Showing papers on "Tridymite published in 2016"

Silicic volcanism on Mars evidenced by tridymite in high-SiO2 sedimentary rock at Gale crater

Abstract: Tridymite, a low-pressure, high-temperature (>870 °C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.% X-ray amorphous material and a bulk composition with ∼74 wt.% SiO2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (∼17 wt.% of bulk sample), tridymite (∼14 wt.%), sanidine (∼3 wt.%), cation-deficient magnetite (∼3 wt.%), cristobalite (∼2 wt.%), and anhydrite (∼1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (∼39 wt.% opal-A and/or high-SiO2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides−perchlorates−chlorates), and has minor TiO2 and Fe2O3T oxides (∼5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a “Lake Gale” catchment environment can account for Buckskin’s tridymite, cristobalite, feldspar, and any residual high-SiO2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.

Agricultural wastes as a resource of raw materials for developing low-dielectric glass-ceramics.

Abstract: Agricultural waste ashes are used as resource materials to synthesize new glass and glass-ceramics. The as-prepared materials are characterized using various techniques for their structural and dielectric properties to check their suitability in microelectronic applications. Sugarcane leaves ash exhibits higher content of alkali metal oxides than rice husk ash, which reduces the melting point of the components due to eutectic reactions. The addition of sugarcane leaves ash in rice husk ash promotes the glass formation. Additionally, it prevents the cristobalite phase formation. These materials are inherently porous, which is responsible for low dielectric permittivity i.e. 9 to 40. The presence of less ordered augite phase enhances the dielectric permittivity as compared to cristobalite and tridymite phases. The present glass-ceramics exhibit lower losses than similar materials synthesized using conventional minerals. The dielectric permittivity is independent to a wide range of temperature and frequency. The glass-ceramics developed with adequately devitrified phases can be used in microelectronic devices and other dielectric applications.

Characterization of Mason Gully (H5): The second recovered fall from the Desert Fireball Network

Abstract: Mason Gully, the second meteorite recovered using the Desert Fireball Network (DFN), is characterized using petrography, mineralogy, oxygen isotopes, bulk chemistry, and physical properties. Geochemical data are consistent with its classification as an H5 ordinary chondrite. Several properties distinguish it from most other H chondrites. Its 10.7% porosity is predominantly macroscopic, present as intergranular void spaces rather than microscopic cracks. Modal mineralogy (determined via PS-XRD, element mapping via energy dispersive spectroscopy [EDS], and X-ray tomography [for sulfide, metal, and porosity volume fractions]) consistently gives an unusually low olivine/orthopyroxene ratio (0.67−0.76 for Mason Gully versus ~1.3 for typical H5 ordinary chondrites). Widespread “silicate darkening” is observed. In addition, it contains a bright green crystalline object at the surface of the recovered stone (diameter ≈ 1.5 mm), which has a tridymite core with minor α-quartz and a rim of both low- and high-Ca pyroxene. The mineralogy allows the calculation of the temperatures and ƒ(O2) characterizing thermal metamorphism on the parent body using both the two-pyroxene and the olivine-chromite geo-oxybarometers. These indicate that MG experienced a peak metamorphic temperature of ~900 °C and had a similar ƒ(O2) to Kernouve (H6) that was buffered by the reaction between olivine, metal, and pyroxene. There is no evidence for shock, consistent with the observed porosity structure. Thus, while Mason Gully has some unique properties, its geochemistry indicates a similar thermal evolution to other H chondrites. The presence of tridymite, while rare, is seen in other OCs and likely exogenous; however, the green object itself may result from metamorphism.

Experimental Study of Slag/Matte/Metal/Tridymite Four Phase Equilibria and Minor Elements Distribution in “Cu-Fe-Si-S-O” System by Quantitative Microanalysis Techniques

Abstract: Laboratory studies have been carried out to determine the slag/matte/metal/tridymite four condensed-phase equilibria in the “Cu-Fe-Si-S-O” system and the minor element distributions between the equilibrated phases at 1200°C. A combined quantitative microanalysis technique including electron probe X-ray microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been developed to accurately characterize the equilibrated system for both major and minor elements. Analysis precision, estimated accuracy and current limitations are discussed. The resulting elemental distributions are plotted against matte grade and compared to FactSage predictions. It was found that the experimental results for major elements are in good agreement with FactSage predictions. The simultaneous distributions of Ag and Au between slag, matte and metal phases are reported for the first time.

