Titanite structure

Sphene-based glass ceramics. These ceramics are composed of titanite as the major crystalline phase (other minor phases may be present) and interstitial Na-Al-Si glass. Importantly, thermodynamic calculations indicate that titanite is stable in the Ca-Na-Cl brines, typically encountered in the Canadian Shield. Furthermore, analyses of natural specimens indicate that the titanite structure is able to accommodate ACTs, REEs, Na, Mn, Sr, Ba in the Ca-site, and Fe and... 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Titanite, CaTiSi05 (C2/c), may incorporate Na+, REE3+ and minor ACT in the Ca site, and Fe3+, Al3+, Nb5+ in the Ti site. The ThOz content of natural titanite may reach 0.28 wt% (Hayward 1988). Measured isomorphic capacities of titanite with respect to U4+, Pu4+, Pu3+, Hf4+, and Gd3+ are (in atoms per formula unit) 0.02-0.05 (Vance et al. 2000) or 0.07 (Stefanovsky et al. 2000b) 0.02 0.05 0.5 0.3 (Vance et al. 2000) or 0.25 (Stefanovsky et al. 2000b), respectively. Due to limited solubility of ACTs and REEs in the structure, titanite is preferably considered as a host phase for these elements when their content is low, such as in titanite-based glass-ceramics developed for Canadian waste (Hayward 1988). 

On the other hand, the role of both, binders for stabilizing structure and the graphite for increasing porosity was analysed. A literature search [18] showed that the most common binders for these reactions were the bentonite and natural silicates (2-4% w/w) with 1,5-3 Kg/mm in order to provide some cohesion and increase the mechanical strength to the zinc titanite and zinc ferrite particles. In the second step, several supported sorbents (10% of active phase) have been prepared and characterised. 

Figure 4 Steady-state temperature structure beneath a periodic topography as calculated using the algorithms of Mancktelow and Grasemann (1997). This simulation assumes a topographic relief of 1.5 km, a topographic wavelength of 20 km, and a uniform denudation rate of 1 mm yr. The topmost sinusoidal line indicates the land surface (defined as having a temperature of 0 °C). Shaded contours are steady-state isotherms from 50 °C to 350 °C at 50 °C intervals. The red contours represent nominal bulk closure isotherms for the muscovite Rb/Sr, apatite fission track, and apatite and titanite (U-Th)/He thermochronometers. Positions A and B and their dashed unroofing paths...

Figure 10. Projection of the structure of titanite (CaTiSiOs) on the (001) plane. Ti cations within octahedral chains parallel to a are interconnected via silica tetia-hedra. Spheres are Ca cations.

Heaney PJ (1994) Structure and chemistry of the low-pressure silica polymorphs. Rev Mineral 29 1-40 Heaney PJ, Salje EKH, Carpenter MA (1991) A dielectric study of the antiferroelectric to paraelectric phase transition in synthetic and natural titanites. (abstr) Trans Am Geophys Union Eos 72 554 Heaney PJ, Veblen DR (1990) A high-temperature study of the low-high leucite phase transition using the transmission electron microscope. Am Mineral 75 464-476 Heaney PJ, Veblen DR (1991b) Observation and kinetic analysis of a memory effect at the a-p quartz transition. Am Mineral 76 1459-1466... 

Taylor M, Brown GE (1976) High-temperature structural study of the Pl a to AHa phase transition in synthetic titanite, CaTiSiOs. Am Mineral 61 435-447 Teufer G (1962) Crystal stracture of tetragonal Z1O2. Acta Crystallogr 15 1187... 

Synthetic titanite, CaTiSiOs, undergoes structural phase transitions near 500 K and ca. 825 K (Salje et al. 1993 Zhang et al. 1996 Kek et al. 1997 Malcherek et al. 1999a). The phases were described by Chrosch et al. (1997) as a for the room-temperature... 

Figure 5. Crystal structure of titanite projected onto the ab-plane in the P21/a phase. The arrows indicate the anti[arallel Ti-displacements in neighbouring Ti06 chains. Made with CrystalMaker, a crystal structures program for Macintosh computers.

