D. H. LINDSLEY Department of Earth and Space Sciences, State University of New Yor\, Stony Broo\, New Yorli 11790

Delimitation of the Hematite-Ilmenite Miscibility Gap

Note: This paper is dedicated to Aaron and Elizabeth solubility—a miscibility gap1—at lower tem- Waters on the occasion of Dr. Waters' retirement. peratures. In the course of a paleomagnetic investiga- tion of hemo-ilmenite samples from Allard ABSTRACT Lake, Quebec, Carmichael (1961) presented Recent analytical data on the compositions a determination of the miscibility gap based of natural hematite-ilmenite solid solutions on solution and exsolution experiments on suggest that the miscibility gap of Carmichael natural material of bulk composition ilm70 (30 (1961) cannot be correct. Attempts to redeter- mole percent Fe203, 70 mole percent FeTiOa). mine the miscibility gap under hydrothermal According to his results, the gap is approximate- conditions have been only partly successful, ly symmetrical (although the reported solubil- for with the oxygen buffers currently available, ity of Fe203 in ilmenitess is several mole per- it is not possible to retain both hematite and cent greater than that of FeTiC>3 in hematiteB8 ilmenite phases in a given experiment. Despite at any given temperature up to 800°C), with the fact that the hydrothermal experiments a consolute point at a composition ilm5o-55 and

do not give the maximum solubilities of Fe203 a temperature of approximately 950°C. Car- in ilmenite and FeTi03 in hematite, they michael's curve has been widely used in the nevertheless show that the published miscibil- interpretation of natural hematite-ilmenite ity gap is too wide (by approximately 15 mole assemblages. percent at 600°C) and that the consolute However, recent electron microprobe temperature is at least 150°C too high. analyses of coexisting natural hematite and ilmenite suggest that Carmichael's curve may INTRODUCTION be in error. For example, both Rumble (1971) In a wide variety of rocks and ores, geologists and Kretchsmar and McNutt (1971) have

have observed lamellar intergrowths of the found extensive solubility of FeTi03 in rhombohedral minerals hematite (end-member hematite8B in rocks believed to have formed at formula Fe203) and ilmenite (end-member 1 formula FeTiC)3), the common plane being The hoop-shaped curve expressing this gap in a phase (0001) [hexagonal setting]. Thus, one finds diagram is popularly referred to as a"solvus." This paper lamellae of hematite solid solution (hematite^) will follow the more rigorous usage that the area within within a host of ilmenite solid solution (ilmen- the hoop—that is, the region where two phases are more stable than one single phase—is a miscibility gap that is ite8S), or lamellae of ilmeniteS3 within a hematite host; more rarely, both types are bounded by two solvi, one giving the composition of ss hematite in equilibrium with ilmenite , [abbreviated found within the same rock. These relations ss 8 hematite (ilm )] and the other giving the composition strongly suggest that there is extensive or com- ss 3B of ilmenite,s in equilibrium with hematite,s [ilmenite,, plete solid solubility between the end members (hem,,)]. The crest of the hoop-shaped curve, where at elevated temperatures, but only limited the two solvi meet, is the consolute or critical point.

Geological Society of America Bulletin, v. 84, p. 657-662, 1 fig., February 1973 657

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relatively modest temperatures. In particular, hematitess. Recently reported (and only partly

Rumble found 30 percent FeTi03 dissolved in calibrated) calcium bearing buffers (heden-

hematitess in metamorphic rocks of the sil- bergite-andradite-quartz-magnetite, and andra- limanite zone. Carmichael's data would require dite-wollastonite-magnetite, Gustafson, 1969; a temperature of at least 850°C to permit such Burt, 1969; Liou and Gustafson, 1971) come solid solution, a temperature that seems clearly close to the desired range aut are also slightly

