Synthetic Titanite Research Articles

Pyrometallurgical processing of quartz-leucoxene to produce synthetic titanite

Pyrometallurgical processing of quartz-leucoxene to produce synthetic titanite

The temperature dependence of the Raman spectra of chromium-doped titanite (CaTiOSiO4 )

Summary The temperature dependence of the Raman bands of Cr4+ modes which show enhanced intensities due to pre-resonance effects is reported from 293 to 673 K in chromium-doped titanite (CaTiOSiO4). Some aspects of the temperature dependence of Raman bands in pure, synthetic titanite which have not been previously published are also included in this study. Two Raman-active components of Ag and Bg symmetry, respectively, of the symmetric Si–O mode in titanite are predicted under P21/a symmetry and also been identified in this phase for the first time. The one component of Bg symmetry disappears just above the antiferroelectric–paraelectric transition at ~500 K in accordance with the predictions under A2/a symmetry for the high-temperature phase. Two resonance-enhanced components of the Cr4+–O stretch are also evident in the P21/a phase and only one could be identified in the A2/a phase, again in accordance with group-theoretical predictions. These observations can be used to characterize the P21/a and A2/a phases of pure synthetic and chromium-doped titanite. The temperature dependence of the Cr4+–O modes can be approximated by two-dimensional Ising behavior with the critical exponent β ≈ 1/8 below 450 K. Between 450 and 498 K, anomalous behavior is observed and this could be due to the appearance of mobile anti-phase boundaries (APBs). Anomalous behavior also persists to temperatures above 500 K. The half-width of the Ti–O stretching mode reflects the influence of the order parameter (Ti–O displacements) as well as mobile anti-phase boundaries. No evidence could be found of the existence of other ions such as Cr4+-ions in Ti-sites and/or Cr3+-ions also in Ti-positions in Cr-doped titanite in the Raman spectra using different laser lines to excite the spectra. Copyright © 2013 John Wiley & Sons, Ltd.

Room temperature single-crystal diffuse scattering and ab initio lattice dynamics in CaTiSiO5

Single-crystal diffuse scattering data have been collected at room temperature on synthetic titanite using both neutrons and high-energy x-rays. A simple ball-and-springs model reproduces the observed diffuse scattering well, confirming its origin to be primarily due to thermal motion of the atoms. Ab initio phonons are calculated using density-functional perturbation theory and are shown to reproduce the experimental diffuse scattering. The observed diffuse x-ray and neutron scattering patterns are consistent with a summation of mode frequencies and displacement eigenvectors associated with the entire phonon spectrum, rather than with a simple, short-range static displacement. A band gap is observed between 600 and 700 cm−1 with only two modes crossing this region, both associated with antiferroelectric Ti–O motion along a. One of these modes (of Bu symmetry), displays a large LO–TO mode-splitting (562–701.4 cm−1) and has a dominant component coming from Ti–O bond-stretching and, thus, the mode-splitting is related to the polarizability of the Ti–O bonds along the chain direction. Similar mode-splitting is observed in piezo- and ferroelectric materials. The calculated phonon dispersion model may be of use to others in future to understand the phase transition at higher temperatures, as well as in the interpretation of measured phonon dispersion curves.

