secondary crenulation cleavage. Nephrites are subsequently sequentially overprinted by porphyroblastic tremolite, diopside, then further tremolite. Crystallisation is controlled by fluctuating activities of SiO2, CaO and H2O in associated fluids. Pervasive dissolution of nephritic tremolite and crystallisation of diopside generates diopsidites containing accessory epidote, uvarovite and zincian chromite formed in equilibrium with H2O-rich fluids. Diopsidites are in turn overgrown by coarse grained (in places > 50 cm long) diopside crystals, interpreted to have infilled an extension fracture that formed during ongoing uplift of the Southern Alps.

Multiple Rodingitization Stages in Alkaline, Tholeiitic, and Calc-Alkaline Basaltic Dikes Intruding Exhumed Serpentinized Tethyan Mantle from Evia Island, Greece

Abstract In Pagondas and Kimi areas, central Evia Island, Greece, variably serpentinized ultramafic rocks of an ophiolitic origin are crosscut by rodingite dikes. Based on their protoliths, these rodingites are distinguished into the following: (i) rodingites of island arc tholeiitic dolerites, (ii) rodingites of Mg-rich tholeiitic gabbros, (iii) rodingites of alkaline basalts, and (iv) rodingites of calc-alkaline basalts. Rodingitization evolved in three successive phases (namely, stages 1, 2, and 3) of increasing metasomatic degree that occurred during the exhumation of the mantle wedge serpentinites. Thermodynamic modeling suggests that stage 1 was developed at ~5 kbar and 450-550°C, whereas at stage 2 and stage 3, gradually decreasing P-T conditions prevailed. Stage 1 affected the protoliths of all rodingite types and was characterized by the crystallization of grossular, diopside, chlorite, and prehnite leading to Ca and LOI enrichments. The observed Si and REE depletions are assigned to the dissolution of primary clinopyroxene and amphibole. Stage 2 mainly affected the rodingites that were derived from alkaline and calc-alkaline basalts, promoting the growth of andradite, vesuvianite, and chlorite at the expense of stage 1 rodingitization mineral phases. At this stage, the marginal zones of rodingite dikes from alkaline and calc-alkaline basalts experienced remarkable Mg and REE enrichments. Stage 3 corresponds to derodingitization and restricted carbonation processes of the previously formed rodingites, which are associated with Fe enrichments. During this stage, significant trace elements including REEs were transferred from the marginal zones to the inner cores of rodingite dikes that were derived from metasomatism of calc-alkaline basalts. Calcite δ18Ο-δ13C isotopic data suggest that carbonation processes were developed at shallow depths below the Tethyan ocean floor, through mixing of seawater and serpentinization-derived fluids. Such multiple stages of rodingitization processes that affected four different lithotypes, as well as the unusual case of alkaline basalts’ rodingitization, would render the Evia rodingites as an important and very rare example of metasomatism.

Dissolution - reprecipitation reactions as a mechanism for magma contamination: An example from interaction of partially melted sanidine megacrysts and clinopyroxene phenocrysts in nephelinite from Graulei, West Eifel Volcanic Field, Germany

Sanidine – diopside – aegirine-augite enclaves in a nephelinite from the West Eifel Volcanic Field in Germany provide new insights into the process of xenolith assimilation. The enclaves originated as sanidine megacrysts which were entrained by the rising nephelinite magma and partly melted at ~1190 °C and 0.5 GPa. The partly melted sanidine captured clinopyroxene phenocrysts (Type 1 cpx) from the nephelinite. These captured phenocrysts underwent dissolution and reprecipitation to form sieve-textured to euhedral near end member diopside crystals (Type 3 cpx) that grew epitaxially on the original phenocryst. The components of the dissolved phenocryst that were not taken up by the diopside were enriched in the potassic melt that formed the enclave, and during quenching formed aegirine-augite crystals (Type 4 cpx) that locally overgrow the Type 3 diopside. Only enclaves > ~ 5 mm preserve the diopside – aegirine-augite – sanidine assemblage. Small Type 2 clinopyroxene-and leucite-rich clusters in the nephelinite are the remnants of assimilation of mm-sized sanidine crystals. The mechanism of phenocryst capture by molten enclaves shows that the assimilation process is more complex than simple dispersal of melt and crystals from disaggregated xenoliths and that crystal transfer from the magma to partly molten xenoliths may play a significant role in the process.

