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. 5. All products in these experiments atoms in the crystals. were identified to be pyrophanite by X-ray The size of pyrophanite crystals attains powder diffraction. These crystals are about 2•`3mm when crystals are synthe tabular with well-developed {0001} face, sized at 950•Ž, but above 950•Ž crystals are similar to ilmenite, and are reddish brown much smaller. and green in color. The colors vary de The cell constants of pyrophanite pending on only fo2: those grown in higher crystals are shown in Table 4 and Fig. 6. fo2 than 10-12 atm are reddish brown, Although ƒ¿ dimensions vary only slightly, whereas those grown in the range from c dimensions increase as lowering fo2.

10-12 to 10-16 atm fo2 show either color, Taking the color changes of pyrophanite and those grown in lower fo2 than 10-16 atm crystals into consideration, the decrease of are green. In green crystals, Mn atoms c dimension in higher fo2 region may be exist in divalent state, because MnO is inferred to the existence of Mn3+ ions in green, while reddish brown crystals may crystals. contain a small amount of Mn3+ ions. Fig. 7 shows the results of EPMA

From Fig. 5, it is seen that the stability analysis of pyrophanite crystals grown at field of Mn2+ in the system Mn-Ti-O seems 900•Ž (sample PFT-37: fo2=10-13.1 atm),

Table 3. Color changes of pyrophanite by heating under various oxygen fugacities 234 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki

Table 4. Cell constants of pyrophanite grown in the Na2B4O2 flux melt at various fo2

Fig. 7. The value of Mn/Ti calculated as

Mn2+ and Ti4+ for convenience about each pyrophanite grown under three kinds of fo2 conditions at 950•Ž.

PFT-41: fo2=10-15.2 atm, PFT-31: fo2= 10-17.4 atm). The values of Mn/Ti are about 1.00, but a weak tendency is noticed that the values increase as increasing fo2. This is similar as in the case of Fe/Ti values Fig. 6. Cell constants of pyrophanite grown under the various fo2 conditions in in ilmenite, although the variation is much the Na2B4O7 flux melt. smaller. It follows from this that the Flux growth of ilmenite and pyrophanite 235

content of Mn3+ ions is very low, though Table 5, Peak intensity ratio of Fe LƒÀ to Fe

it may drastically change the color and the Lƒ¿ line on ilmenite grown under three kinds of fo2 conditions at cell dimension. 900•Ž. These values are in average ones of measurements of 4•`6 times. 3) Changes in X-ray emission spectra

X-Ray emission spectra or fluorescent

spectra are very informative in assigning

the electronic structures due to chemical

bond. Fischer(1965) showed the changes

in the L-emission spectra by oxidation of

the first-series transition metals. It was

shown that the wavelengths of the Fe La

and Lf and also Mn Lƒ¿ and LƒÀ lines shift

to the shorter side. Albee and Chodos (1970)

reported that the relative peak intensity of

the Fe Lƒ¿ and the Fe LƒÀ(or Mn Lƒ¿ and Mn

LƒÀ) X-ray emission line can provide semi

quantitative information on Fe2+: Fe3+ (or

Mn2+: Mn3+) ratios in oxides and silicates . According to their results, intensity ratio

of Fe LƒÀ to Fe Lƒ¿ line (Fe LƒÀ/Lƒ¿ value)

decreases according to the increase of Fe3+ Fig. 8. Peak intensity ratio of Fe LƒÀ to Fe ions. Furthermore, the Fe Lƒ¿ and LƒÀ X- Lƒ¿ line on ilmenite grown under ray emission spectra of Fe and Fe-Ti oxides three kinds of fo2 conditions at 900•Ž.

have been investigated in detail at various operating voltages (O'Nions and Smith sec. was obtained at each wavelength. 1971; Smith and O'Nions, 1971; Pavicevic The influence of contamination on the sur et al., 1972). face of sample was decreased by a cold trap In the present study, evaluation of the method. Interference from high order Fe K oxidation state has been tried by measuring line and Ti K line were removed by pulse the Fe L or Mn L emission spectra for Fe height analysis. and Mn in ilmenite and pyrophanite , re The results of measurements of the spectively. These spectra were obtained Fe L line are shown in Table 5 and Fig. 8. by using defocused beam(•`40ƒÊm) of 0 .3 These values are averaged ones of measure

ƒÊA beam current at 15kV (Ohtsuka et al., ments of several times (4-6 times). The

1979). The X-ray intensity was counted measured samples of ilmenite are grown at for 10 sec. each wavelength at intervals of 900•Ž (sample PFT-75, PFT-71, PFT-76). about 0.005 A (near the peak position) or These three ilmenite crystals have similar about 0.024 A (far the peak position) from variety in wavelengths of the Fe La line. about 17.1 A to 17 .8 A in the Fe L line These wavelengths differ apparently from

