Corundum Crystals Research Articles

Charged oxygen interstitials in corundum: first principles simulations

AbstractCombining supercell models and hybrid B3PW exchange‐correlation functionals, ab initio simulations on quasi‐stable configurations of interstitial $O_i^-$ ions in α‐Al2O3 (corundum) crystals and possible migration trajectories have been modelled. We have studied crystalline distortion around migrating $O_i^-$ including interatomic distances and the effective atomic charges, as well as redistributions of the electronic density. Unlike neutral interstitial atom Oi studied by us previously, migrating $O_i^-$ ion does not form dumbbells with the nearest regular $O_{reg}^{q -}$ oxygen ions, due to the strong Coulomb interaction with the nearest $Al_{reg}^{q+}$ cations as well as stronger repulsion between $O_i^-$ and adjacent regular $O_{reg}^{q -}$ ions. We have also estimated the energy barrier for migration between the nearest quasistable configurations of interstitial $O_i^-$ ion. One of these configurations is an octahedron formed by six nearest $O_{reg}^{q -} $ anions in the centre of which $O_i^-$ ion is located, unlike a neutral Oi atom which prefers to migrate directly towards one of adjacent $O_{reg}^{q -} $ anions with the dumbbell formation. As the result, the barrier for $O_i^-$ ion migration in corundum has been found to be by ∼70% smaller than that of Oi atom. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation of Glass-Ceramics by Reactive Crystallization with Waste Glass and Mullite

Diopside glass-ceramics were prepared by reactive crystallization with mixed powder of waste glass and mullite. The effects of mullite content and sintering temperature and holding time on the crystal phase and morphology of diopside glass-ceramics were investigated. The reactive crystallizing behavior between waste glass powder and mullite was studied using energy dispersive spectrdmeter during the sintering and holding process. The result show that the optimum preparation conditions were as follows: the addition of 30 wt% mullite used as crystallization promoting agent, sintering temperature of 900 °C and holding time of 2h. The main difference between reactive crystallization method and the traditional method of preparing diopside glass-ceramics was the reactive crystallization mechanism. The diopside and corundum crystals were formed via an interaction between glass powder and mullite instead of direct precipitation from the parent glass during the calcining and holding process. With the increasing of sintering temperature and holding time, the Ca and Mg element concents of the glass were increased, on the contrary, that Si element concent was decreased in the mullite. It shows that the Si element diffused from mullite to glass and precipitated diopside crystals.

Surface termination structure of α-Ga2O3 film grown by mist chemical vapor deposition

The surface structure of α-Ga2O3(0001) grown on an α-Al2O3(0001) substrate by mist chemical vapor deposition was studied by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). The minimum step height observed in the AFM image was 0.21 ± 0.01 nm, coinciding with the height of three atomic layers of α-Ga2O3(0001). It was revealed by CAICISS analysis that the surface of α-Ga2O3(0001) is terminated by a Ga layer followed by an O layer, which is consistent with the surface termination of α-Al2O3(0001). A structural model taking surface relaxation into account was also constructed by fitting the simulated curve for the azimuth angle dependence of the Ga intensity to the experimental dependence. The resultant structural model is similar to the model of an α-Al2O3(0001) surface, which indicates analogous behavior in corundum crystals.

Archean Xenocrysts in Modern Volcanic Rocks from Kamchatka: Insight into the Basement and Paleodrainage

AbstractWe report U-Pb zircon ages and in situ δ18O values for crystals of zircon, rutile, and corundum erupted in the course of a March 2009 phreatomagmatic explosion of Koryaksky volcano, Eastern Volcanic Front, Kamchatka, Russia. Zircon crystals display a wide spectrum of crystallization ages ranging from Cenozoic to Archean, including single grains and grain-age clusters, ca. 1.7, 11, 37–56, 89–99, 152, 1461, 1946, 2584–2734, and 3314 Ma. The older end of this spectrum represents the oldest known zircon ages from Kamchatka. Oxygen isotopic values span from normal to low δ18O in zircon (3.3‰ to 5.5‰), corundum (4.3‰ to 5.5‰), and rutile (2‰ to −2.8‰). As Koryaksky volcano is built over Cretaceous-Eocene and younger crust, the presence of older zircon crystals requires that these accessory minerals with their diverse ages and δ18O values were derived from heterogeneous detrital components in sediment that contaminated the magma shortly before or during the phreatomagmatic eruption. We also present a com...

