Wollastonite Crystals Research Articles

Atomistic simulation of the surface structure of wollastonite and adsorption phenomena relevant to flotation

Atomistic simulation techniques have been used to calculate the surface structure and stability of wollastonite crystal and its adsorption behaviour in the presence of molecular and dissociated water, and two widely used collector head group molecules of methanoic acid and methylamine. Seven predominant surfaces have been modelled and their calculated surface energies correspond well with their preferred morphological domination. Surfaces are identified having fourfold and threefold coordination of surface silicon. Threefold surface silicon are stabilized by addition of hydroxyl ion on them and proton on surface oxygen. Stable surfaces thus obtained are subjected to surface Ca 2+ replacement by 2H + by transforming 2O 2− to 2OH −. Surface energy and reaction energy values indicate wollastonite surface stabilized to a great extent by adsorbing water in dissociated form. The Ca 2+ replacement from the first few layers of the surface is energetically more favourable in acidic condition. Three Miller indexed surfaces terminating with fully coordinated silicon were reconstructed to make the surface free of lone oxygen, and these surface cuts were chosen to carry out simulation work for adsorption of molecules. A comparison of surface energies revealed that all the surfaces become stabilized in the presence of added molecules but the presence of methylamine decreases surface energy to lowest values. Adsorption of dissociated water is preferred by {100} and {102} surfaces, while {001} surface adsorbs methylamine strongly as the results show highly negative adsorption energies. In terms of pure molecule adsorption, the preferred adsorption sequence for all the surfaces is methylamine>methanoic acid>water. For {100} and {102} predominant surfaces, the difference in adsorption energy values is not much and we conclude that the collectors having long-chain hydrophobic alkyl chain, the two head group molecules considered, cannot render enough hydrophobicity due to inadequate adsorption on wollastonite. Thus the presence of activators/modifiers in actual wollastonite flotation practice is substantiated.

Atomistic simulation of the surface structure of wollastonite

Abstract Atomistic simulation techniques have been used to calculate the surface structure and stability of wollastonite crystal. Seven predominant surfaces have been modelled and their calculated surface energy corresponds well with their morphological domination. The surface energy, hydration energy and reaction energy values indicate wollastonite surfaces stabilized to great extent by adsorbing water in dissociated form. The Ca 2+ replacement from the first few layers of the surface is found to be energetically more favourable, elucidating high dissolution phenomena of wollastonite mineral.

The formation of columnar fiber texture in wollastonite rims by induced stress and implications for diffusion-controlled corona growth

The evolution of columnar fiber texture was studied in wollastonite reaction rims synthesized by the reaction calcite + quartz=wollastonite + CO2. Experiments were performed at 850 to 950 °C at 100 MPa in dry CO2 and were evaluated by scanning and transmission electron microscopy. Rim growth rates are interpreted as controlled by the diffusion of the SiO2 component through the rims from the quartz–wollastonite to the wollastonite–calcite interface. The temperature dependence of rim growth rates yields an apparent activation energy of 314 ± 53 kJ mol−1. The columnar fibrous wollastonite crystallizes at the quartz–wollastonite interface and comprises the largest parts of the rims. Ultimately, at the growth front strain contrast centers are present in the quartz. The strained volume extends about 200 nm into the quartz grains. We suggest that this might signify deformation of the quartz lattice due to wollastonite crystallization.

Nucleation and Crystallization of New Glasses from Fly Ash Originating from Thermal Power Plants

The nucleation and crystallization kinetics of new glasses obtained by melting mixtures of a Spanish carbon fly ash with glass cullet and dolomite slag at 1500°C has been evaluated by a calculation method. These glasses, whose microstructure was examined by TEM carbon replica, were susceptible to controlled crystallization in the 800°–1100°C range. The resulting glass‐ceramics developed acicular and branched wollastonite crystals or a network of dendritic pyroxene mixed with anorthite feldspar (SEM and EDX analysis). The time–temperature–transformation curves (processing of the XRD data) showed the crystallization kinetics and the critical cooling rate to be in the 12°–42°C/min range.

