Crystallization Of Leucite Research Articles

Study of the crystallization of leucite in feldspar glass matrix

Feldspathic glass–ceramics reinforced with leucite are usually used in dental prosthesis. This study focused on leucite crystallization kinetics due to its importance to the end product of a dental crown processing. Leucite grains were nucleated and grown from feldspathic glass frit powders with particle size smaller than 45 μm. The nucleation and crystallization kinetics of leucite crystals in the glass matrix was investigated under isothermal and non-isothermal conditions through differential thermal analysis. The samples were also characterized by X-ray diffraction and scanning electron microscopy. The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy (E) of leucite crystallization was determined by the Kissinger method and the Avrami parameter (n) indicated that surface crystallization is the dominant mechanism in the glass.

Dental Ceramics/Bioactive Glass Composites: Characterization and Mechanical Properties Investigation

Apart from a good bioactive behavior, composite materials for dental applications should attain high mechani- cal properties. Thus, the aim of this work was the fabrication and characterization of novel sol-gel derived dental ceram- ics/bioactive glass composites and the investigation of their mechanical properties such as flexural strength and Weibull modulus. Fourier Transform Infrared Spectroscopy (FTIR) and Surface Scanning X-ray Diffractometry (XRD) revealed the crystallization of leucite. Flexural strength values were in the range of 5-50MPa and it was followed a linear rela- tionship between the amount of dental porcelain in the com- posites and the strength of the materials. In conclusion, den- tal ceramics/bioactive glass composites exhibit mechanical integrity and can be potentially used in restorative dentistry.

Thermal evolution and crystallization kinetics of potassium-based geopolymer

In this paper crystallization kinetics of potassium-based geopolymer (Si/Al = 2.5, denoted as KG2.5) upon heating are investigated by differential thermal analysis (DTA) under non-isothermal conditions at various heating rates. From 35 to 700 °C KG2.5 shows a significant weight loss due to both evaporation of free water and condensation of hydroxyl groups. Crystallization of KG2.5 starts at a low temperature of ∼960 °C according to XRD and DTA analysis. KG2.5 also exhibits a low onset temperature of viscous sintering stage and on heating to 950 °C the sintering is completed. In the DTA graphs, the exothermic peaks which are caused by leucite crystallization shift to higher temperatures with increasing heating rate. The activation energy value of crystallization for leucite is found to be 455.9 kJ/mol and the corresponding Avarami constant is 3.89 indicating the three-dimensional crystal growth mechanism.

Kinetic Analysis of Isothermal Crystallization of Potassium Aluminosilicate Ceramics (Leucite and Kalsilite) from Amorphous Potassium Aluminosilicate Precursors

Kinetics of the isothermal crystallization of tetragonal modification of leucite and hexagonal modification of kalsilite from differently prepared precipitated amorphous aluminosilicate precursors were investigated at three different temperatures. Kinetic analyses of the transformation processes have shown that the crystallization of leucite occurs the same way as the crystallization of kalsilite and follows a pseudo-zero-rate kinetic equation. The apparent activation energies calculated by the Arrhenius equation were 373 kJ/mol for the crystallization of kalsilite and 403 kJ/mol for the crystallization of leucite, which is explained by the different microstructures of the prepared precursors.

Petrography, geochemistry and geodynamic environment of potassic alkaline rocks in Eslamy peninsula, northwest of Iran

Eslamy peninsula, 360 km2 in area, is located in the eastern coast of Urmieh lake in the northwest of Iran. This peninsula is a complex stratovolcano with a collapsed center, which is elevated due to later intrusions of sub-volcanic masses with trachytic to microsyenitic composition. The composite cone consists of a sequence of leucite tephrite, tephrite, leucite basanite, basanite and related pyroclastic rocks. Magmatic activities in the Eslamy peninsula begin with potassic alkaline to ultrapotassic and basic, silica-undersaturated shoshonitic rocks and they are followed by intrusions of lamprophyric dykes and end with acidic magmatism including trachytic, microsyenitic, syenitic and phonolitic domes. The original magma of the Eslamy peninsula rocks has a potassic alkaline nature (Roman type) rich in LREE and LILE and depleted of HREE. These characteristics suggest that the origin of magma can be from deep mantle with a garnet lherzolite composition, a low partial melting rate which has been contaminated by crustal materials in its way up. Fractional crystallization of olivine, diopsidic clinopyroxene and leucite played an important role in the evolution of magmas. Scrutinizing the geodynamic environment of Eslamy peninsula rocks in discrimination diagrams indicates that these rocks must have been formed in a post-collision magmatic arc setting.