incompatible with the silicate minerals present. reducing with respect to hematitess. Complete Rumble concludes (1971, p. 163)' 'Redetermina- redetermination of the hernatite-ilmenite mis- tion of the ilmenite-hematite solvus is evi- cibility gap evidently will require either a new dently required before the discrepancies can be buffer or the use of the hydrogen diffusion resolved." technique (Shaw, 1963). However, experi- mental data obtained usir.g available buffers PRESENT RESULTS cast serious doubt on the validity of Car- A potentially effective way to redetermine michael's curve and, therefore, are presented the miscibility gap would be to use hydro- even though the results do not provide an thermal experiments, in the hope that a water- actual determination of the miscibility gap. rich fluid phase might speed up reaction rates. Results of these experiments are given in Such experiments require that the oxygen Table 1. Of particular interest for present pur- fugacity of the fluid be maintained within a poses are those experiments starting with range where both rhombohedral phases are equimolar mixtures of encl-member hematite stable—that is, hematite8S is not reduced and and ilmenite, for if any solid solution is formed, ilmenitess is not oxidized. The required range its composition sets limits on the extent of of oxygen fugacity turns out to be quite the miscibility gap (Fig. 1). To interpret the narrow; it falls between the known solid-solid experimental results correctly, one must buffer curves (Eugster and Wones, 1962; recognize that the equilibrium solubility of Huebner, 1971). Thus, the magnetite-hematite FeTiC>3 in hematiteos (rutile and/or pseudo- and MnO-MiijOi buffers oxidize ilmenitess, brookitess) is always less than that of FeTiC>3 in whereas the nickel-nickel oxide buffer reduces hematitess (ilmss). In the same way the TABLE 1. HYDROTHERMAL EXPERIMENTS ON THE MUTUAL SOLUBILITY OF HEMATITE AND ILMENITE tartlng P T Buffer Duration Products iterials (kb) Ce) Days Hours I + H 1 900 MnO-MnsO, 0 22 Ilmjs + P I + H 1 850 MnO-MnaO, 2 15 Ilmis + P + R I + H 1 800 MnO-MnjO, 4 20 Ilmss + P + R I + H 2 750 Mn0-Mtls0„ 9 0 Ilms, + P + R I + H 2 700 MnO-MnjO, 46 23 IlmS3 + R I + H 2 600 MnO-MtisO, 46 0 Ilmso + R I + H 1 800 Wo-Mt-Ar.d 5 0 Ilm5i, + Uspn I + H 1 600 Hd-And-Q-Mt* 145 14 Ilm;s±! + Usps Ilmso 1 600 Hd-And-Q-Mt* 145 14 Ilmso (unchanged) I + H 1 900 Hd-And-C-Mt 3 20 Ilm62 + UspJS Umso 1 900 Hd-And-C-Mt 3 20 Ilm59 + Usp2i I + H 1 900 Wo-Mt-Ard 5 17 Ilm66 + Usp28 Ilm50 1 900 Wo-Mt-Ard 5 17 Ilm63 + Uspis I + H 1 700 Hd-And-Q-Mt« 91 17 Ilm»2 + R Ilmso 1 700 Hd-And-Q-Mt* 91 17 Urn»» + R Ilm5o 1 700 Hd-And-Q-Mt 45 17 limits + R I + H 1 800 Hd-And-Q-Mt* 31 19 Ilm»s + R Umso 1 800 Hd-And-Q-Mt* 31 19 limits + R

Notes: Starting materials; I + H is equimolar mixture of synthetic Ilmenite and hematite grouid together. xlra5o is a single-phase solid solution, 50 mole percent each of FeTio . and FaiOi. Productsi Ilm3s is ilmenite-hematite solid solution with 35 mole percent FeTiOi. Usp25 is ulv'ispinel-magneti te fFe2riOit - Fe3Oit) solid solution with 25 mole percent Fe2T-Oh. All compositions are ± 2 ir.ole percent. R, rutile (Tio2) / p, pseudobrookitess. Buffers.- t/o-Mt-And; wollastonite-magnetite-andradite. Hd-And-Q-Mti hedenbergite-andradite-qlartz-magnetlte.