Features of OSL and TL properties of anion-defective corundum crystals exposed to thermo-optical treatment

The results of systematic investigation of the mechanisms and role of phototransfer in TL and OSL processes in α-Al2O3−δ crystals under controlled filling of the main and deep traps depleted at 450 and 720 K, respectively, are presented.Optical depletion spectra of the main and deep traps were measured and compared. It was found that green light stimulation only depletes the main trap, while blue light depletes both the main and deep traps. Optical depletion of the deep trap results in phototransfer of charges that can be realized by 2 mechanisms: either through the main trap to recombination centers or directly to the latter. However, the dominant contribution to the OSL signal is provided by phototransfer through the main trap. It is also found that UV emission at 3.7 eV dominates over F-emission at 3.0 eV in the phototransferred TL spectrum. It is assumed that this UV emission is not related to the F+ centers.

鲁西昌乐新生代碱性玄武岩中刚玉/蓝宝石的矿物学特征及其成因探讨

本文利用电子探针、LA-ICP-MS等现代测试方法系统的分析山东昌乐刚玉/蓝宝石矿物的元素分布特征，探讨了新生代碱性玄武岩中蓝宝石的成因。昌乐刚玉/蓝宝石矿物中总体杂质含量较低，总含量为1.73%~0.21%，不同样品元素含量具有差异，Fe元素是昌乐蓝宝石中含量最高的杂质元素，也明显高于其他产地的玄武岩型蓝宝石。昌乐蓝宝石中Ga元素含量均高于100 ppm，表明昌乐刚玉显示岩浆型成因，裂纹较发育的刚玉叠加了明显的后期熔体渗透交代作用。通过刚玉边部共生的碱性长石初步估算，刚玉形成的条件应接近或大于1000˚C/23 km。 On the basis of EPMA and LA-ICP-MS modern analysis methods, we systemically investigate the che- mical characteristics of the Changle corundum/sapphire minerals, and explore the origin of the Changle corundum/sapphire in Cenozoic alkali basalts. The general content of impurity in the Chan-gle corundum/sapphire is low, which is 1.73%~0.21% in total, and is variable in different samples. Among the impurity elements in Changle samples, the concentration of Fe is the highest, and it’s significantly higher than the basalt type sapphires from other origin places. Ga content is higher than 100 ppm in the Changle corundum/sapphire, showing a magma type in origin. The fracture developed corundum presents a significant late melt infiltration and alteration. By the associated alkali feldspar within the margin of corundum crystal, preliminary estimate indicates that the formation of corundum/sapphire conditions should be close to or more than 1000˚C/23 km.

Mineralogical characteristics and the gem quality of corundum in Khe tre area, Eakar, Daklak

Corundum in Khe Tre area essentially distributes in pegmatite bodies that intruded the granite of Ea Dui complex ((P-Ted) and metamorphic rocks of Ea Rock formation (PR2er). Corundum mainly developed within pegmatite bodies and in contact metasomatic endozone between pegmatite and granite or gneiss. Mineralogical composition of pegmatite is as follows; Major minerals: alkali feldspar (orthoclas, microlin) 60 -70 %, quartz ~ 15 %; Minor minerals: plagioclase, biotite, muscovite Accessory minerals: garnet, zircon, sphene, apatite,...The corundum here was found in intergrowth with feldspar and characterized by dark to dark blue (very rare) hue, opaque diaphaneity, subhedral to anheral crystals and few mineral inclusions. The size of corundum crystals varies from about 2 cm to 5 cm. Rough corundum crystals are rarely of gem value. Most of them can be used only as abrasibve material. For improvement of corundum to be used as gemstones, heat treatment method was proposed.

Induced formation of corundum crystals in supercritical water fluid

The formation of fine-crystalline corundum (α-Al2O3) from hydrargillite in supercritical water fluid (SCWF) at 400°C and 26.8 MPa in the presence of corundum particles was studied. It was found that the α-Al2O3 particles added to the reaction medium accelerated the formation of fine-crystalline corundum due to an increase in the rate of nucleation. In this case, both the buildup of added corundum grains, and the generation of new corundum crystals from boehmite were observed. The formation of new crystals depended on distance between the grains of the additive. Depending on this distance, the newly formed crystals consisted of two or three fractions with different average particle sizes. It was concluded that the buildup of the fused corundum particles in SCWF occurred due to the uptake of corundum nuclei formed in a surrounding layer of boehmite. For the added particles of fused corundum with an average size of 2.82 µm, the thickness of a layer from which the buildup occurred was 17.47 µm. The corundum nuclei formed at large distances from the growing particles became the centers of growth for new corundum crystals of the smallest sizes. The role of SCWF consisted in increasing the spatial and structural mobility of the reactants.