Leucite crystals: Surviving witnesses of magmatic processes preceding the 79AD eruption at Vesuvius, Italy

Crystals in volcanic rocks are sensitive records of magma chamber and conduit conditions under volcanoes. Plagioclase is an invaluable tool to identify ascent rates for calc-alkaline magmas, but may be absent in alkaline melts. In contrast, leucite is common in alkaline magmas and is potentially useful to investigate storage and ascent conditions prior to volcanic eruptions. Leucite microphenocrysts are ubiquitous within the products from all phases of the 79AD eruption of Vesuvius. Steady-state (isobaric–isothermal) and dynamic (decompression) experiments on white phonolitic pumice from the opening (EU1) and lower Plinian (EU2) phases of the eruption were performed at temperature conditions ranging from 800 to 850 °C to test the possibility that leucites within this ‘white’ magma formed during ascent. However, multiple-step decompression (MSD) experiments using a decompression rate of 0.25 MPa/s failed to crystallize leucite even at pressures well below its stability domain. On the other hand, single-step decompression (SSD) experiments from 150 MPa to 25 MPa result in leucite crystallization after a ~ 12 h lag period, but the skeletal habit and size distribution differ from those seen in natural pumices. Instead, euhedral leucites texturally matching those observed in 79 AD samples formed after 5 days under isobaric and isothermal (IB–IT) experimental conditions. Crystallization conditions derived from the latter experiments suggest the magma reservoir was thermally zoned with cooler EU1 ( T = 830–840 °C) overlying slightly hotter EU2 ( T = 850–925 °C) magma. Two models for natural crystallization conditions are consistent with the experimental data: either leucites formed at ~ 4 km depth ( P ~ 100 MPa) in a steady storage environment inside a magma saturated with H 2O-rich vapor, or, alternatively, the white magma was initially undersaturated with respect to H 2O and leucites formed during a slow depressurization event prior to the eruption. Leucite crystallization seemingly adheres to the classical nucleation theory, and supports a compositional (i.e. H 2O) control on surface tension. Derived leucite growth rates reach ~ 10 − 7 mm s − 1 minimum, comparable to the fastest growth rates observed for plagioclase crystals in calc-alkaline magmas.

Kinetic studies on leucite precursors

AbstractKinetic studies were performed on two types of leucite precursors. These precursors were prepared using a hydrothermal method at 150°C; the reaction time was 1.5 hours. In order to obtain precursors having different amounts of seed the molarity of KOH was changed. These intermediate products were subsequently calcinated from 5 minutes to 72 hours at temperatures of 850°C, 900°C, 950°C, 1000°C and 1050°C. The crystallinities of the powders were calculated by X-ray diffraction analysis. The crystallization curves for the synthesis of leucite exhibited a typical sigmoidal characteristic. Using different kinetic equations it was found that the Avrami-Eroféev model is the most appropriate to describe the experimental data. Using the Avrami-Eroféev model n reaches an average value of 2.9 which is connected with the three-dimensional growth of nuclei. The calculated activation energy of crystallization of leucite was 385 kJ mol-1 for non-seeded precursors and 246 kJ mol-1 for seeded precursors, respectively.

Experimental determination of radium partitioning between leucite and phonolite melt and 226Ra-disequilibrium crystallization ages of leucite