Luminescence de cristaux de d�vitrivication de wollastonite dans des verres non dop�s et dop�s en Eu, Dy et Tm

Wollastonite crystals (CaSiO 3 ), pure or doped with rare earth ions, were grown by a devitrification process of a ternary SiO 2 -Na 2 O-CaO glass. The nature of point defects in these crystals was studied. Concerning the non-doped crystals, two trap centers were revealed by thermoluminescence (TL) and identified by electron spin resonance (ESR) using preheating experiments: one is a hole center HC 1 and the other one an electron center whose main characteristic feature is g = 2.0020. Cathodoluminescence (CL) studies showed an important emission band considered as intrinsic. As for the doped crystals (Eu, Dy. Tm), most CL emission bands were identified. With TL, it is shown that Eu acts in wollastonite crystals as an electron trap and also as an emission centre.

Effect of silicon carbide whisker reinforcement on CaO–ZrO2–SiO2glass–ceramic system

AbstractAn industrial frit formulated in the new CaO–ZrO2–SiO2 glass–ceramic system was studied as a matrix for whisker reinforced composites. The frit was ball milled in acetone and wet ultrasonically mixed with 5, 10, 20, and 30 vol.-% SiC whiskers in order to overcome whisker agglomeration and obtain intimate mixing of the two phases. The samples were hot pressed at 14 MPa in graphite dies, using a N2 atmosphere, for 2 h at 1280°C. In order to investigate the effect of whiskers as a reinforcement, flexural strength as well as crack configuration and propagation were taken into consideration. Whisker orientation perpendicular to the hot pressing direction was found by SEM observation, and no carbon layer at the whisker/matrix interface was detected by EPMA. Further characterisation of the specimens involved physical (density, elastic modulus) and microstructural properties (XRD, SEM, TEM). The result of glass devitrification was inter locked wollastonite crystals.

Kinetics of wollastonite nucleation in CaO⋅SiO2 glass

The nucleation and growth of wollastonite crystals have been investigated in stoichiometric CaO⋅SiO2 glass. In the temperature range of 1043 to 1073 K, the nucleation rate was evaluated from crystallite densities, determined by optical microscopy on samples subjected to double-stage heat treatment. Turnbull’s method was used to analyze the nucleation rates in terms of various theoretical models: the classical theory (CNT), a semiempirical density functional approach (SDFA) based on the model of Bagdassarian and Oxtoby, and the phenomenological diffuse interface theory (DIT). When either the viscosity or the transient time of nucleation is used in determining the rate constants, a strongly temperature dependent interfacial free energy is obtained that approaches a linear behavior at high temperatures. Accordingly, Turnbull’s plot is curved; however, the high-temperature asymptotic behavior is well described by both the SDFA and the DIT. The DIT analysis indicates a homogeneous process, as opposed to the heterogeneous mechanism suggested by the SDFA. The melting point values of the dimensionless interfacial free energy (Turnbull’s α) deduced from the slope of Turnbull’s plot are 0.79–0.85 (DIT) and >1.06–1.14 (SDFA). The somewhat too high values in the latter case originate from a broad (∼200 Å) interfacial region predicted by the SDFA that exceeds considerably the interface width from computer simulations. Attempts to remove this problem by adjusting the parameters of the SDFA failed, suggesting that a more accurate free energy functional needs to be introduced into the model. Possible origins of the nonlinear Turnbull plot are discussed.

Effect of fluoride on sinterability of a silicate glass powder

Partial substitution of fluorine for oxygen was utilised to increase the crystallisation tendency in glasses based on the Nu 2O-CaO-Al 2O 3-SiO 2 system. The glass without fluoride is surface-nucleated and forms wollastonite crystals growing one-dimensionally into the bulk glass. The addition of fluoride facilitates both viscous flow and crystallisation in bulk glass. However, the enhancement of crystallisation due to fluorine can affect processing by the powder route. In the optimum process the glass should sinter to full density before the onset of crystallisation. To achieve this, the relative rates of crystallisation and densification must be optimised by controlling the heating rate. Values of flexural strength as high as 210 MPa can be achieved in fluorine-containing samples of glass-ceramics where both the sintered density and microstructure have been optimised.