*Buffer assemblage lacked hedenbergite at conclusion of run and, therefore, was oxidizing relative to the complete assemblage.

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900-

800- <3 P,R O o

700-

600-

100 80 60 40 20 Fe Ti 0, Fe203 llmenite Hematite Mole Percent Fe Ti 0, Figure 1. T-X diagram for the join ilmenite- hematite. The remaining ilmenite was oxidized to

hematite, showing the miscibility gap reported by hematite3S plus rutile (R) or pseudobrookiteS9 (P) or Carmichael (1961; hoop-shaped curve with dashed both. Triangles enclosing dots show the solution of crest) and the results of hydrothermal (PH20 = 1 to 2 Fe2C>3 in ilmenite; the remaining Fe2C>3 was partially kbar) experiments (Table 1) that are inconsistent with reduced and reacted with some FeTiOa to yield mag- it. Starting materials for all experiments shown were netite-ulvospinel solid solutions. To be consistent with equimolar mixtures of hematite and ilmenite. Height the above results, the miscibility gap should lie ap- and width of symbols correspond approximately to proximately within the shaded area. The irregular shape uncertainties in temperature and composition, respec- of this area emphasizes that it merely delimits the pos- tively. The rectangle indicates homogenization of the sible range of the miscibility gap and does not represent end members to the composition ilm49±2 plus traces of the gap itself. rutile. Open triangles show the solution of FeTiC>3 in

solubility of Fe2C>3 in ilmenite (magnetiteS3) component in the host phase is correspondingly is always less than that of Fe203 in ilmenite less than the maximum possible. (hemS3). This solubility relation becomes clear Thus the solution experiments illustrated in when one considers that the maximum solubil- Figure 1 serve to constrain or delimit the ity of a component in a phase occurs when the possible position of the miscibility gap. The activity of the component being dissolved is at shaded area in Figure 1 shows the approximate a maximum, namely when there is present a region within which the miscibility gap would second phase having the composition of that be consistent with the new data but does not component. In these experiments, the com- represent the form of that gap. Unless the gap ponent being dissolved does not exist as a is highly asymmetric, the consolute tempera- phase because the oxygen fugacity is either too ture must lie at or below 800°C, as shown by high or too low to permit its existence. As a an experiment at that temperature in which result, the activity of the component being equimolar ilmenite plus hematite were homog-

dissolved is always somewhat less than the enized to a single-phase ilmenite49±2 (plus maximum value, and the solubility of that traces of rutile). The much greater solubility

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of FeTi03 in hematite88 compared to Cfr- diffraction, comparing the positions of selected michael's solvus is more in keeping with the peaks with those of synthetic standards (Lind- analytical data of Rumble and of Kretchsmar sley, 1963); a few compositions so determined

and McNutt; Rumble's hematite88 with 30 were confirmed by cDmputing unit cell param- percent FeTiC>3 need not have formed at eters a and r and comparing these to the temperatures above 600°C, a more reasonable parameters determined for the same standards value than the 850° required by Carmichael's (Lindsley, 1965). Th: method has internal con- curve. sistency inasmuch as the standard compositions Although the new experimental data for were prepared in much the same way as were the unknowns. The technique probably gives hematite88 are more compatible with the compositions of natural samples, the data point compositions to ±2 mole percent when applied to synthetic sample!.. Carmichael's technique at 600°C for ilmenitess is not. Both Rumble and Kretchsmar and McNutt found much was dual: he determined compositions of il- menitess by comparison of ¿104 with synthetic more restricted solubility of Fe2C>3 in ilrrenite88 than is suggested by this experiment. Careful standards and of hematite88 by Curie point examination of the run product suggests that measurements. Since the Curie temperatures of it is a valid solution experiment, but caution intermediate hematke-ilmenite solid solutions dictates against placing a great deal of credence are now known to be dependent upon the in its significance until it can be confirmed by thermal history of the specimen, this method other experiments. of determination might be one source of dis- crepancy. The presence of impurities in the The only data of Carmichael that are com- natural specimens may also bias the x-ray pletely incompatible with my own are the determinations of ilmenite,». points for hematitess (ilm8S) labelled "exsolving, 100 days" (1961, p. 524). All others could be However, the most likely source of dis- rationalized by assuming incomplete reaction. crepancy lies in the different experimental For example, note the great disparity in com- techniques used. The most obvious difference is positions (up to 20 mole percent) for the my use of water-rich fluids at pressures of 1 and "exsolving 100 days" and "dissolving 100 days" 2 kb. Nevertheless, assuming that water does runs bracketing his ilmeniteSs (hemS8) solvus. not enter any of the solid phases involved, its The discrepancies between Carmichael's presence can only serve to speed reaction rates,