Radium and other trace element (i.e. Cs, Rb, Ba, Sr, Cd, Cu, Pb, Zn, La, Lu, Nd, Sm, Y, Sn) partition coefficients have been experimentally determined at atmospheric pressure for leucite in equilibrium with a phonolitic melt. Ra bulk concentrations of ∼ 0.25 ppm are sufficient for precise LA-ICP-MS measurements of Ra in leucite and melt. Partition coefficients increase for the alkalis from Rb to Cs, and for the alkaline earths from Sr to Ba to Ra, with the maximum of both lattice strain parabolas at a radius of 1.89 ± 0.01 Å, i.e. near Cs and Ra. At 1190 °C, D Ra leucite/melt was determined to 2.3 ± 0.7 and 1.8 ± 0.5 for two different experiments, the average D Ra/ D Ba is 4.2 ± 0.5. Hitherto, crystallization ages calculated from the isotopic 226Ra disequilibrium assumed that D Ra equals D Ba. The experimentally determined partition coefficients lead to an order of magnitude younger 226Ra crystallization ages for leucite. Employing our partitioning data in combination with the isotopic results from Black, S., Macdonald, R., DeVivo, B., Kilburn, R.J., Rolandi, G. U-series disequilibria in young (A.D. 1944) Vesuvius rocks: Preliminary implications for magma residence times and volatile addition. J. Volcanol. Geotherm. Res. 82, 97–111, leucite crystal ages from several phonothephrite lavas of the 1944 eruption of Vesuvius become undefined, testifying for a more complicated process than equilibrium leucite crystallization and subsequent undisturbed evolution of these lavas.

X-Ray pair distribution function analysis of a metakaolin-based, KAlSi2O6·5.5H2O inorganic polymer (geopolymer)

The atomic structure of geopolymers is often described as amorphous with a local structure that is equivalent to that of crystalline zeolites. However, this structural relationship has never been quantified beyond a first-nearest-neighbor bonding environment. In this study, the short to medium range (∼1 nm) structural order of metakaolin-based KAlSi2O6·5.5H2O geopolymer was quantified and compared to zeolitic tetragonal leucite (KAlSi2O6) using the X-ray atomic pair distribution function technique. Unheated KAlSi2O6·5.5H2O was found to be structurally similar to leucite out to a length of ∼8 A, but had increased medium range disorder over the 4.5 A 300 °C, changes in the short to medium range structure were observed due to dehydration and removal of chemically bound water. Crystallization of leucite occurred in samples heated beyond 1050 °C. Refinements of a leucite model against the PDF data for geopolymer heated to 1100 °C for 24 h yielded a good fit.

Magma–carbonate interaction: An experimental study on ultrapotassic rocks from Alban Hills (Central Italy)

The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO < 3 wt.%) are characterised by low SiO 2 concentration (< 50 wt.%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H 2O and CaCO 3 in the starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO 3 in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidence showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO 3-bearing experiments (i.e., simulating magma–carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important.

Nanocrystalline Seeding Effect on the Crystallization of Two Leucite Precursors

The effect of nanocrystalline leucite seeding with leucite precursors prepared by sol–gel and hydrothermal methods on the leucite crystallization process and the microstructure of its sintered porcelain was studied. The introduced seeds lowered the crystallization temperature of leucite by 100° and 50°C for the precursor prepared by hydrothermal and sol–gel methods, respectively. The crystallization process was changed after the seeds were introduced. As the transition phase during leucite crystallization, kalsilite did not appear after the seeds were added. When the seeded hydrothermally derived precursor was treated at 650° and 700°C, part of the cubic leucite was stabilized to room temperature. This stabilization was due to the crystallization of nanocrystalline leucite on the seeds at a low temperature. The leucite synthesized by the hydrothermal method with seeding at 800°C had an average particle size of 0.4 μm that grew to about 0.6 μm in the sintered porcelain.

Seeded Crystallization of Leucite

The leucite crystallization kinetics from a hydrothermally derived precursor seeded with nano‐crystalline leucite was investigated by X‐ray diffraction and non‐isothermal differential thermal analysis. The nano‐crystalline leucite was prepared by high‐energy milling of high‐purity leucite powder and the leucite precursor was prepared by the hydrothermal method of silica sol, aluminum nitrate, and potassium nitrate. After the seeds were introduced, the crystallization temperature of the precursor was lowered by 100°C and the transition phase kalsilite did not appear during the crystallization process. When the seeded precursor was heat treated at 700°C, a small amount of cubic leucite was stabilized to room temperature. The seeded precursor showed an exothermic peak between 800° and 920°C under different heating rates. The activation energy for the growth of leucite from the seeded precursor was 256(SD9) kJ/mol.