Apatite–wollastonite glass-ceramics part I Crystallization kinetics by differential thermal analysis

Apatite crystallization in the apatite–wollastonite glass exhibits a three-dimensional bulk mechanism with an Avrami parameter (n)≈3, while that of the second phase (wollastonite) shows a two-dimensional surface mechanism (planar growth) with n≈2. A strong effect of glass particle size on the wollastonite crystallization temperature is observed. The peak temperature is lower when the particle size is smaller.

Chlorine-rich pyrometamorphic magma at White Island volcano, New Zealand

The 25 January 1987 phreatomagmatic eruption of Congress Vent on White Island volcano ejected paralava bombs derived from pockets of pyrometamorphic magma. The current White Island magmas are calc-alkaline basaltic andesite/andesite, but the paralavas are mostly highly peralkaline with molar(Na2O + K2O)/Al2O3 ratios from 1 to 22 recorded in matrix glasses. The glass compositions are highly variable, though quench crystals of tridymite, wollastonite and green clinopyroxene are ubiquitous, while albite occurs only in the least peralkaline samples. All have high alkali contents (Na2O + K2O > 10%) and chlorine mostly > 1% and up to 1.7%. The paralava glasses represent the most Cl-rich volcanic melts yet discovered. They display a strong positive correlation between Cl and total Fe, but a negative correlation between Cl and normative alkali feldspar content. It is likely that the Fe content of melts was the dominant control over Cl solubility, not their alumina-alkali systematics. Possible parent rocks for the pyrometamorphic magmas include halite-cemented crater-lake sediments and acid-sulfate hydrothermally altered vent breccias and tuffs, as found in the eruption deposits together with the paralava bombs. Fusion probably occurred at temperatures between 830 and 1000 °C and at a pressure below 5 MPa in a regime where silicate melts were buffered by immiscible NaCl-saturated aqueous vapor and hydrosaline melt. The conditions required to form the paralava bombs appear to be uncommon, but it is likely that similar pyrometamorphic melts could be generated at many volcanoes where shallow magma penetrates a hydrothermal system.

Mechanical and biological properties of two types of bioactive bone cements containing MgO-CaO-SiO2-P2O5-CaF2 glass and glass-ceramic powder.

In this study two types of bioactive bone cement containing either MgO-CaO-SiO2-P2O5-CaF2 glass (type A) or glass-ceramic powder (type B) were made to evaluate the effect of the crystalline phases on their mechanical and biological properties. Type A bone cement was produced from glass powder and bisphenol-a-glycidyl methacrylate (BIS-GMA) resin, and type B from glass-ceramic powder containing apatite and wollastonite crystals and BIS-GMA resin. Glass or glass-ceramic powder (30, 50, 70, and 80 by wt %) was added to the cement. The compressive strength of type A (153-180 MPa) and B (167-194 MPa) cement were more than twice that of conventional polymethylmethacrylate (PMMA) cement (68 MPa). Histological examination of rat tibiae showed that all the bioactive cements formed direct contact with the bone. A reactive layer was seen at the bone-cement interface. In specimens with type A cement the reactive layer consisted of two layers, a radiopaque outer layer (Ca-P-rich layer) and a relatively radiolucent inner layer (low-calcium-level layer). With type B cement, although the Ca-P-rich layer was seen, the radiolucent inner layer was absent. Up to 26 weeks there was progressive bone formation around each cement (70 wt %) and no evidence of biodegradation. The mechanical and biological properties of the cements were compared with those of a previously reported bone cement containing MgO-free CaO-SiO2-P2O5-CaF2 glass powder (designated type C).

A heat-generating bioactive glass-ceramic for hyperthermia.

Glass plates of the chemical composition: CaO (29.0), SiO 2 (31.0), Fe 2O 3 (40.0), B 2O 3 (3.0), P 2O 5 (3.0) in weight ratio were heated to 1050 degrees C at a rate of 5 degrees C/min and then cooled to laboratory temperature. The resulting glass-ceramic containing magnetite and wollastonite crystals showed high-saturation magnetization. The bonding ability of this new glass-ceramic to bone tissue was evaluated using rabbit tibiae, and compared with glass of the same composition. This glass-ceramic formed a Ca, P-rich layer on its surface and bonded tightly with bone within 8 weeks of implantation. However, the glass did not form this Ca, P-rich layer, nor had it bonded with bone at 25 weeks. The bone-heating ability of this glass-ceramic was investigated by applying a max. 300-Oe, 100-kHz magnetic field. The granules of the glass-ceramic filled in the rabbit tibiae heated the whole surrounding bone to more than 42 degrees C and maintained this temperature for 30 min. Bioactive ceramics reinforce the mechanical strength of bone tissue. Furthermore, this heat-generating bioactive glass-ceramic can be used for hyperthermic treatment of bone tumors.