(from 19.0 A to 19.7 A in the Mn L line), those of Fe2O3 and Fe2TiO6 (Table 6). The and this measurement was repeated 10 LƒÀ/Lƒ¿ value is found to differ very times. Then , the X-ray intensity for 100 slightly, that is, the LƒÀ/Lƒ¿ value of 236 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki

Table 6. Wavelengths of Fe L emission lines on Fe-metal and Fe-Ti oxides

Fig. 9. Peak intensity ratio of Mn LƒÀ to Mn Lƒ¿ line on pyrophanite grown under three kinds of fo2 conditions at 950•Ž.

Table 7. Peak intensity ratio of Mn LƒÀ to Mn according to the study of Albee and Chodos Lƒ¿ line on pyrophanite grown under (1970). three kinds of fo2 conditions at 950•Ž. These values are in average It is possible to evaluate the oxidation ones of measurements of 4•`6 times state in Fe- or Mn-oxides to some extent by a designed measurement.

Acknowledgements: The authors are very grateful to Mrs. Michiko Kobayashi of National Institute for Research in Inorganic Materials for the chemical analy ses of ilmenite crystals. ilmenite grown under at higher fo2 (sample

PFT-75) tends to be smaller (Fig. 8). References This tendency is concordant with the ex istence of Fe3+: sample (PFT-75) contains Albee, A. A. and Chodos, A. (1970), Semiquantita tive electron microprobe determination of 3.6 wt% Fe3+ ions (Table 2). It is signifi Fe2+/Fe3+ and Mn2+/Mn3+ in oxides and sili cant that the tendency of decrease of cates and its application to petrologic problems. LƒÀ/Lƒ¿ value is observed, although the Amer. Mineral., 55, 491-501. Baruskova, M.L., Kuznetsov, V. A. and Malin decrease is only slightly. ovskaya, E. K. (1973), Crystallization of As to pyrophanite crystals (sample divalent metal titanates in high temperature PFT-37, PFT-41, PFT-31), the same solutions. Soviet Phys. Cryystallogr., 17, 1113- 1115. measurements as ilmenite were carried out Buddington, A. F. and Lindsley, D. H. (1964), (Table 7, and Fig. 9). The three pyrophanites Iron-titanium oxide minerals and synthetic have similar wavelength in Mn Lƒ¿ line. equivalent. J. Petrol., 5, 310-357. Carmichael, I. S. E., Turner, F. J. and Verhoogen, A difference of LƒÀ/Lƒ¿ value is observed J. (1974), Igneous Petrology., McGraw-Hill, among them (Fig. 9). Pyrophanite grown New York, 739p. at higher fo2, which is reddish brown in Darken, L. S. and Gurry, R. W. (1945), The system color, has smaller LƒÀ/Lƒ¿ value than that iron-oxygen: I, The wustite field and related equilibria. J. Amer. Chem. Soc., 67, 1398- grown at lower fo2. This tendency may be 1412. interpreted by the existence of Mn3+ ion, Deer, W. A., Howie, R. A. and Zussman, J. (1962), Flux growth of ilmenite and pyrophanite 237

RockForming Minerals, vol. 5, Non-silicates., puter controlled EPMA. Advan. X-ray Chem. John Wiley, New York. Anal. Japan, 11, 13-17, (in Japanese). Fischer, D.W. (1965), Changesin the soft X-ray O'Nions, R. K. and Smith, D. G. W. (1971), In L emissionspectra with oxidation of the first vestigations of the LII,III, X-ray emission series transition metals. J. Appl. Phys., 36. spectra of Fe by electron microprobe. Part

2048-2053. 2. The Fe LII,III spectra of Fe and Fe-Ti Fujiki,Y. and Ohtsuka, Y. (1977),Flux growth of oxides. Amer. Mineral., 56, 1452-1463. ilmenite and pyrophanite under controlled Pavicevic, M., Ramdohr, P. and El Goresy, A.

