Titanites

Sphen, m. sphene. titanite. sphenoidisch, a. sphenoid, sphenoidal, spicken, v.t. lard smoke oil (wool). 

Average of reported values for zircon, andalusite, silli-manite, staurolite, topaz, titanite, thortveitite, muscovite, apophyllite, hardystonite, analcite, carnegieite, sodalite, danburite, scapolite, and cristobalite mean deviation 0.02 A. 6 Average for BP04 (1.54 A.) and KHjPO, (1.56 A.). For KjSOi and other sulfates. i For Mg(C104)-6H,0. 

Other accessories that may play an important role in the fractionation of some U-series elements, include, monazite, apatite, allanite, titanite, thorite and chevkinite. Hermann (2002) has recently determined experimentally the partitioning of U, Th and lanthanides between allanite and granitic melt at 2.0 GPa and 900°C. He finds D ] = 20 and Z)tii = 60, confirming that allanite can play an important role in controlling U-Th budgets in silicic melts. The very high Z La in the same experiment (-200), indicates that allanite will also be an important host for Bi and Ac. 

Titanium is the most abundant metal in the earth crust, and is present in excess of 0.62%. It can be found as dioxy titanium and the salts of titanium acids. Titanium is capable of forming complex anions representing simple titanites. It can also be found in association with niobium, silicates, zircon and other minerals. A total of 70 titanium minerals are known, as mixtures with other minerals and also impurities. Only a few of these minerals are of any economic importance. 

Titanium - the atomic number is 22 and the chemical symbol is Ti. The name derives from the Latin titans, who were the mythological first sons of the earth . It was originally discovered by the English clergyman William Gregor in the mineral ilmenite (FeTiOj) in 1791. He called this iron titanite menachanite for the Menachan parish where it was found and the element menachin. It was rediscovered in 1795 by the German chemist Martin Heinrich Klaproth, who called it titanium because it had no characteristic properties to use as a name. Titanium metal was first isolated by the Swedish chemists Sven Otto Pettersson and Lars Fredrik Nilson. 

Several minerals have been identified in polished thin sections of various rock types including massive sulphide, gossan, intermediate crystal-lapilli tuff, and felsic ash tuff (Fig. 2). These minerals include apatite, monazite, zircon, allanite, titanite, xenotime, magnetite, cassiterite, cobaltite-gersdorffite, rutile, ilmenite, goethite, sphalerite, galena, arsenopyrite, chalcopyrite, pyrite, and pyrrhotite. These minerals range in size from 20 pm to 250 pm and represent potential Indicator minerals. 

The rocks of the NS-N dyke group range from nepheline syenite to sodalite-bearing nephelinolite. The primary phases in the NS-N dyke group are k-feldspar and nepheline, with variable abundances of amphibole, plagioclase, phlogopite, calcite, sodalite, titanite, cancrinite, apatite, clinopyroxene, zircon, chlorite, quartz, pyrochlore, and opaque phases. 

Retrograde metamorphic processes, linked to hydrothermal fluid circulation, finally produced a new mineral assemblage constituted mainly by tremolite-actinolite> epidote + chlorite + quartz + sericite + titanite + hematite. The amphibole appears as green fibrous crystals over clinopyroxene and other anhydrous minerals. 

Biotite and magnetite are also usually present and visible in hand specimen, muscovite may be present, and more rarely other oxides may be seen. Field estimates of modes ranged from 20-35 vol.% quartz, 15-35 vol.% plagioclase, 30-50 vol.% potassium feldspar, and 1-10 vol.% biotite. Accessory minerals include magnetite, muscovite, monazite, xenotime, zircon, apatite, epidote, ilmenite, titanite, allanite, molybdenite, and galena. The major U and Th minerals are uraninite and uranothorite. 

Titanium occurs in nature in the minerals rutile( Ti02), ilmenite (FeTiOs), geikielite, (MgTiOs) perovskite (CaTiOs) and titanite or sphene (CaTiSi04(0,0H,F)). It also is found in many iron ores. Abundance of titanium in the earth s crust is 0.565%. Titanium has been detected in moon rocks and meteorites. Titanium oxide has been detected in the spectra of M-type stars and interstellar space. 

Figure 4-34 is a phase diagram for the system titanite-anorthite. Suppose a crystal of titanite is initially in contact with a crystal of anorthite. The two are heated to 1350°C. Either phase by itself would not melt. But because the temperature is higher than the eutectic point of the two phases, at the interface there is melting. As melting proceeds, a thin melt layer would form between the two crystals. The melting of the two phases continues and the rate may be controlled by different factors. The rate would depend on the controls, as outlined below. 

Figure 4-34 Phase diagram of the titanite-anorthite system calculated using thermodynamic data of Robie and Hemingway (1995). The horizontal axis is mass fraction of An. At 1350°C, titanite is in equilibrium with a melt of composition A, and anorthite is in equilibrium with a melt of composition B. In other words, melt A is saturated with respect to titanite, and melt B is saturated with respect to anorthite.

Figure 4-35 Concentration profile during melting of two crystals controlled by (a) interface reaction and (b) diffusion in the melt. The open circles indicate melt compositions in equilibrium with titanite (position A in Figure 4-34) and with anorthite (B in Figure 4-34).

If there are many grains of anorthite and titanite in a melt matrix, then the total mass dissolution rate of anorthite is... 

If the melting is controlled by diffusion in the melt (which means extremely rapid interface reaction rate), the melt composition at the titanite-melt interface would be A, and that at the anorthite-melt interface would be B (Figure 4-35b). Treat the diffusion as binary diffusion. The diffusive flux across the melt at steady state is... 

Figure A4-1 Comparison of oxygen diffusivity in various minerals under hydrothermal conditions- Mineral names (from high to low diffusivity) An, anorthite Ah, albite Bt, biotite Ms, muscovite Phi, phlogopite Cc, calcite Qz, quartz Ap, apatite Mt, magnetite Hb, hornblende Tr, tremolitel Tt, titanite Di, diopside Rut, rutile Aim, almandine.

Zhang X.Y., Cherniak D.J., and Watson E.B. (2006) Oxygen diffusion in titanite lattice diffusion and fast-path diffusion in single crystals. Chem. Geol. 235, 105-123. 

Titanite (formerly called sphene ) is an orthosihcate mineral with the chemical formula CaTi0Si04, where approximately 20% of the oxygens can be partially replaced by OH (hydroxyl) and F. Titanite has a monochnic symmetry... 

The natural titanite in our study consisted of nine samples. Concentrations of potential luminescence impurities in one sample are presented in Table 4.12. The laser-induced time-resolved technique enables us to detect Sm +, Nd, Tm ", Pr ", Er ", Eu " and Cr " emission centers (Figs. 4.33-4.34). 

Fig. 4.33. a-d Laser-induced time-resolved luminescence spectra of titanite demonstrating Cr, Eu and Sm centers... 

Table 4.12. Concentrations of rare-earth elements in titanite sample (ppm)...

Fig. 4.36. Luminescence (right) and excitation (left) spectra of titanite (1), vuon-nemite (2), epistolite (3), natisite (4), penkvUsite (5), Ti-silicate (6), vino-gradovite (7), leucosphenite (8), Ti-carbonatesUicate (9), fersmanite (10) (Gaftetal. 1981)...

Exciting closed shell transition metal complexes such as tungstates, molybdates, vanadates, titanites etc. This way of excitation and its transfer to the rare-earth elements has been known since the late fifties. 

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