The stability of hydroxyapatite in an optimized bioactive glass matrix at sintering temperatures

The stability of sintered hydroxylapatite particles was studied in glass matrices of the system SiO2-CaO-P2O5-Na2O-Al2O3-B2O3 at sintering temperatures between 700 and 1000 °C. The results from X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDX) analysis, transmission electron microscopy (TEM) and electron diffraction showed that above 700 °C the Na+ ions diffuse into the hydroxylapatite particles which then transform into rhenanite. The glass matrix undergoes crystallization yielding wollastonite crystals and a silica rich matrix.

Combeite (Na2.33Ca1.74others0.12)Si3O9from Oldoinyo Lengai, Tanzania

The third occurrence of combeite is in lapilli ejected from the active carbonatite volcano Oldoinyo Lengai, Tanzania. The combeite occurs as euhedra in equilibrium with nepheline, Na-melilite, Tiandradrite, and Ti-magnetite, and as coronas replacing corroded clinopyroxene and wollastonite crystals. Garnet and nepheline euhedra do not have combeite coronas. The combeite of the coronas is associated with minor Na-melilite, nepheline, and calcium silicates (rankinite? and larnite?). The coronal combeite is attributed to incomplete reaction between Ca-bearing silicates and sodium carbonatite magma. The mean of 11 analyses, , indicates solid solution towards the component from the idealized composition. This is consistent with experimental phase equilibria in the system.

Formation of a high-strength bioactive glass-ceramic in the system MgO-CaO-SiO2-P2O5

Formation of a high-strength bioactive glass-ceramic in the system MgO-CaO-SiO2-P2O5 was investigated by observing the microstructure of the crystallized products. Crystallization of the parent glass in a bulk form led to the occurrence of large cracks in the crystallized product. This was attributed to the precipitation of fibrousβ-wollastonite crystals growing perpendicular to the outer surfaces of the glass after uniform precipitation of fine oxyapatite/fluoroapatite crystals. On the other hand, crystallization of the same glass in a powder compact led to the formation of a crack-free dense crystallized product due to uniform precipitation of both apatite and wollastonite fine crystals throughout the glass article. The uniform precipitation of the wollastonite crystals was attributed to the simultaneous formation of fine crystals in the individual glass particles.

Stable isotope studies of metasomatic Ca-Fe-Al-Si skarns and associated metamorphic and igneous rocks, Osgood Mountains, Nevada

Garnet-pyroxene skarns were formed 90 m.y. B.P. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation. The metasomatic replacement followed contact metamorphic recrystallization of the Preble. The sources, temperature, and variation in H2O/CO2 ratios of the metasomatic fluid are interpreted from 269 analyses of oxygen, carbon, hydrogen, and sulfur isotopes in whole rocks, minerals and inclusion fluids. Skarns formed in three mineralogical stages. Oxygen isotope data indicate that temperatures during the crystallization of garnet, pyroxene and wollastonite (Stage I) were least 550 ° C, and that the metasomatic fluid had an $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035 in the massive skarns, and ≦ 0.12 in vein skarns up to 3 cm thick. Pore fluids in isotopic equilibrium with garnet in calc-silicate metamorphic rocks, on the other hand, had $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.15. The metasomatic fluids of Stage I were derived primarily from the crystallizing magma. The isotopic composition of magmatic water was δ18O =+9.0, δD= −30 to −45. Oxygen isotope temperatures of greater than 620 ° C were determined for the granodiorite. Isotopic and chemical equilibria between mineral surfaces and the metasomatic fluid were approached simultaneously in parts of the skarn several meters or more apart, while isotopic and chemical disequilibria (i.e. zoning) have been preserved between 20 to 40 μm-thick zones in grandite garnet. More Fe-, or andradite-rich garnet crystallized in more H2O-rich C-O-H fluids ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.01) than present with grossularite-rich garnet ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035). Stage II was marked by the replacement of garnet and pyroxene by quartz, amphibole, plagioclase, epidote, magnetite, and calcite. Many of the replacement reactions took place over a relatively narrow range in temperature (480–550 ° C), as indicated by 18O fractionations between quartz and amphibole. Meteoric water comprised 20 to 50% of the metasomatic fluid during Stage II. Calcite was formed along with pyrite, minor pyrrhotite, and chalcopyrite during Stage III, although the crystallization of pyrite and calcite had begun earlier, during Stages I and II, respectively. Carbon and sulfur isotope compositions of calcite and pyrite indicate a magmatic source for most of the C and S in the metasomatic fluids of Stage III. By the end of Stage III, meteoric water constituted as much as 100% of the metasomatic fluid. Minerals from grandiorite and skarn do not show large depletions in 18O because the oxygen isotope composition of the metasomatic fluid was buffered by the calcareous wall rocks and the grandiorite. Meteoric water in the vicinity of the Osgood Mountains during the Late Crectaceous (δ18Ocale. ≃ −14.0, δD = − 107) was slightly enriched in 18O and D relative to present-day meteoric water (δ18O = 15.9, δD = − 117)

