Oct., 1960
ST4BILITY r ~ E L . 2 T l O N S OF I l l O S OXIDES
1451
STABILITY RELATIONS OF IRON OXIDES: PHASE EQUILIBRIA IN THE SYSTEM Fe304-Fe20sAT OXYGEN PRESSURES UP TO 45 ATMOSPHERES' BY BERTPHILLIPS -4ND ARNULFhfUAN Contribution No. 69-60f i om College of Mineral Industries, The Pennsylvania State University, University Park, Pennsylvania Received March $ 1 , 1960
Phase equilibrium data for the system Fe~Oa-FesOjhave been obtained a t oxygen pressures up to 45 atmospheres in the temperature range from 1194 to 1588". The crystalline phases magnetite and hematite coexist in equilibrium with liquid and gas (oxygen pressure -16 atmospheres) in a eutectic situation a t 1566". Magnetite contains oxygen in rxcess of the stoichiometric (Fe3OI)amount, up to a maximum of 40 weight yo Fe203.
Introduction Iron oxides have attracted a large amount of attention in earth sciences as well as in mineral technology and metallurgy for many decades. Added impetus to the interest in these oxides has been supplied recently by rapid advances in the fields of ferromagnetic and ferroelectric oxide materials. The major contributions to knowledge of phase relations in the system Fe-0 were made by Greig, Posnjak, Merwin and Sosman2 and by Darken and of other investigations and comG ~ r r y . Review,a ~ parisons of data obtained in these are given in the two above-mentj oned papers. All these studies were restricted to parts of the system where the oxygen pressure is below one atmosphere. The present study was carried out a t higher oxygen pressures and dealt specifically with determination of the eutectic between magnetite and hematite. Experimental General Procedure.-Phase relations were determined by the quenching technique. Mechanical mixtures of hematite and magnetite were sealed in 80 --eight Yo platinum-20 weight '% rhodium tubes and held for 15 to 30 minutes a t Yelectetf temperatures until equilibrium was established among gas and condensed phases. The samples were then quenched rapidly to room temperature and the phases present determined by microscopic and X-ray techniques. Starting Materials.--The mixtures used in the present investigation were prepared from hematite ("Baker Analyzed" 99.4 weighl, % ' Fe403) sintered in air a t 1300°, and magnetite. The magnetite was prepared by heating a mixture of m s t i t e ("FeO") and hematite a t 1420" in air for 16 hours. The product analyzed 25 6 weight 70 FeO, 74.4 weight yo Fe203(=82.3 weight Fe304, 17.7 weight % Fez(&). For checking purposes, additional mixtures were prepared from sintered hematite and m s t i t e . The results of equilibration runs on mixtures of identical composition prepared from different starting materials n-ere in excellent agreement Furnaces and Temperature Control.--A vertical tube quench furnace with 80 weight % platinum-20 weight % rhodium alloy resistance winding was used in the experiments. Teniperature constancy was maintained by a commercial electronic control instrument activated by a platinum-90 weight % platinum/lO weight Vo rhodium thermocouple whose junction was kept close to thr hot (1) Paper based on parts of a dissertation submitted by the senior author (B.P.) in partial fulfillment of requirements for the degree of Doctor of Philosophy in Geochemistry at The Pennsylvania State University, June, 1959. A t the time this work was done, the authors were graduate fellow in geochemistry and associate professor of metallurgy, respectively. The Pennsylvania State University. Bert Phillips i s now senior scientist, Tem-Pres Research, Inc., State College, Pennsyl wania. (2) J. W. Greig, E. Posnjak, H. E. Merwin and R. R. Sosman, Am. J. Sei.. 80 (5th series), 239 (1935). (3) L. 9. Darken a n d R. W. Gurry, J . A m . Chem. Soc., 6 7 , 1398 (1945): 68, 798 (1946).