Leucite crystallization kinetics with kalsilite as a transition phase

The leucite crystallization kinetics via kalsilite from the hydrothermally-derived precursor was investigated by XRD and non-isothermal DTA. The leucite precursor was prepared by hydrothermal method from silica sol, aluminum nitrate and potassium nitrate. The crystallization of the precursor experienced two steps. Kalsilite crystallized at first, and then it reacted with the redundant SiO 2 in the precursor and finally the leucite formed. On the DTA curve, the two exothermic peaks were corresponding to the two times of crystallization. Determined by the Kissinger equation, the activation energy of kalsilite crystallization and leucite crystallization via kalsilite was 103(SD 8) kJ/mol and 125(SD 4) kJ/mol respectively and both the crystallizations were nucleation and three-dimensional growth, which follows the diffusion-controlled mechanism with increasing nucleation rate.

Crystallization of oxyapatite in glass-ceramics

The aim of the presented study was to investigate the crystallization of oxyapatites in glass-ceramics. The crystallization was analysed for three different glasses from the SiO 2–B 2O 3–Al 2O 3–Y 2O 3–CaO–Na 2O–K 2O–F system with different F and B 2O 3 content after heat treatment. To characterize the microstructure and the phase formation, we used scanning electron microscopy (SEM) and room temperature X-ray diffraction (RT-XRD). The main results of this study showed that the oxyapatite crystallization is different in comparison to that of fluorapatite. We detected that the oxyapatite crystallizes according to the surface crystallization mechanism. Also we determined leucite as a secondary crystal phase in dependence of the B 2O 3 and F content. The results showed that a two-fold controlled surface crystallization of oxyapatite and leucite took place.

The crystallization of an aluminosilicate glass in the K 2O–Al 2O 3–SiO 2 system

The aims of the study were to explore the nucleation and crystallization kinetics of an aluminosilicate glass in K2O-Al2O3-SiO2 system and to characterize it. A starting glass composition of wt%; 64.2% SiO2, 16.1% Al2O3, 10.9% K2O, 4.3% Na2O, 1.7% CaO, 0.5% LiO and 0.4% TiO2 was heated in an electric furnace and later quenched to produce glasses. The glass powders were heat treated using differing heat treatment schedules and quenched. Dta, Xrd, Eds and Sem analyses were used to characterize and explore the crystallization kinetics of the glasses. Phase separation of the glasses was identified and characterized in the glasses. Tetragonal leucite, cubic leucite and sanadine glass-ceramics were produced. Fine leucite crystals (1 microm2) were crystallized with minimal matrix microcracking. Amorphous phase separation appeared to be an important precursor to nucleation and crystal growth in the alkali aluminosilicate glasses explored. It was possible to control the crystallization of tetragonal leucite and sanidine phases by selected heat treatment of glass powders and monoliths, resulting in the production of fine grained tetragonal leucite glass-ceramics.

Isothermal phase transformations of a dental porcelain

The purpose of this investigation was to determine if the change in the leucite weight fraction during an isothermal heat treatment could be estimated by observing the deformation of PFM strips in a high-heating-rate, computer-controlled bending beam viscometer (BBV). Specimens of a commercial body porcelain were fired according to the manufacturer's instructions-50 disk specimens for quantitative X-ray diffraction (XRD) and 100 bimaterial strip specimens for BBV. The XRD specimens were annealed at temperatures between 650 and 1000 degrees C, and leucite weight fraction was measured using an alumina internal standard. The BBV specimens were annealed in the BBV using time-temperature schedules designed to elucidate the leucite crystallization behavior between 700 and 1000 degrees C. Timoshenko's equation for a bimaterial thermostat was used to estimate the change in the thermal expansion of the porcelain near room temperature. Changes in leucite weight fraction were determined from these thermal expansion changes. The means and SDs were compared to values obtained by quantitative XRD. Good agreement was obtained between values of leucite weight fraction derived from beam deformation and those determined by quantitative XRD (p> or =0.45). The anneal sequence showed that the increase in leucite weight fraction at 800 or 900 degrees C is reversible by an anneal at 1000 degrees C. The BBV technique yields comparable results to quantitative XRD and provides the opportunity to efficiently monitor porcelain leucite changes nondestructively over multiple heat treatments. This technique could prove useful for testing firing schedules designed to stabilize the leucite content in dental porcelain.