Brace AD, Taylor W, Murray AF (1980) Precursor order and Raman scattering near displacive phase transitions. J Phys C Solid State Phys 13 483-504 Brace A, Cowley RA (1981) Structural phase transitions. Taylor and Francis, London Chrosch J, Bismayer U, Salje EKH (1997) Anti-phase boundaries and phase transitions in titanite an X-ray diffraction study. Am Mineral 82 677-681... 

Higgins JB, Ribbe PH (1977) The structure of malayaite, CaSnSi04, a tin analog of titanite. Am Mineral 62 801-806... 

Hollabaugh CL, Foil FF (1984) The crystal structure of Al-rich titanite from Grisons, Switzerland. Am Mineral 69 725-732... 

Etude par diffusion inelastique des neutrons. J Phys 40 1185-1194 Kek S, Aroyo M, Bismayer U, Schmidt C, Eichhom K, Krane HG (1997) Synchrotron radiation study of the crystal structure of titanite (CaTiSiOs) at lOOK, 295K and 530K Model for a two-step stractrual trarrsitioa Z Kristallogr 212 9-19... 

Meyer H-W, Bismayer U, Adiwidjaja G, Zhang M, Nistor L, Van Tendeloo G (1998) Natural titanite and malayaite Structural investigations and the 500 K anomaly. Phase Transitions 67 27-49 Petzelt J, Dvorak V (1976) Changes of infrared and Raman spectra induced by structural phase transitions 1. General considerations. J Phys C Solid State Phys 9 1571-1586... 

Salje EKH, Graeme-Barber A, Carpenter, MA, Bismayer U (1993) Lattice parameters, spontaneous strain and phase transitions in Pb3(P04)2. Acta Ciystallogr B49 387-392 Salje EKH, Schmidt C, Bismayer U (1993) Structural phase transition in titanite, CaTiSiOs A Ramanspectroscopic study. Phys Chem Miner 19 502-506 Speer JA, Gibbs GV (1976) Crystal structure of synthetic titanite CaTiSiOs, and the domain textures of natural titanites. Am Mineral 61 238-247... 

Zhang M, Salje EKH, Bismayer U (1996) Structural phase transition near 825 K in titanite evidence from infrared spectroscopic observation. Am Mineral 82 30-35 Zhang M, Meyer H-W, Groat LA, Bismayer U, Salje EKH, Adiwidjaja G (1999) An infrared spectroscopic and single-crystal X-ray study of m ayaite, CaSnSiOs. Phys Chem Miner 26 546-553... 

Farges F, Ewing RC, Brown GE Jr. (1993) The stracture of aperiodic, metamict (Ca,Th)ZrTi207 (zirconolite) An EXAFS study of the Zr, Th, and U sites. J Mater Res 8 1983-1995 Fleet ME, Henderson GS (1985) Radiation damage in natural titanite by crystal structure analysis. Mater Res Soc Symp Proc 50 363-370... 

A considerable number of materials called titanates are known, some of which are of technical importance. Nearly all of them have one of the three major mixed metal oxide structures (page 54), and indeed the names of two of the structures are those of the titanium compounds that were the first found to possess them, namely, FeTi03, Hmenite, and CaTi03, perovskite. Other titanites with the ilmenite structure are MgTiOa, MnTi03, CoTi03... 

Titanite (formerly called sphene ) is an orthosilicate mineral with the chemical formula CaTi0Si04, where approximately 20 % of the oxygens can be partially replaced by OH (hydroxyl) and F. Titanite has a monoclinic symmetry with a space group P2i a. It has optically positive character with a= 1.84—1.95, P= 1.87—2.034, 7 = 1.943—2.11, very high refractive indices (1.843-1.950) and extreme birefringence (0.100-0.192). The structure of titanite consists of chains of octahedral sites occupied by Ti" ", cross-linked by isolated Si04 tetrahedra. Large... 

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