Oriented surface nucleation in diopside glass

Oriented surface crystallization on polished diopside glass surfaces has been studied with scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and laser scanning microscopy. An orientation preference of [001] parallel to the glass surface was detected for separately growing diopside crystals even as small as 700 nm in size. This finding shows that crystal orientation occurs in the outermost surface layer without crystal-crystal interaction and indicates that the crystal orientation is a result of oriented nucleation. Depending on surface preparation, monomodal crystal orientation distributions with [100] perpendicular to the surface or bimodal distributions with [100] and [010] perpendicular to the glass surface were detected. It was also shown that the degree of crystal orientation increases with decreasing surface roughness. The observed orientation of diopside crystals could be explained in terms of the interfacial energies of different crystal faces.

Carbonation of calcium-magnesium pyroxenes: Physical-chemical controls and effects of reaction-driven fracturing

The weathering of primary silicates, and their carbonation in particular, is key for the geochemical cycling of elements, strongly affecting the C cycle and the long-term regulation of the Earth's climate. The knowledge on the controlling factors and mechanisms of aqueous carbonation of primary silicates is however still far from complete. This precludes a better understanding of their chemical weathering in nature and is a strong handicap to implement effective Carbon Capture and Storage (CCS) strategies. Here, dissolution-carbonation reactions of two abundant Ca-Mg pyroxenes, augite and diopside, have been investigated in experiments conducted at hydrothermal conditions, in the presence/absence of different carbonate sources (NaHCO3 and Na2CO3). We show that the main reaction products are low-magnesium calcite and amorphous silica. A higher conversion of augite (∼38 wt%) than diopside (∼15 wt%) was achieved. The presence of abundant Fe and Al (and minor Na) in the former pyroxene strongly enhances the release of cations to the solution, and contributes to the formation of abundant secondary crystalline and amorphous silicates as well as carbonates (Na-phillipsite and magnesium silicate hydrate, MSH, as well as calcite). In particular, Na-phillipsite nucleates in etch pits exerting a crystallization pressure ∼100 MPa exceeding the tensile strength of augite (i.e, failure under tension due to breaking of inter-atomic bonds as well as opening and propagation of subcritical fractures or flaws) and causing extensive fracturing. This takes place via an interface-coupled dissolution-precipitation mechanism, despite the bulk system being undersaturated with respect to this phase. Limited reaction-induced fracturing was also observed following MSH precipitation within diopside crystals. Reaction-induced fracturing increases exposed reactive surface area and creates channels for solution flow, thereby contributing to the progress of the reaction via a positive feedback loop. Ultimately, our results help to understand differences in the kinetics and mechanisms of chemical weathering of these two abundant rock forming inosilicates relevant for CCS strategies, showing that secondary phase formation (other than carbonates) are fostered by moderately alkaline pH and the presence of alkali metals, resulting in reaction-driven fracturing that enables the progress of silicate carbonation for an effective, safe, and permanent CO2 mineral storage. We also show that under our experimental conditions the precipitation of amorphous silica and calcite cannot generate sufficient pressure as to create fracturing, an effect that limits carbonation of Mg-Ca-Fe pyroxenes.

Physico-mechanical properties and microstructure of multi-phase ceramic composites based on zircon and dolomite mixtures

Four composites containing zircon and dolomite were designed by adding dolomite from 5wt% to 35wt% at the expense of zircon content. Densification parameters in terms of bulk density, apparent porosity and linear change were determined at different firing temperatures (1200°C–1400°C). Cold crushing strength of sintered composites, phase composition and microstructure were investigated. Samples contain 35wt% of dolomite and fired at 1200°C for 2 hours exhibited higher porosity (AP ∼ 51.25%) than other samples and can be used as porous ceramics. This is due to CO2 evaporation through the thermal decomposition of dolomite. Dense ceramics can be obtained by adding 5wt% of dolomite and fired at 1400°C for 2 hours (bulk density ∼3.67 g/cm3 and apparent porosity ∼4.5%). Only zirconia and diopside crystalline phases were detected at composite containing 35wt% of dolomite and fired at 1400°C. Due to the liquid phase sintering process, the densification parameters of the sintered samples have been enhanced by increasing the temperature. The mechanical properties of the sintered samples were improved due to the transformation toughening mechanism of tetragonal zirconia. Microstructure and EDAX analysis of the sintered composites show the presence of sub-prismatic zircon and rounded zirconia crystals as well as irregularly dark diopside crystals.

Mineralogická charakteristika granátického skarnu z lokality Trohanka pri Prakovciach (gemerikum, Slovenská republika)

Garnet skarn mineralization was recently studied at the Trohanka locality near Prakovce (Gemeric Unit, Eastern Slovakia). Ca-skarn forms lenticular bodies in green schist environment. It mainly consists of zonal garnets, pyroxenes, amphiboles and magnetite accumulations. Studied garnets are rich in andradite component (up to 89.95 mol. %) with minor grossular component (6.83 - 39.67 mol. %). Strong oscillatory zoning in andradite is caused by substitution of Fe3+ and Al3+. Most pyroxenes are rich in the hedenbergite component. In some cases, euhedral diopside crystals with marginal transition zones (composed of diopside with lower content of Mg2+ and higher content of Fe2+) were found in hedenbergite matrix. Amphiboles are dominantly represented by ferro-actinolite and ferro-hornblende in association with isolated euhedral crystals of ferro-tschermakite and ferro-pargasite. Indistinct chemical zonality of amphibole euhedral crystals is caused by presence of ferro-pargasite in the central parts and ferro-tschermakite in the peripheral parts of crystals.