Tantalum-bearing titanite: synthesis and crystal structure data

Synthetic titanite, CaTiOSiO4, and the series of (Ca1−x Na x )(Ti1−x Ta x )OSiO4 and Ca(Ti1−2x Ta x Al x )OSiO4 solid solutions have been prepared by ceramic methods, and their crystal structure determined by the Rietveld analysis. At ambient conditions, titanite can contain up to 20 mol% NaTaOSiO4 or 60 mol% Ca(Al0.5Ta0.5)OSiO4. These limits might differ in natural samples due to combination with substitutions involving fluorine and/or hydroxyl replacing oxygen together with vacancies at cationic sites. All cations located at the vii X- and vi Y-sites in the structures of tantalian titanite are disordered. Expansion of the bond from 1.618 to 1.621 A in CaTi0.8Ta0.1Al0.1OSiO4 and CaTi0.6Ta0.2Al0.2OSiO4 to 1.644 A in the CaTi0.4Ta0.3Al0.3OSiO4 titanite suggests the possible presence of some Al3+ in the tetrahedral site replacing Si4+ in the latter. All tantalian titanites crystallize in the space group A2/a. This implies that both single-site and complex double-site substitutional schemes induce P21/a → A2/a phase transition(s). The (Ca1−x Na x )(Ti1−x Ta x )OSiO4 substitution scheme incorporates larger cations at both the vii X and vi Y sites, whereas the Ca(Ti1−2x Ta x Al x )OSiO4 scheme involves only vi Y-site (Al3+,Ta5+) cations with a slightly smaller “average” radius. Unit cell dimensions change insignificantly or increase incrementally with increase of average cationic radii in the (Ca1−x Na x )(Ti1−x Ta x )OSiO4 series, and with an insignificant decrease in the viR Y average cationic radii in the Ca(Ti1−2x Ta x Al x )OSiO4 series. Both Ta-doped titanite and CaTiOSiO4 consist of distorted polyhedra with the XO7, YO6 coordination polyhedra and the SiO4 tetrahedron in tantalian titanite being less distorted compared to those of the pure CaTiOSiO4.

Preparation of a Synthetic Titanite Glass Calibration Material for In Situ Microanalysis by Direct Fusion in Graphite Electrodes: A Preliminary Characterisation by EPMA and LA‐ICP‐MS

This paper describes a technique for the preparation of a titanite (CaTiSiO5) glass calibration material for use in in situ microanalysis of major, minor, and trace elements in geological materials. The starting composition was a titanite matrix doped with minor and trace elements at ∼ 200 μg g‐1. The elements Sc, Y, REEs, Th and U were added in the form of nitrates in solution, and the elements V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Hf and W were added as solid oxides. The synthetic titanite glass was produced by direct fusion by resistance heating in graphite electrodes at 1600‐1700 °C, and quenched in air. Backscattered electron images indicate good homogeneity, with no signs of separate phases or vesicles, and analysis of the major elements Ca, Ti and Si by electron microprobe showed relative standard deviations between 0.5 and 0.7%, based on six independent measurements. Deviations from nominal concentrations for Ca, Si and Ti were measured to ‐1.2, ‐3.3 and ‐0.8%, respectively. The homogeneity of the trace elements in the glass was assessed by LA‐ICP‐MS analyses, using NIST SRM 610, 612 and 616 as external calibrators, and Ca as the internal standard element. Determinations were made both with a quadrupole mass spectrometer and a sector field instrument, and both raster and spot modes of analysis were used. For the majority of doped elements, precision was better than 10%, and relative deviations from nominal values were, with few exceptions, between 5 and 10%.

COMPOSITION AND PARAGENESIS OF Na-, Nb- AND Zr-BEARING TITANITE FROM KHIBINA, RUSSIA, AND CRYSTAL-STRUCTURE DATA FOR SYNTHETIC ANALOGUES