data and my own for hematiteS8 (ilm88) are so but not to change equilibrium values. The great as to require explanation; possible causes presence of water might also influence the rate include differences in starting materials, in of ordering of Fe and Ti. However, the inter- experimental technique, and in determining mediate solid solutions are probably disordered compositions of phases. (Certainly errors in at temperature and have the hematite space measuring temperature could not account for group (R3r), and thus this effect would not be discrepancies in excess of 100°C!) My starting important. Rumble (1971) has calculated the

materials included commercial Fe203 dried at effect of pressure on the hematite-ilmenite 950°C (hematite), and ilmenite and ilm5o miscibility gap and has found it to be small. For synthesized at 1100° to 1150°C from stoichio- example, at 6003C, there is a difference of only

metric mixtures of Fe, Fe203, and Ti02. Car- 5 mole percent in the calculated solubility of michael used Allard Lake hemoilmenite "as is" FeTi03 in hematite at lkb and at lOkb pres- for solution experiments; he homogenized it sure. The use of hydro -.hernial conditions in the by heating to 1250° for exsolution experiments. present experiments probably cannot explain Carmichael's analysis shows his material to the discrepancy between the results. be unusually pure: Mg(0.1 to 1.0 percent), Carmichael's use of sealed silica-glass tubes Si(0.1 to 1.0 percent), and Al(0.05 to 0.5 per- would seem to indicate that his samples—unlike cent) were mainly ascribed to silicate con- mine, which were oxidized or reduced depend- taminants; Ca, Cr, Cu, Mn, Ni, V, Zn, and Zr ing on the buffer—remained on the hematite- are each less than 0.1 percent. Probably dif- ilmenite join. Several points would suggest, ferences in chemistry cannot explain the dis- however, that his samples were slightly crepancies, although there remains a small oxidized during the experiments. Experiments possibility that trace constituents might in- designed to exsolve previously homogenized fluence the ordering of Fe and Ti in inter- material (ilmio) yielded small amounts (which mediate solid solutions. could not be detected ay x-rays) of hematite88, My determinative technique involved x-ray as indicated by Curie point measurements, plus