Research in the Area of Preparing Materials Based on Fuzed Quartz HCBS. Part 10. Some Properties of Cristobalite-Containing Materials 1

Abstract: The effect of repeated nonisothermal heating to 1500°C on shrinkage and crystallization of quartz ceramic with added alkali (0.5% Na2O) and monolithic fuzed quartz is studied. Marked (up to 10%) tridymite formation is detected in specimens with added alkali and with a predominant cristobalite content. Some properties are provide for specimens after single-stage heating to 1500°C. Depending on additive content specimens have open porosity of 6 – 9% and ultimate strength in compression of 130 – 185 MPa.

Oxidation behavior of metallurgical silicon slag under non-isothermal and isothermal conditions

Abstract: Metallurgical silicon slag is the by-product of metallic silicon industry. The utilization of silicon slag is very limited due to the absence of related researches. In this work, the oxidation behavior of metallurgical silicon slag was studied by means of phase variation under the non-isothermal and isothermal conditions. The crystal structure and phase composition of silicon slags were investigated by X-ray powder diffraction, simultaneous thermal analysis, scanning electron microscopy and thermogravimetry. During the oxidation process, the CO2 emission was progressively enhanced with increasing the oxidation temperature, and the exothermic oxidation primarily occurred above 700 °C. The non-isothermal oxidation of silicon slag powder was a complicated process, involving the oxidation, crystal precipitation and disproportionation processes. The graphite phase was completely removed at 1000 °C. The SiC and cristobalite phases greatly increased at 750 °C and decreased with further increasing the oxidation temperature. For the isothermal oxidation, the Si and SiC phases generally reduced with increasing the soaking time. The cristobalite and tridymite phases increased with the oxidation time. The mass change in slag powders suggested that the isothermal oxidation process consisted of the initial redox reaction and subsequent oxidation reaction.

High-Temperature, Perhaps Silicic, Volcanism on Mars Evidenced by Tridymite Detection in High-SiO2 Sedimentary Rock at Gale Crater, Mars

Abstract: The Mars Science Laboratory (MSL) rover, Curiosity, has been exploring sedimentary rocks within Gale crater since landing in August, 2012. On the lower slopes of Aeolis Mons (a.k.a. Mount Sharp), drill powder was collected from a high-silica (74 wt% SiO2) outcrop named Buckskin (BK). It was a surprise to find that the Buckskin sample contained significant amounts of the relatively rare silica polymorph tridymite. We describe the setting of the Buckskin sample, the detection of tridymite by the MSL Chemistry and Mineralogy (CheMin) X-ray diffraction instrument, and detection implications. Geologic setting: The Buckskin outcrop is part of the Murray formation exposed in the Marias Pass area. The formation was previously studied by CheMin in the Pahrump Hills member [1] where three samples of drill fines were analyzed (Confidence Hills (CH), Mojave2 (MJ) and Telegraph Peak (TP) [2]). Assuming approximately horizontal bedding, the Buckskin outcrop is approx.15 m stratigraphically above the bottom of the Pahrump Hills member. Mudstone, generally characterized by fine lamination, is the dominant depositional facies [1]. Buckskin Mineralogical and Chemical Composition: The CheMin instrument and XRD pattern analysis procedures have been previously discussed [3-6]. The diffraction pattern used for quantitative XRD analysis (Fig. 1) is the sum of the first 4 of 45 diffraction images. The remaining images are all characterized by both on-ring and off-ring diffraction spots that we attributed to poor grain motion and particle clumping. Coincident with particle clumping was a significant decrease in the intensity of the tridymite diffraction peaks (Fig. 2a). The derived mineralogical composition of the crystalline component (derived from the first 4 diffraction images) is given in Table 1. The tridymite is well-crystalline and its pattern is refined as monoclinic tridymite (Fig 1). Mineral chemical compositions were derived from XRD unit cell parameters or obtained from stoichiometry. The XRD-calculated amorphous component was 50 +/- 15 wt%. We constrained the value to 60 wt% because it is the minimum value necessary to give a positive Al2O3 concentration for the amorphous component using APXS data for the post-sieve dump pile (Table 2). The amorphous component has high SiO2 (approx.77 wt%) and high anion (SO3+P2O5+Cl ~10 wt%) concentrations. Calculation shows that a cation-anion balance is achieved if the cations in the amorphous component except SiO2 and TiO2, which do not readily form salts, are assumed to be present as amorphous mixed-cation sulfates, phosphates, and chlorides (or perchlorates/ chlorates).