We describe titanite with unusually high contents of Na, Nb, and Zr from a hydrothermal natrolite-rich vein cutting kalsilite‐ nepheline syenite at Mount Rasvumchorr, Khibina peralkaline complex, in the Kola Alkaline Province, Russia. This titanite is associated with astrophyllite, ceriobetafite, yttrobetafite, henrymeyerite, and banalsite. We recognize four generations of titanite: nearly stoichiometric titanite-I, titanite-II, containing up to 16.4 wt.% Nb2O5 (0.25 apfu Nb) and 3.2% Na2O (0.21 apfu Na), titanite-III, with up to 9% ZrO2 (0.15 apfu Zr) and zoned from 12.4 to 2.2% Nb2O5, and (Nb‐Zr)-poor titanite-IV, with up to 3.6% Al2O3 and 2.2% Fe2O3. In titanite-II, substitution of Nb at the octahedral site is accompanied by Na at the seven-fold site and, probably, by (F,OH) ‐ and vacancies. A compositional analogue of titanite-III, and synthetic titanite containing 0.25 apfu Zr, similar to the most Zr-rich titanite known, have been prepared by standard ceramic techniques, and their crystal structure determined by Rietveld refinement of powder X-ray-diffraction patterns. The synthetic variants doped with Zr, or with Zr, Na, and Nb, adopt space group A2/a, and consist of distorted YO7 polyhedra, XO6 octahedra, and SiO4 tetrahedra. The (Ca0.85Na0.15) (Ti0.70Zr0.15Nb0.15)OSiO4 analogue of natural titanite from the Rasvumchorr zeolite vein contains the least-distorted coordination polyhedra. The polyhedra in Ca(Ti0.75Zr0.25)OSiO4 are moderately distorted, and the most strongly distorted polyhedra are those in the CaTiOSiO4 end-member.

Crystal chemistry of titanite-structured compounds: the CaTi1-xZrxOSiO4 (x≤0.5) series

Inhomogeneous aggregates of late-stage titanite enriched in Zr have been described recently from post-magmatic parageneses in silica-undersaturated rocks. In the natural samples, simple isovalent substitution of the large Zr ([vi]R4+=0.72 A) for Ti ([vi]R4+=0.605 A) is limited to an empirical maximum of 0.25 afu (15.3 wt.% ZrO2). As the natural material is not suitable for crystallographic study, a series of CaTi1-xZrxOSiO4 titanite samples have been synthesized by standard ceramic methods at ambient pressure in air, and their crystal structure determined by Rietveld refinement of laboratory powder X-ray diffraction patterns. All of the synthetic Zr-doped titanite varieties adopt space group A2/a and consist of distorted CaO7 polyhedra together with less distorted (Ti1-xZrx)O6 octahedra and SiO4 tetrahedra. Cell dimensions and atomic coordinates together with volumes and distortion indices are given for all polyhedra. The empirical limit for Zr substitution in synthetic (F,OH)-free titanite is 0.5 afu (29.6 wt.% ZrO2). The existence of a Zr analogue of titanite in nature is considered to be unlikely.

Solid solution of rare earth elements in synthetic titanite: a reconnaissance study

Titanite varieties doped with rare earth elements (REE) have been prepared by ceramic synthesis and quenching in air. Their crystal structure was determined by Rietveld analysis of the powder X-ray diffraction patterns. Two different substitution schemes, Ca1− x Na x /2Sm x /2TiSiO5 and Ca1− x Dy x Ti1− x $Fe_{x^{3+}}$ SiO5, are studied at x = 0.2. Both synthetic varieties of titanite adopt space group A2/a. This implies that both single-site and complex multivalent substitutional schemes destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P21/a→A2/a phase transition. Unit cell dimensions obtained for the REE-bearing titanite varieties are as follows: a = 7.0541(1)A; b = 8.7247(1)A; c = 6.5664(1)A; β = 113.732(1)° for Ca0.8Na0.1Sm0.1TiSiO5; and a = 7.0021(1)A; b = 8.7256(1)A; c = 6.5427(1)A; β = 113.294(1)° for ${\rm Ca}_{0.8}{\rm Dy}_{0.2}{\rm Ti}_{0.8}{\rm Fe}_{0.2}^{3+} {\rm SiO}_5$ . Both REE-doped titanite samples and a control sample of the pure titanite end member have similar unit cell parameters and consist of polyhedra distorted to a similar extent with the exception of more-distorted SiO4 tetrahedron in CaSiTiO5. The structural data suggest that the Ca1− x Na x /2Sm x /2TiSiO5 and Ca1− x Dy x Ti1− x Fe x SiO5 solid solutions adopting the titanite structure might extend to x sufficiently greater than 0.2 and involve both heavier and lighter trivalent rare earth elements.