Ab initio simulations on migration paths of interstitial oxygen in corundum

Ionizing radiation produces in Al2O3 (corundum) crystals primary Frenkel pairs of complementary defects (in oxygen sublattice these are oxygen vacancies and interstitial oxygen ions, VO−Oi). The interstitial Oi atoms begin to migrate above certain temperature and create the dumbbell pairs with regular oxygen atoms (Oreg−Oi). We have calculated the optimal dumbbell configurations and optimized further migration paths (i.e., Oi interstitial can break the bond with one Oreg atom and moves towards another, one of four next-neighbor Oreg atoms). To simulate all possible Oi migration trajectories, we have performed large-scale hybrid DFT-LCAO PBE0 calculations on 2×2×1 supercells of defective α-Al2O3 crystals using CRYSTAL14 computer code. The limiting barrier height for oxygen interstitial 3D migration is estimated as 1.3eV.

Corrosion Behavior of Al2O3‐C Refractories with Casting Mould Meniscus Slags at 1 550 °C

The degradation behavior of alumina (10–20 wt%) carbon refractories was investigated during high temperature (1 550 °C) contact with a ternary casting mould meniscus slag containing 50% MnO, 25% SiO2, and 25% Al2O3 and a binary slag containing 40% MnO and 60% SiO2. Using the sessile drop approach, the transient progress of corrosion reactions was investigated as a function of time and refractory composition. An in‐depth characterization and analysis of reacted assemblies was carried out using optical and scanning electron microscopy (SEM). Optical micrographs showed a significant penetration of both slags within the refractory substrate, with very little slag remaining behind after 15 min of contact. The depth of penetration of ternary slag was found to increase with carbon concentration in the refractory. An all‐round corrosion was observed with extensive refractory degradation over a period of time. Corrosion was determined to a great extent by dissolution reactions in the interfacial region; alumina dissolution from the refractory resulted in the formation of needle‐like corundum crystallites which transformed to a cohesive network of faceted corundum crystals on extended contact. A number of reaction products containing Mn and Si were also observed.

Ab initio simulations on Frenkel pairs of radiation defects in corundum

Large scale first principles periodic calculations based on the density functional theory within the localized atomic orbital approach (DFT-LCAO) using the hybrid exchange- correlation potential B3PW have been performed in order to study the structural and electronic properties of radiation-induced Frenkel pairs Oi+VO in corundum crystal. As an initial approach, we have used conventional 2x2x1 supercell for defective α-Al2O3 lattice containing 120 atoms. After relaxation of the ideal supercell structure, the optimized doi-vo distance has been found to be ~4.5 Å while the formation energy of Frenkel pair has achieved 11.7 eV. The interstitial Oi atom, both single and a component of Oi+VO pair, spontaneously forms a dumbbell with the adjacent atom in the regular oxygen sublattice (doi-o = 1.404 Å) with the induced charge -1.1 e. On the whole, possibilities of supercell model for proper description of Frenkel pairs with changing inter-defect distance and space orientation inside corundum crystal are rather limited. This is why the alternative cluster model must be developed for this aim.

Microstructure and Thermal Shock Behaviour of Alumina Composite Ceramic for Heat Transmission Pipeline

Al2O3 composite ceramics materials used for heat transmission pipeline were prepared by pressureless sintering in air. Calcined bauxite, talc and commercial alumina powder were used as the main raw materials, partially stabilized zirconia was also added to improve the thermal shock resistance of the samples. The effects of composition and sintering temperature on density, phase composition, microstructure and thermal shock behavior of samples were evaluated. It shows that the B2 sample which was sintered at 1400 °C for 2 h has optimal properties with porosity 0.4%, bulk density 3.2 g•cm-3.The bending strength increases 19.46 MPa after 30 times thermal shock cycles from 1100 °C to room temperature. The main phases of B2 are corundum and spinel, meanwhile, a small amount of monoclinic zirconia and α-quartz are also detected according to XRD pattern. Microstructure analysis reveals that spinel crystals are interlocked by prismatic corundum crystals, bright white beaded monoclinic zirconia particles are distributed uniformly, and it is beneficial to improve the thermal shock resistance of sample.

Role of Magnesia as an Additive on Sintering and Micro-Structural Characteristics of Indian Bauxite

Raw Indian Bauxite from Saurashtra region, Gujarat is selected for experimental studies to produce refractory material after sintering with addition of chemical grade magnesia powder of variable fractions (4wt%, 8wt%, 12wt %) as sintering aid for 4hours after mixing in rubber lined pot. Powder X-Ray diffraction study confirmed the formation of spinel (magnesio- aluminate phase) along with corundum as major crystalline phase. Physical properties like bulk density, apparent porosity is measured for the prepared samples. The maximum bulk density (3.57gm/cc) is obtained with 4wt% MgO containing batch with 0.53% of apparent porosity fired at 1500oC. Maximum shrinkage (29.59%) is attained at 1550oC with no additive batch. The elongated corundum crystals and rectangular spinel phase is observed in the thermally etched polished surface of the magnesia containing bauxite batch.