Electron-microscopic analysis of disorder in wollastonite

1- and 2-dimensional high-resolution lattice images have confirmed, and extended details of the ultramicro-structure of defective and apparently perfect crystals of wollastonite deduced from X-ray analysis. Ordering of monoclinic and triclinic polytypes has been directly observed, and faulted regions of apparently perfect crystals have been identified. Evidence has also been found for lamellae of one polytype terminating within another.

Some Observations on the Formation of Wollastonite from Calcite and Quartz

Mixtures of powdered calcite and quartz were reacted for various lengths of time at a total pressure of 2000 bars, 650 °C, in an H2O–CO2 fluid. Extent of formation of wollastonite was determined by partial chemical analysis of run products. Experimental scatter precludes detailed analysis of the reaction mechanism.The data show that approximately 40% reaction takes place in the first 24 h, and that after this initial period the rate decreases. Examination of run products shows the calcite to be surrounded by radiating wollastonite crystals. This suggests that the wollastonite cover forms during the initial stages of reaction and, by shielding the calcite, suppresses the reaction.Single cleavage flakes of calcite were packed in −325 mesh quartz and allowed to react under the same conditions as the powders. A scanning electron microscope was used to examine and photograph the fragment surfaces at the conclusion of the experiments.Examination of the photographs shows the calcite to be completely covered with a thin layer of wollastonite crystals. Details of the morphology suggest that the wollastonite grew outward from nucleation centers and that the solution of calcite and quartz may have been accelerated near growing wollastonite. Although no details of the reaction mechanism can be deduced, the model favored here is that growth was most rapid near the calcite–wollastonite interface and the mantling effect slowed the reaction by preventing the transport of silica to the calcite surface.

The effect of vanadium oxides on the crystallisation of silicate glasses

The influence of vanadium oxides as catalysts for nucleation and crystal growth in CaO-MgO-Al2O3-SiO2 glasses has been investigated. The effect of varying the total vanadium content and the ratio of oxidised to reduced vanadium ions has been observed. No internal nucleation was observed, but the rate of growth of crystals of anorthite and wollastonite from the surface was increased by additions of V2O5 and V2O3. The values of the growth rates and the analysed V5+ content in both oxidised and reduced glasses suggest that V5+ ions are the most active species. Increasing concentrations of V2O5 in the glass gave maxima in the growth rates between 2 and 4 wt % for crystallisation temperatures between 900 and 980° C.

Activity of the carbonatite volcano Oldoinyo Lengai, 1966

From 1960 to August, 1966, the activity of Oldoinyo Lengai took the form of quiet extrusion of carbonatite lava. In August, 1966, the style of activity changed abruptly and violent ash eruptions took place. The activity varied from minor emissions of ash to major Plinian and Vulcanian type eruptions. A new ash-cone built up within the crater and ash was widely distributed on the slopes of the volcano and over the surrounding countryside. The ash consists of sodium carbonate mixed with crystals of nepheline, pyroxene, wollastonite, apatite, melanite and pyrite. Also blocks of ijolite and melteigite were ejected during the activity.