Spontaneous strain in synthetic titanite, CaTiOSiO4

AbstractLattice parameters of synthetic titanite powder, CaTiOSiO4, have been determined between room temperature and 1023 K. Only the e11 and e13 components contribute significantly to the strain tensor associated with the antiferroelectric-paraelectric phase transition at Tc = 487 K. A finite strain component e13 is observed in the paraelectric phase for 487 K < T < 825 K. The disappearance of this shear strain marks the isosymmetric transition near 825 K. The temperature evolution of the volume strain and of e11 is proportional to the squared order parameter observed in single-crystal diffraction experiments. The magnitude of the volume strain is sufficiently large to relate the observed near tricritical behaviour of the antiferroelectric-paraelectric phase transition to strain coupling.

Software development for studies of diffuse scattering using CCD-detectors and synchrotron radiation sources

A graphical-user-interface based software system was developed to cover advanced data processing requirements which arise from studies of diffuse scattering in disordered minerals using synchrotron radiation sources and CCD-detectors. The software includes interfaces to standard applications, procedures for numerical processing of large data sets, corrections for sample external scattering and detector-specific distortions, different scaling options to correct the data set against the varying primary beam intensity as well as procedures to reconstruct arbitrary slices in reciprocal space on a regular grid. The software system was successfully applied in studies of diffuse scattering in disordered REE-doped germanates, phase-transition studies of synthetic titanite and studies of the thermal recrystallization behaviour of radiation-damaged (metamict) minerals.

In situ powder diffraction study of titanite (CaTiOSiO4) at high pressure and high temperature

A set of powder diffraction data was collected for synthetic titanite (CaTiOSiO 4 ) at simultaneously high pressures and high temperatures in a P-T field between 275 K to 650 K and room pressure to 4.9 GPa, respectively. With these data it was possible to relate the A 2/ a high-pressure phase at >3.5 GPa (room temperature) to the A 2/ a high temperature phase observed above 825 K (room pressure). The slope of the phase transition is −180 K/GPa. The data also allowed the extraction of P-V-T equations of state with the following parameters: for P 2 1 / a: K 298,0 = 113.4(3), (∂ K T ,0 /∂ T ) P = −0.061(3) GPa/K, V 298,0 = 369.04(2) A 3 , α 0 = 2.07(5)/10 5 K. For A 2/ a: K 298,0 = 135.2(2), (∂ K T ,0 /∂ T ) P = −0.073(1) GPa/K, V 298,0 = 367.12(2) A 3 , α 0 = 2.8(2) / 10 5 K, where K is bulk modulus, V is volume, and α is thermal expansivity. A structural analysis based on Rietveld refinements revealed that the polymerized CaO 7 polyhedra dominantly affect the structural response to changing pressure and temperature. The TiO 6 octahedra rotate almost rigidly in response to the compression of the CaO 7 polyhedra with which they share edges. The SiO 4 tetrahedra show a strong angular distortion with only little change in bond lengths.

An infrared spectroscopic and single-crystal X-ray study of malayaite, CaSnSiO 5

Powder infrared spectroscopy and X-ray diffraction techniques on single crystals were used to study the thermal behaviour of malayaite, CaSnSiO5. Infrared spectra show a discontinuity in the temperature evolution of phonon frequencies and absorbance near 500 K. However, crystal structure data collected at 300, 450, 550, 670, and 750 K show no evidence of a symmetry-breaking phase transition and no split positions. The most obvious change with heating is a tumbling motion of the SnO6 octahedra and an increase of the anisotropic displacement factors of Ca. The thermal evolution of the mean-square vibrational amplitude of the Ca atom shows a pronounced change in slope near 500 K. The evidence suggests that the 500 K anomaly in malayaite is more similar in character to the 825 K (β-γ) transition as opposed to the 496 K (α-β) transition in synthetic titanite.