Origin of gem corundum in calcite marble: The Revelstoke occurrence in the Canadian Cordillera of British Columbia

The calcite marble-hosted gem corundum (ruby, sapphire) occurrence near Revelstoke, British Columbia, Canada, occurs in the Monashee Complex of the Omineca Belt of the Canadian Cordillera . Corundum occurs in thin, folded and stretched layers with green muscovite + Ba-bearing K-feldspar + anorthite (An 0.85–1 ) ± phlogopite ± Na-poor scapolite . Other silicate layers within the marble are composed of: (1) diopside + tremolite ± quartz and (2) garnet (Alm 0.7–0.5 Grs 0.2–0.4 ) + Na-rich scapolite + diopside + tremolite + Na,K-amphiboles. Non-silicate layers in the marble are either magnetite- or graphite-bearing. Predominantly pink (locally red or purple) opaque to transparent corundum crystals have elevated Cr 2 O 3 (≤ 0.21 wt.%) and variable amounts of TiO 2 ; rare blue rims on the corundum crystals contain higher amounts of TiO 2 (≤ 0.53 wt.%) and Fe 2 O 3 (≤ 0.07 wt.%). The associated micas have elevated Cr, V, Ti, and Ba contents. Petrography of the silicate layers show that corundum formed from muscovite at the peak of metamorphism (~ 650–700 °C at 8.5–9 kbar). Because the marble is almost pure calcite (dolomite is very rare), the corundum was preserved because it did not react with dolomite to spinel + calcite during decompression. The scapolite-bearing assemblages formed during or after decompression of the rock at ~ 650 °C and 4–6 kbar. Gem-quality corundum crystals formed especially on borders of the mica-feldspar layers in an assemblage with calcite. Whole rock geochemistry data show that the corundum-bearing silicate (mica-feldspar) layers formed by mechanical mixing of carbonate with the host gneiss protolith ; the bulk composition of the silicate layers was modified by Si and Fe depletion during prograde metamorphism . High element mobility is supported by the homogenization of δ 18 O and δ 13 C values in carbonates and silicates for the marble and silicate layers. The silicate layers and the gneiss contain elevated contents of Cr and V due to the volcanoclastic component of their protolith. • First detailed characterization of prograde gem corundum formation in calcite marble • Corundum present in mica-feldspar layers in calcite marble • Common protolith of Cr,V-bearing marble assemblages and host rock • Isotope homogenization between the host rock and the marble • Corundum formed by prograde muscovite breakdown reactions

Morphology and crystal structures of solar and presolar Al2O3 in unequilibrated ordinary chondrites

Corundum, the thermodynamically stable phase of alumina (Al2O3), is one of the most refractory dust species to condense around evolved stars. Presolar alumina in primitive chondrites has survived various kinds of processing in circumstellar environments, the interstellar medium (ISM), the Sun’s parent molecular cloud, and the protosolar disk. The morphology and crystal structure of presolar alumina grains may reflect their formation and evolution processes, but the relative importance of these two types of processes is poorly understood. In this study, we performed detailed morphological observations of 185 alumina grains extracted from unequilibrated ordinary chondrites (Semarkona, Bishunpur, and RC075). We also performed electron back-scattered diffraction analyses of 122 grains and oxygen isotopic analyses of 107 grains. Dissolution experiments on corundum and transition alumina phases were carried out to examine the possibility of the alteration of surface structures of alumina grains by the chemical separation procedures of chondrites.The average size of the alumina grains was 1μm, and neither whiskers nor extremely flat grains were observed. About one-third of the grains had smooth surfaces, while ∼60% of the grains had rough surfaces with 10–100nm-sized fine structures. The rough-surface grains have varieties of morphology and crystallinity, suggesting that the rough surface structures are secondary in origin. Electron back-scattered diffraction patterns from 95% of alumina grains matched with α-Al2O3 (corundum), and more than 75% of the alumina grains are single crystals of corundum. Nine presolar alumina grains with anomalous oxygen isotopic compositions were found among 107 alumina grains, and most of them were characterized by rough surface structures. While most of the presolar alumina grains were corundum, the relative abundance of amorphous or low-crystallinity grains is higher in presolar alumina grains than in solar alumina grains. The dissolution experiments showed that all phases except for corundum dissolved during the acid treatments of chondrites. This suggests that smooth surface structures of corundum grains were originally formed in space, and that original surfaces of alumina that had been damaged by energetic particle irradiation in the ISM or the protosolar disk were lost during chemical separations to form the rough surface structures, and that amorphous or low-crystallinity alumina grains in chondrites have acid-resistant structures different from sol–gel-synthesized amorphous alumina. The present results also imply the possible presence of acid-soluble alumina phases, undiscovered by chemical separations, in chondrites.