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ilmeniteS8 that was slightly richer in Fe203 than Computer Center. Discussions with Douglas was the starting material. Mass-balance con- Rumble and S. E. Haggerty have been most siderations require that the entire charge must helpful. have become slightly oxidized, and that small portions—probably surface films on individual REFERENCES CITED grains—were extensively oxidized to Fe203- rich hematitess that was not in equilibrium with Burt, D. M., 1969, New oxygen buffers in the the ilmenitess. (If this hypothesis is correct, system Ca-Fe-Si-O [abs.]: EOS (Am. Geophys. the ilmenite83 of the experiments must either Union Trans.), v. 50, p. 339. be slightly cation-deficient or must contain Carmichael, C. M., 1961, The magnetic properties undetected small amounts of rutile or pseudo- of ilmenite-haematite crvstals: Royal Soc. brookite or both.) In this regard, note the London Proc., v. A263, p. 508-530. Eugster, H. P., and Wones, D. R„ 1962, Stability following statement by Carmichael (1961, p. relations of the ferruginous biotite, annite: 522; italics added): "The crystals of haemo- Jour. Petrology, v. 3, p. 82-125. ilmenite were heated in sealed quartz tubes Gustafson, W. I., 1969, Stabilities of andradite and containing a small amount of air to prevent hedenbergite and their relation to skarn reduction of the haematite to magnetite." On equilibria [abs.]: Geol. Soc. America, Abs. for the basis of high-temperature phase-equilibrium 1968, Spec. Paper 121, p. 122-123. studies (Webster and Bright, 1961; Taylor, Huebner, J. S., 1971, Buffering techniques for 1964), we now know that ilni7o will be oxidized hydrostatic systems at elevated pressures, in at oxygen fugacities greater than approximately Ulmer, G. C., ed., Research techniques for IO-6 atm—a value certain to be exceeded by high pressure and high temperature: New "a small amount of air." Although this sentence York, Springer-Verlag, p. 123-177. refers directly to the initial homogenization Kretchsmar, U. H„ and McNutt, R. H„ 1971, A study of the Fe-Ti oxides in the Whitestone experiments, it appears to apply to the exsolu- Anorthosite, Dunchurch, Ontario: Canadian tion experiments as well. If so, surface oxidation Jour. Earth Sei., v. 8, p. 947-959. of the crystals by the air sealed within the Lindsley, D. H., 1963, Fe-Ti oxides in rocks as capsules might well account for the traces of thermometers and oxygen barometers: Car- Fe203-rich hematitess in the exsolution experi- negie Inst. Washington Year Book 62, p. 60- ments as detected by Curie point measure- 66. ments. As already noted, these are just the 1965, Iron-titanium oxides: Carnegie Inst. compositions that are completely incompatible Washington Year Book 64, p. 144-148. with my experiments and with the recent Liou, J. G., and Gustafson, W. I., 1971, Experi- analytical data. The conclusion of this paper mental study of the reaction: andradite -f- 2 is that the published miscibility gap for the quartz = 2 hedenbergite -f wollastonite -j- hematite-ilmenite system should be rejected. }^02 and its petrologic application [abs.]: EOS (Am. Geophys. Union Trans.), v. 52, p. I regret that I cannot offer a better one at 927. present. Rumble, Douglas, 1971, Fe-Ti oxide minerals and the behavior of oxygen during regional meta- ACKNOWLEDGMENTS morphism: Carnegie Inst. Washington Year Book 70, p. 157-165. It is appropriate that this report appear in a Shaw, H. R., 1963, Hydrogen-water vapor mix- volume honoring Aaron Waters, for my pro- tures: Control of hydrothermal atmospheres longed involvement with the Fe-Ti oxides by hydrogen osmosis: Science, v. 139, p. 1220— stems from my doctoral thesis in which I tried 1222. to understand the petrology and magnetic Taylor, Robert W., 1964, Phase equilibria in the properties of some oxidized basalts. As my system Fe0-Fe203-Ti02 at 1300°C: Am. Mineralogist, v. 49, p. 1016-1030. advisor, he encouraged the work for the thesis Webster, A. H., and Bright, Norman F. H., 1961, as well as my post-doctoral research on the The system iron-titanium-oxygen at 1200°C phase equilibria of these oxide minerals. It is a and oxygen partial pressures between 1 atm special pleasure to thank him in this way. and 2 X 10-11 atm: Am. Ceramic Soc. Jour., Most of the experiments reported here were v. 44, p. 110-116. performed at the Geophysical Laboratory, Washington, D. C., with the continued support of P. H. Abelson. Work at Stony Brook was MANUSCRIPT RECEIVED BY THE SOCIETY MAY 15, supported by National Science Foundation 1972 Grant GA-21379 and by the Stony Brook REVISED MANUSCRIPT RECEIVED JULY 24, 1972

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