A high temperature diffraction study of synthetic titanite catiosio4

The results of a high temperature single crystal X-ray diffraction study of synthetic titanite within the stability field of the A2/a (C2/c) paraphase (T > 500 K) are reported. The structure has been refined using a conventional model and one in which the Ca atom is disordered over two positions. A break in thermal expansion near T c = 825 K correlates with an effective volume contraction of the Ti octahedron. When refined with split Ca position a reorientation of the Ca displacement vector at T c is found, resulting in a more symmetrical structural arrangement of the disordered Ca cations with respect to the surrounding Ti cations. In the conventional model this reorientation is seen as a break in the thermal elongation of the shortest Ti-Ca distance. The observed temperature of the isosymmetrical structural instability is in agreement with previous observations based on Hard Mode IR and Raman spectroscopic measurements. A possible mechanism causing the observed structural changes and similarities to the thermal behaviour of the iso-structural malayaite, CaSnSiO5, are discussed.

The β-γ phase transition in titanite and the isosymmetric analogue in malayaite

X-ray diffraction technique and hard mode infrared spectroscopy were used to study the thermal behaviour of synthetic titanite, CaTiSiO5, two natural titanite samples and iso-structural malayaite, CaSnSiO5. Both pure minerals show a thermal anomaly near 500 K. Specific heat measurements show an intermediate phase in synthetic titanite. Impurities in titanite tend to suppress the discontinuity near 500 K. The β-γ transition near 820 K is not affected by Fe and Al defects in the natural material. Infrared studies in malayaite show a change of the temperature dependence of the phonon frequencies. X-ray experiments show weak structural rearrangements with heating but no evidence of a symmetry breaking phase transition near 500 K The character of the 500 K anomaly in malayaite is more similar to the β-γ transition (825 K) than the α-β Mtransition (496 K) in synthetic titanite.

Incorporation of rare earth elements in titanite; stabilization of the A2/a dimorph by creation of antiphase boundaries

The atomic arrangement of a natural rare-earth-rich titanite and two synthetic rare-earth-doped titanites have been refined in space group A2/a, and the atomic arrangement of an undoped P2 1 /a synthetic titanite was also refined for comparison. Previous work has shown that titanite possesses a domain structure, with domains formed of like-displaced Ti atoms in the [100] octahedral chains. P2 1 /a titanite results when the crystal is formed of a single domain, but as Ti-reversal sites occur in the octahedral chain the apparent A2/a structure results from the average of antiphase domains. Antiphase boundaries occur at O1, which is alternately overbonded or underbonded at the boundaries, depending on the displacement of the neighboring Ti atoms. Type 2 antiphase boundaries exist where two Ti atoms are displaced away from the intervening O1 atom and are energetically unfavorable because of underbonding of that O1 atom. However, substitution of a trivalent rare earth element in the adjacent Ca (super 2+) site relieves that under-bonding, favoring the creation of type 2 antiphase boundaries and stabilization of the A2/a dimorph. The results of high-precision crystal structure analyses demonstrate that rare earth substituents for Ca stabilize the A2/a dimorph at lower substitution levels than required for octahedral substitutions.

Radiation damage in natural titanites

Natural titanites which have incurred radiation damage from the decay, over geologic time, of U and Th incorporated in the samples have been studied by powder X-ray diffraction, differential thermal analysis and infrared spectroscopy. In agreement with previous studies of ionirradiated synthetic titanite, X-ray diffraction indicated that titanite is two to three times more sensitive than zircon to damage from α-decay processes. Differential thermal analysis of samples which were deduced to have accumulated 10–30 percent of the α-fluence required to render the structure completely X-ray amorphous gave a series of exothermic peaks in the 200–800° C range of temperature. In such samples, X-ray and density measurement showed that atomic displacement damage anneals during heating periods of the order of one h in the temperature range 500–800° C. The density of titanite rendered X-ray amorphous by irradiation was estimated to be ∼8 percent lower than that of crystalline titanite.