oxygenfugacity. J. Japan. Assoc.Min. Petr. (1972), Electron microprobe investigations of Econ. Geol.,72, 394-397. the oxidation state of Fe and Ti in ilmenite Ginley,D.S. and Baughman,R.J. (1976),Prepara in Apollo 11, Apollo 12, and Apollo 14 tion and Czochralskicrystal growth of the crystalline rocks. Proc. third Lunar Sci. iron titanates, FeTiO3,Fe2TiO4, and Fe2TiO6. Conf., Geochim. Cosmochim. Acta, Supple. 3, Mat. Res. Bull., 11, 1539-1544. vol. 1, 295-303. Lindh,A. (1972),A hydrothermal investigationof Smith, D. G. W. and O'Nions, R. K. (1971), Investi the system FeO-Fe2O3-TiO2.Lithos, 5, 325- gations of the LII,III, X-ray emission spectra 343. of Fe by the electron microprobe. Part I: Lindsley, D.H. (1962), Investigations in the Some aspects of the Fe LII,III, spectra from system FeO-Fe2O3-TiO2.Carnegie Inst. of metallic iron and hematite. J. Phys. D: Washington,Year Book,61, 100-106. Appl. Phys., 4, 147-159. Lindsley,D. H. (1963), Fe-Ti oxides in rocks as Stickler, J. J. and Heller, G. S. (1962), Antifer thermometers and oxygen barometers. romagnetic resonance in MnTiO3. J. Appl. CarnegieInst. of Washington, Year Book,62, Phys., 33, 1302-1303. 60-66. Taylor, R. W. (1963), Liquidus temperatures in the Lindsley, D.H. (1965), Iron-titanium oxides. system FeO-Fe2O3-TiO2. J. Amer. Ceram. CarnegieInst. of Washington, Year Book,64, Soc., 46, 276-279. 144-148. Taylor, R. W. (1964), Phase equilbria in the system Lindsley, D.H. (1973), Delimitation of the FeO-Fe2O3-TiO2 at 1300•Ž. Amer. Mineral., hematite-ilmenite miscibility gap. Geol.Soc. 49, 1016-1030. Amer.Bull., 84. 657-661. Tsusue, A. (1973), The distribution of manganese Lindsley,D. H. and Lindh (1974),A hydrothermal and iron between ilmenite and granitic magma investigationof the system FeO-Fe2O3-TiO2: in the Osumi Peninsula, Japan. Contrib. a discussionwith new data. Lithos, 7, 65-68 Mineral. Petrol., 40, 305-314. Matsuoka,K. (1971), Syntheses of iron-titanium Wanklyn, B. M., Wondre, F. R. and Davison, W. oxidesunder hydrothermal conditions. Bull. (1976), Flux growth of crystals of some Chem.Soc. Japan, 44, 719-722. magnetic oxide materials: Mn7SiO12, CuO, Muan, A. and Osborn, E.F. (1965),Phase equi MCr2O4, MTiO3, Ni2NbBO6, MMOO4 and Li2M2

libria amongoxides in steelmaking.xx+236 p, (MOO4)3, (M=Mn, Co, Ni). J. Mater. Sci., Addison-WesleyPublishing Co. 11, 1607-1614. Newmann,E. R. (1974), The distribution of Mn2+ Webster, A. H. and Bright, N. F. H. (1961), The and Fe2+between ilmenites and magnetitesin system iron-titanium-oxygen at 1200•Ž and igneousrocks. Amer.J. Sci., 274, 1074-1088. oxygen partial pressures between 1 atm Ohtsuka,Y., Nishida,N., Okudera, S. and Fujiki, and 2•~10-14 atm. J. Amer. Ceram. Soc. 44, Y. (1979),Chemical state analysis with com 110-116. 238 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki

イ ル メ ナ イ ト,パ イ ロ フ ァ ナ イ トの フ ラ ッ ク ス 合 成

大塚 芳郎・藤木 良規・鈴木 淑夫

イ ル メ ナ イ ト(FeTiO3)お よび パ イ ロフ ァ ナ イ ト(MnTiO3)単 結晶 を種 々 の 酸 素分 圧 条 件 下 で,フ ラッ クス法 により合成 した 。 合 成 し た イ ル メ ナ イ トの格 子 定 数 は,合 成 時 の酸 素 分 圧 に対 応 して 変 化 し,ま た よ り高い酸 素分 圧 条 件 下 で合 成 し た もの は数%のFe3+を含 む こ とが 認 め られ た 。 パイロ フ ァナ イ トは,合 成 時 の酸 素 分 圧 で その 色 が変 化 し,よ り低 い酸 素 分 圧 条 件 下 で 合成 し た もの は緑色 を 呈 し,よ り高い 酸 素分 圧 条 件 下 の ものは 赤 褐 色 を呈 し た 。格 子 定数 に 関 し て は,イ ル メ ナ イ トの場合 と同様 に 酸 素 分 圧 条 件 に 対応 し て変 化 す る こ とが 認 め られ た 。 き らに,こ れ らの単 結 晶 に つ い て, FeLお よびMnL線 の 測 定 を行 い, EPMAに よ る状 態 分析 を試み た。