Gemological, physical and chemical properties of prase opals from Hanety Hill (Tanzania)

Abstract: Chemical, physical and gemological properties of some green “prase” opals from Hanety Hill in Central Tanzania were investigated. The color of the opals ranges from clear green to apple green, diaphaneity from translucent to semitransparent, luster is vitreous, and all result inert to UV lamp radiations. Specific gravity values are between 2.11-2.13, refraction indices between 1.439 and 1.458, comparable with literature data. XRD and FTIR analyses show the opals belong to the CT type, with tridymite higher than cristobalite. SEM observations revealed a lepispheric and mammillary structure formed by spherules with a diameter around 10-12 μm, on their turn composed of amorphous silica microspheres. The opals show a homogeneous chemical composition with very high contents of Ni and lower of Mg, Ca, Fe and transition elements like Zn, Cr and Co. Al and K are nearly absent. The chemical composition of the opals reflects that of serpentine rocks. Because of its abundance, the cromophore element responsible for the green color is Ni. Prase opals may have formed through the process of low temperature metamorphism, which generated the serpentinite from a magmatic ultrafemic rock.

Nanocomposites on the Base of Synthetic Opals and Nanocrystalline Phases of Bi-containing Active Dielectrics

Abstract: Nanocomposites based on bare synthetic opals soaked with the Bi12SiO20, Bi2TeO5 and NaBi(MO4)2 melts are characterised by employing scanning electron microscopy, Bragg light reflection and FTIR and Raman spectroscopy technique. Crystalline state of the substance inside opal pores is proved for all composites. The formation of bismuth orthosilicate and tridymite and its phases is proposed in ‘opal–Bi12SiO20’ and ‘opal–Bi2TeO5’ composites, respectively. Raman spectrum of ‘opal–NaBi(MoO4)2’ is not essentially changed compared to that of the single crystal, except for a shift of one Raman band and appearance of one weak new band. The reasons for spectral intensity redistribution revealed by micro-Raman surface scanning may be both the composition inhomogeneity and the structural light focusing together with concentration of the exciting radiation field nearby the surface and bulk defects.

Report on the natural occurrence of a silica-clay nanocomposite

Abstract: A natural occurrence of a silica/clay nanocomposite material was investigated by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). High-resolution images show that this nanocomposite material consists of 5–20 nm thick slabs of smectite and tridymite/cristobalite layers with coincident normals. In spite of the brittle glass-like appearance of the nanocomposite material its colloidal properties are similar to those of pure smectite but partial loss of expansion capacity was detected upon glycerol solvation. The structural relationship between smectite and silica is interpreted based on the smectite structure model of Edelman and Favejee (1940) which supposes reversed tetrahedra in the SiO4 layer of the TOT structure. This structure model explains the presence of silica impurities in bentonites used as raw material and several geological standard montmorillonites.

Study of interaction in silica glass via model potential approach

Abstract: Silica is one of the most commonly encountered substances in daily life and in electronics industry. Crystalline SiO2 (in several forms: quartz, cristobalite, tridymite) is an important constituent of many minerals and gemstones, both in pure form and mixed with related oxides. Cohesive energy of amorphous SiO2 has been investigated via intermolecular potentials i.e weak Van der Waals interaction and Morse type short-range interaction.We suggest a simple atom-atom based Van der Waals as well as Morse potential to find cohesive energy of glass. It has been found that the study of silica structure using two different model potentials is significantly different. Van der Waals potential is too weak (P.E =0.142eV/molecule) to describe the interaction between silica molecules. Morse potential is a strong potential, earlier given for intramolecular bonding, but if applied for intermolecular bonding, it gives a value of P.E (=−21.92eV/molecule) to appropriately describe the structure of silica.