Carbonate melts in the hydrous upper mantle

Carbonatite compositions resulting from melting of magnesian calcite + olivine + clinopyroxene were experimentally determined in the system CaO–MgO–SiO2–CO2–H2O as a function of temperature and bulk H2O contents at 1.0 and 1.5 GPa. The melting reaction and melt compositions were found to be highly sensitive to H-loss or -gain during experiments. We hence designed a new hydrogen-trap technique, which provided sufficient control to obtain consistent results. The nominally dry solidus temperatures at 1.0 and 1.5 GPa are 1225–1250 °C and 1275–1300 °C, respectively. At 1.0 GPa, the solidus temperature decreases with H2O increasing to 3.5 wt% (1025–1050 °C), then remains approximately constant at higher H2O concentrations. Our nominally dry solidus temperatures are up to 140 °C higher than in previous studies that did not take measures to limit hydrogen infiltration and hence suffered from H2O formation in the capsule. The near-solidus anhydrous melts have 7–8 wt% SiO2 and molar Ca/(Ca + Mg) of 0.78–0.82 (XCa). Melting temperatures decrease by as much as 200 °C with increasing $${\text{X}}_{{{\text{H}}_{{2}} {\text{O}}}}$$ in the coexisting COH-fluid. Concomitantly, near-solidus melt compositions change with increasing bulk H2O from siliceous Ca-rich carbonate melts to Mg-rich silico-carbonatites with up to 27.8 wt% SiO2 and 0.55 XCa. The continuous compositional array of Ca–Mg–Si carbonatites demonstrates the efficient suppression of liquid immiscibility in the alkali-free system. Diopside crystallization was found to be sensitive to temperature and bulk water contents, limiting metasomatic transformation of carbonated upper mantle to wehrlite at 1.0–1.5 GPa to < 1175 °C and < 7 wt% bulk H2O.

Nucleation delay in the anorthite-diopside binary system: Models and experiments

Nucleation delay was calculated for crystallization of anorthite (CaAl2Si2O8) and diopside (CaMgSi2O6) from melt, and for crystallization in 3 melts in the anorthite-diopside system using classical nucleation theory and values for equilibrium and transport properties from the literature. Calculations were compared to experiments in the literature and new experiments performed on diopside. Only one value, the activation energy of transport for diopside melt, was adjusted to fit the experiments. The activation energies, interfacial energies, and size of the structural unit of growth used for the end-member compositions were applied to calculate nucleation delays for three mixtures inside the binary. The calculated and experimental nucleation delays were within ~10x of each other, substantiating the use of classical nucleation theory to calculate nucleation delays in this system. Differences between measurements and calculations are probably due to uncertainties in equilibrium and transport properties of the system, particularly the activation energy for transport.

Crystallization Behavior and Optical Properties of Glass-Ceramics Containing Nano-Diopside Crystals

Diopside is a ceramic material with excellent properties including a low dielectric constant, high thermal conductivity, low sintering temperature below 1000 °C, and high mechanical strength. It has been applied to wireless and optical communications, substrates for touch panels, lenses for UV-LED, building materials, and so on. In this study, glass-ceramics containing nano-sized diopside crystals were fabricated, and their transmittance at visible light and photoluminescence were evaluated. In particular, TiO2 was added as a nucleating agent to suppress the surface crystallization phenomenon and Mn was used as a dopant to emit red light. The glass-ceramics were prepared by heat treatment at a temperature lower than the maximum crystal growth temperature (TP) calculated from the non-isothermal analysis method using differential thermal analysis (DTA) for the formation of nano-sized crystals. For glass containing 20 wt% of TiO2, the Avrami constant was calculated to be 2.23 and the activation energy required for crystal growth to be 549 kJ/mol, reflecting typical bulk crystallization behavior. Glass-ceramics with high light transmittance up to 70% were obtained by inducing the bulk crystallization behavior, and the diopside crystal size was less than 10 nm, which was equal to or higher than that of commercialized transparent glass-ceramic products. Glass-ceramic specimens doped with Mn showed luminescence of 736∼766 nm wavelength at excitation light of 365 nm wavelength. The emission peak intensity increased with the amount of dopant added, but gradually decreased with increasing crystallinity of the diopside phase.