Crystallization of leucite as the main phase in glass doped with fluorine anions

The crystallization of a multicomponent glass containing 1.63 wt% of F− anions was studied. The results show in powder glass with particle sizes less than 0.15 mm, that surface crystallization is dominant, whereby two phases: leucite and dioside are formed. In glass powder or particle size about 0.15 mm, three phases, phlogopite, diopside and leucite, are formed, accompanied by an abrupt decrease in the resistance of the glass to crystallization. If the particle sizes are in the range 0.15 to 0.45 mm, both surface and volume crystallization are significant, while with particle sizes >0.45 mm, volume crystallization is dominant. Two nucleation temperatures, Tn1 = 655°C and Tn2 = 675°C, were determined in the temperature range of 600–710°C. These temperatures satisfy the condition that Tn ≥ Tg. Crystallization of bulk glass occurs in the temperature range of Tc = 870–1100°C, the crystal phases appearing in the sequence: phlogopite, followed by diopside, followed by leucite. Kinetical and microstructural studies show that the crystallization process is controlled by volume diffusion.

Volatile content and degassing processes in the AD 79 magma chamber at Vesuvius (Italy)

The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) was investigated through the study of melt inclusions (MI) in crystals of different origins. FTIR spectroscopy and EMPA were used to measure H2O, CO2, S and Cl of the different melts. This allowed us to define the volatile content of the most evolved, phonolitic portion of the magma chamber and of the mafic melts feeding the chamber. MI in sanidine from phonolitic and tephri-phonolitic pumices show systematic differences in composition and volatile content, which can be explained by resorption of the host mineral during syn-eruptive mixing. The pre-eruption content of phonolitic magma appears to have been dominated by H2O and Cl (respectively 6.0 to 6.5 wt% and 6700 ppm), while magma chamber refilling occurred through the repeated injection of H2O, CO2 and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO2 degassing probably occurred during the decompressional path of mafic batches towards the magma chamber, while sulphur was probably released by the magma following crystallization and mixing processes. Water and chlorine strongly accumulated in the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been strongly compositionally controlled, and Cl release was inhibited by groundmass crystallization of leucite, which shifted the composition of the residual liquid towards higher Cl solubilities.

Water-enhanced crystallization of leucite in dental porcelain

Objective: The purpose of this study was to determine whether long-term exposure of dental porcelain to saliva during temporary cementation of a porcelain-fused-to-metal (PFM) restoration could enhance leucite crystallization if the restoration is refired. Such water-enhanced leucite crystallization in dental porcelains could lead to porcelain–metal thermal incompatibility problems. Methods: Six commercial dental body porcelains and the Component No. 1 (leucite-containing) frit of the Weinstein et al. [13] patent were studied. For each porcelain, 30 coupon specimens were randomly assigned to a treatment group. Ten specimens were placed in artificial saliva, 10 in distilled water, and 10 in a desiccator and were stored for six months. At the end of the six months, an additional 10 coupons of each porcelain were prepared to serve as a control. All 40 specimens of each porcelain were randomized and subjected to one additional firing. Leucite weight fraction was determined by quantitative X-ray powder diffraction analysis via an internal standard technique. Results: Comparisons among the treatments via the least-squares-means test-adjusting for porcelain showed that the saliva group mean leucite weight fraction was significantly higher than that of the other groups. The change in porcelain thermal expansion that would be associated with a leucite change in this range would be between 0.2×10 −6 K −1 and 0.3×10 −6 K −1. Significance: The results of this work constitute the first demonstration that moisture absorbed by a porcelain can act as a glass modifier and enhance the crystallization of the glass during subsequent firing. The effect was sufficiently large to generate thermal expansion changes that would exceed the maximum safe mismatch between porcelain and metal.

Synthesis of Needlelike Leucite Crystals Using Potassium Sulfate Flux.

When a powder mixture consisting of Al2(SO4)3, SiO2 and K2SO4 was heated in a closed alumina crucible, a reactant mass remained at the bottom of the crucible. Needlelike leucite crystals were obtained by treating this reactant mass with hot HCl. The effect of the mixing ratio of the raw materials, the heating temperature and the duration of heating at the selected temperature on the formation of needlelike leucite crystals was investigated. When a powder mixture, consisting of Al2(SO4)3:SiO2:K2SO4=1.0:2.8:1.9 (molar ratio), was heated at 1200°C for 3h, the obtained needlelike leucite crystals were 1-2μm in length and 0.1-0.3μm in width. Needlelike leucite crystals were formed by the crystallization of leucite on small mullite crystals obtained by heating, followed by growth of the leucite crystals in a liquid phase of the K2SO4 flux as the main component.