Characteristics Analysis of Plane Processing of Sapphire Corundum Crystals

In order to solve the problems of plane processing of the sapphire corundum crystals, an orthogonal test design was put forward to investigate the key factors that influencing the quality of grinding and polishing, and the optimal plane processing parameters were obtained by visual analysis. The samples of the sapphire corundum crystals were processed using the optimal plane processing parameters, and the surface roughness was also detected using Mahr roughness instrument, the test results show that the plane quality is improved dramatically.

新潟県魚沼層群 SK030 火山灰層中のジルコン双晶：形態，累帯構造，内部構造

Twin crystals of zircon in SK030 volcanic ash layer in Uonuma Formation, which is Cenozoic strata in Niigata Prefecture, central Japan, were examined mineralogically. Zircon in the SK030 layer was along with monazite, xenotime and corundum crystals. Major twin types were contact twin and penetration twin. Based on the twin forms, the former is subdivided into Fantype I and II, and the latter into Wedgetype I and II. Cathod luminescence (CL) images of the zircon twins showed growth layer. In zircon crystals with the penetration twin, complex growth patterns were observed. Base do n high resolution transmission electron micorscopic images and reciprocal lattice analysis of the corresponding ED patterns of the twin structure, the twin plane was determined as {112} plane.

Modeling of the elastic electronic polarization of l-24 cordierite ceramic

A procedure for modeling the elastic electronic polarization of a specimen of L-24 cordierite ceramic is examined. The results of simulation modeling of the dielectric frequency spectra of oxide crystals of corundum and quartz, which are components of L-24 ceramic, and modeling of a commercial sample of this ceramic are presented.

Shock Induced Reaction of Ni/Al Nanopowder Mixture

Nanopowder Ni/Al mixture (mixed in Al:Ni = 2:1 stoichiometry) was shock compressed by employing single and two-stage light gas gun. The particle size of Al and Ni are 100-200 nm and 50-70 nm respectively, morphologies of Al and Ni are sphere like either. Recovered product was characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. According to the XRD spectrum, the mixed powder undergo complete reaction under shock compression, reaction product consist of Ni2Al3, NiAl and corundum structure Al2O3 compound. Grain size of Ni-Al compound is less than 100 nm. With the shock pressure increasing, the ratio of Ni2Al3 decreased obviously. The corundum crystal size is 400-500 nm according to the SEM observation. The results of shock recovery experiments and analysis show that the threshold pressure for reaction of nano size powder Ni/Al mixture is much less than that of micro size powder.

Experimental constraints on permeable gas transport in crystalline silicic magmas

The gas and fluid transport in magmas via permeable flow through interconnected bubble networks controls the rate of outgassing from magmas ascending in volcanic conduits and the fluid transport in the mushy boundary layer of magma reservoirs. Hence, clarifying its mechanism and rate is crucial to understanding the explosivity of volcanic eruptions and the evolution and dynamics of a magma reservoir. Recent experimental studies have determined the gas permeabilities in crystal-free rhyolite and basalt. However, no experimental study has investigated the effect of the crystal contents on the permeable gas transport in magmas. In this study, we performed decompression experiments for hydrous rhyolitic melts having crystallinities of 30 and 50 vol% to examine the effect of crystals on the bubble microstructure and gas permeability during magma vesiculation. Size-controlled (100-meshed) corundum crystals were used as an analog of the phenocrysts in silicic magmas. Microstructural analyses using X-ray CT showed that bubbles coalesce and their connectivity increases with a decrease in the final pressure after the decompression, that is, an increase in the vesicularity. As long as the vesicularities of melt part in the crystal-free basis (melt vesicularity) were similar, no clear effect of the crystallinity on the degree of bubble coalescence and connectivity was observed at melt vesicularities 68 vol%). This result indicates that the permeable fluid transport through a deep volcanic conduit, which has been proposed on the basis of the observations of volcanic gases and natural products, is so slow that other processes, like shear deformation or magma convection, may be needed to explain the observations.