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Flux Growth and State Analysis of Ilmenite and Pyrophanite

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Flux Growth and State Analysis of Ilmenite and Pyrophanite J. Japan. Assoc. Min. Pety. Econ. Geol. 78, 229-238, 1983 Flux growth and state analysis of ilmenite and pyrophanite YOSHIROOHTSUKA* Instituteof Geoscience,The Universityof Tsukuba, Ibaraki305, Japan YOSHINORIFUJIKI NationalInstitute for Researchesin InorganicMaterials, Ibaraki 305, Japan ANDYOSHIO SUZUKI Instituteof Geoscience,The Universityof Tsukuba,Ibaraki 305 , Japan Ilmenite and pyrophanite crystals were grown by the isothermal evaporations of an Na2B4O7 flux melt under various oxygen fugacities controlled by the mixed gas of CO2 and H2. Most crystals obtained were tabular with well-developed {0001} faces . Ilmenite crystals grown under higher fo2 conditions contain Fe3+ ions in several percents , and have smaller cell dimensions than those grown under lower fo e conditions. Colors of pyrophanite crystals also seem to be affected by foe conditions, and cell constants decrease with increas ing foe, indicating that Mn3+ ions exist in pyrophanite grown under higher fo e conditions.I ntensity ratio (LƒÀ/Lƒ¿) of the Fe L (or Mn L) X-ray emission line slightly decreases for ilmenite (or pyrophanite) crystals grown under higher fo2 conditions , which seems to be consistent with the suggested existence of Fe3+ (or Mn3+) in ilmenite (or pyrophanite) crystals. Introduction Three solid solution series exist in Lindsley, 1964; Matsuoka, 1971; Lindh, the system Fe-Ti-O; hexagonal ilmenite 1972; Lindsley and Lindh, 1974). Budd (FeTiO3)-hematite (Fe2O3) series (a series), ington and Lindsley (1964) reported that ƒ¿ cubic ulvospinel (Fe2TiO4)-magnetite(Fe3O4) and ƒÀ series may be used as a geothermom series (ƒÀ series) and orthorhombic pseudo eter and an oxygen barometer according brookite (FeTi2O6 Fe2TiO5) series (ca series). to the following equation. Minerals of the ƒ¿ and ƒÀ series are common X Fe2TiO4+(1-X)Fe3O4+1/4O2 accessory minerals in igneous and meta =XFeTiO3+(3/2-X)Fe2O3 morphic rocks, and are important for rock magnetism and redox parameter. The temperatures and oxygen fugacities Phase relations in the join FeO-Fe2O3- during the solidification process of a magma TiO2 have been studied under I atm (Web may be evaluated on the basis of chemical ster and Bright , 1961; Taylor, 1963, 1964) compositions of coexisting iron-titanium and hydrothermal conditions (Lindsley , oxides in an igneous rock (Carmichael et al., 1962, 1963, 1965, 1973; Buddington and 1974). Therefore, it will be useful if the (Manuscript received February 28, 1983) * Present address: Low Level Waste Management Laboratory , Division of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki. 230 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki effect of oxygen fugacity upon the properties of these crystals is clarified in the system Fe-Ti-O. Pyrophanite and ilmenite can form a complete series of solid solutions (Deer et al., 1962). Content of MnO in ilmenite decreases with increasing temperature (Tsusue, 1973; Newmann, 1974). Recently, single crystals of ilmenite have been synthesized for interest in electri cal properties (Ginley and Baugman, 1976), and pyrophanite crystals for interest in magnetic properties (Stickler and Hell, 1962; Baruskova et al., 1973; Wanklyn et al., 1976). Fig. 1. Diagram showing stability of various In the present study, single crystals of Fe-oxides and Mn-oxides as a function ilmenite and pyrophanite were grown by of oxygen fugacity and temperature, and the relation among oxygen fuga the isothermal evaporation of the Na2B4O7 city, CO2/H2 ratio and temperature flux melt under various oxygen fugacities of under total pressure 1 atm (after steady state, and the effect of oxygen fuga Muan, 1965). city on each crystal was examined. (1:1 in molar ratio). Reaction time of crystal growth was 240-340 hours. The Experimental identification of products was made by X- In this study, oxygen fugacity was ray powder diffraction. controlled by the mixed gas flow method Cell constants of these crystals were (Darken and Gurry, 1945). In Fig. 1, the calculated from the data of X-ray powder fo2 variance curves with temperature at diffraction. The chemical analyses and the different CO2/H2 ratios and the equilibrium chemical state analyses were made with curves of Fe/FeO, FeO/Fe3O4 and Fe3O4/ EPMA system in the Chemical Analysis Fe2O3 (or Mn/MnO and MnO/Mn3O4) are Center of the University of Tsukuba. This shown. Ilmenite and pyrophanite crystals system is composed of the JEOL JXA-50A were synthesized by isothermal evaporation EPMA and ELIONIX ACPS-XR computer of the Na2B4O7flux melt using a 30ml Pt- control system with a DEC PDP-11 mini crucible under the various controlled oxygen computer, and is controlled by a modified fugacities. The grown crystals were sep BASIC language "EASS". arated by dissolving flux in HNO3 solution. Firstly, Fe2O3and TiO2 powders (1:2 in Results and discussion molar ratio), and MnO2 and TiO3 powders 1) Syntheses of ilmenite (1:1 in molar ratio) were prepared as starting Fig. 2 shows the results of syntheses materials for the synthesis of ilmenite and of ilmenite. Under lower fo, conditions pyrophanite, respectively. Each starting ilmenite coexists with rutile, while under material was mixed well with Na2B4O7flux higher fo2 conditions only pseudobrookite Flux growth of ilmenite and pyrophanite 231 is formed. The phase boundary between rutile abounds in lower fo2 region. ilmenite and pseudobrookite is parallel to Most ilmenite crystals are tabular in the fog variance curve at a fixed ratio of shape and are black in color. Generally, CO2/H2 with temperature (Fig. 2). This the size of ilmenite crystals increases as the boundary is approximately parallel to the FeO/Fe3O4 equilibrium curve shown in Fig. 1. The coexistence of rutile and ilmenite indicates that TiO2 component be comes more excess than FeO in the reaction of FeO with TiO2. It seems that the solubilities of the two components in the Na2B4O7flux vary with fo2, respectively. This is supported by the observations that Fig. 2. Results of ilmenite syntheses by isothermal evaporation method under Fig. 3. Cell constants of ilmenite grown under the various oxygen fugacities and the various fo2 conditions in the temperatures. Na2B4O7 flux melt. Table 1. Cell constants of ilmenite grown in the Na2B4O7 flux melt at various fo2 232 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki decrease of growth temperature, attaining 71: fo2=10-16.3 atm), while it is larger up to 1-2mm at 900•Ž. The tabular than 1.0 in those grown at higher fo2 (sample crystals have well-developed {0001} face PFT-75: fo2=10-14.1 atm). The increase similarly as those synthesized by the slow of Fe/Ti value can be interpreted by the cooling method (Fujiki and Ohtsuka, 1977). existence of Fe3+ (hematite component). Fig. 3 shows the cell constants of The composition of hematite-ilmenite ilmenite crystals against fo2. Both ƒ¿ and c solid solution series may be expressed dimensions show tendencies to increase with Fe3+2-2xFe2+xTixO3, where x is the mole lowering fo2, from 5.085 A to 5.092 A and fraction of ilmenite. Mole fraction of ilme from 14.08 A to 14.10 A, respectively (Table nite in sample (PFT-75) is calculated to be 1). Lindsley (1965) reported that the cell 0.97 from Fe/Ti=1.06. The results of wet dimensions of Fe2O3-FeTiO3 series decrease chemical analysis indicate that sample with the increase of Fe2O3 component. (PFT-75) contains 3.6 wt% Fe3+, and Accordingly, it can be considered that the sample (PFT-71) and (PFT-76) contain less increase of Fe3+ in ilmenite crystals causes amount of Fe3+ ions (Table 2). The varia a tendency to decrease the cell dimensions tion of cell dimensions is concordant with in this experiment. the increase of Fe3+ in ilmenite crystals. The composition of ilmenite crystals The value of Fe/Ti measured with EPMA grown at 900•Ž was analyzed by both EPMA for sample (PFT-75) varies widely as com and wet chemical analysis (Table 2, Fig. 4). pared with sample (PFT-71) or (PFT-76) The average value of Fe/Ti, which is calculat (Fig. 4). The S.D. (standard deviation) ed as Fe2+ and Ti4+ for convenience from for the data of PFT-75 is 0.04, while it is the results of EPMA analyses, of each 0.01 in sample (PFT-71) and (PFT-76). crystal is shown in Fig. 4. The Fe/Ti This may be inferred to the heterogeneity ratio of ilmenite crystals grown at lower of Fe3+ distribution in ilmenite crystals. fo2 is nearly equal or smaller than 1.0 From these results, most suitable condi (sample PFT-76: fo2=10-18.4 atm, PFT- tion of fo2 to synthesize the ilmenite crystal was found to be C02:H2=1:1. 2) Syntheses of pyrophanite The results on pyrophanite are shown Table 2. Chemical compositions of ilmenite grown at 900•Ž at three different fo2 Fig. 4. The value of Fe/Ti calculated as Fe2+ and Ti4+ for convenience about each ilmenite grown under three kinds of fo2 conditions at 900•Ž. Flux growth of ilmenite and pyrophanite 233 to be narrower than that in the system Mn-O. Furthermore, in order to examine the relation between fo2 and the color of crystals, both crystals were heated under the controlled fo2 conditions (Table 3). Heat-treatment in higher fo2 region caused the color change from green to reddish brown. But reddish brown color did not change to green by heating in lower fo2 region. However, when these crystals were heated under the conditions of H2 gas flow, Fig. 5. Results of pyrophanite syntheses by the reddish brown color changed to brown isothermal evaporation method under the various oxygen fugacities and ish green. These observations suggest that temperatures. the color changes of pyrophanite crystals are caused by the valence states of Mn in Fig.
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