spot of the furnace. Temperatures within the furnaces m-ere measured before and after each run with another platinum-90 weight % platinum/lO weight % rhodium thermocouple calibrated frequently against melting points defined as follows: diopside (CaMgSiz06),13915 " ; pseudowollastonite (CaSiO,), 1544". Temperatures thus defined are on the Geophysical Laboratory Scale, which is almost identical t o the 1948 International Scale up to approximately 1550". Temperatures above 1550" have been converted to the 1948 International Scale. Control of Oxygen Pressures.-High oxygen pressures were obtained in the present investigation by decomposition of one of the condensed phases (hematite) present. In most cases a simple sealed tube technique was used. Mechanical mixtures of sintered hematite and magnetite were funneled into 80 weight yo platinum-20 weight yorhodium tubes which beforehand had been sealed a t one end. The tubes, approximately 15 mm. long and Kith an inside diameter of 2.5 mm., were filled to approximately one-half of capacity. The upper, unfilled part was closed by squeezing the tube, and the sample-containing part was compressed as much as possible to keep free space in the tube a t a minimum. The partially flattened tubes were then heated t o a dull red for two t o five seconds to remove adsorbed water from the sample and excess air from the tube before sealing the upper end of the tube in a hydrogenoxygen flame. Chemical analysis was performed on two of the samples in order to check that no appreciable compositional change took place during the short heating before sealing of the tube. The removal of water and air from the sample was desirable in order t o obtain a high ratio of oxygen to total gases in the closed tubes a t the high temperatures of the subsequent equilibration runs. The oxygen pressure is controlled by temperature and composition of the sample enclosed. If other gases are present, the maximum oxygen pressure that the tubes will withstand before bursting (-18 atmospheres) will decrease, thus limiting appreciably the range over which the present technique may be used. When the sealed tubes are heated in the quench furnace, some hematite is decomposed t o provide the oxygen pressure of the gas phase corresponding to equilibrium with the condensed phases present. Because the densitv of the gas phase is very low in comparison with that of condensed phases, and because the volume of gas in the tube is small, the compositional changes of condensed phase5 caused by the decomposition is small under the experimental c*onditions used in the present investigation (-two w i g h t Fed04 as oxygen pressures up to 16 atmospheres are produced). A slightly modified technique was used in a few runs in the present investigation in an attempt to extend the study to still higher oxygen pressures. The mixture of hematite and magnetite was placed in an 80 weight % platinum-20 weight %rhodium tube approximately 20 nim. long and mith an inside diameter of approximately one mm. The tube was filled to three fourths of its capacity and treated in the same manner as just described until it had been sealed. This sealed tube was then dropped into another 80 weight yo platinum-20 weight yo rhodium tube approximately nine inches long and with an inside diameter of 2.5 mm. The bottom end of the latter tube had been sealed beforehand and the upper end was attached to a tank of nitrogen through a series of fittings and valves. Pressures up to approximately 27 atmospheres could be exerted in this outside tube while it was in a furnace a t 1600" if the section of the outer tube kept a t this temperature was reinforced by
BERTPHILLIPS AND ARNULFMUAN
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melting several layers of platinum-rhodium alloy onto its surface. In this way the inside tube could withstand internal pressures of up t o npprouimatcly 45 ntmospheres before bursting.
Results and Discussion The equilibrium data obtained in the present investigation are presented in Table I. These data have been combined with those of Greig, et CLZ.,~ and of Darken and Gurry3 to construct the phase diagram shown in Fig. 1. ;2 eutectic is seen to exist a t 156G0, with magnetite, hematite, liquid of composition 59 weight yoFe203,41 weight, yo Fe3Q4and gas with oxygen partial pressure of approximately 16 atmospheres coexisting in equilibrium. The boundary of the hematite field above 145.5" i s sketched as a dash line forming a smooth continuation of the curve established by Greig, et CLZ.,~ ,zt lower temperatures. A rough estimate of the oxygen pressure of the gas phase present a t the eutectic situation was arrived a t by direct testing of the pressures which
Fig. 1.-Diagram illustrating phase relations in the system FeaO4-Fe2O3, based on data in the literature up to a n oxygen pressure of one atmosphere and on data obtained in the present investigation up to approximately 45 atmospheres. Heavy solid and dash lines are boundary curves and lighter dash-dot lines are oxygen isobars. The various point symbols used are explained in the framed insert at the bottom of the diagram.
could be withheld by the tubes. Nitrogen gas supplied to the outer stage of the two-stage pressure device described in a previous section of this paper caused 2.5 mm. tubing (not reinforced) to fail a t approximately 18 atmospheres a t 1577". By combining this observation with the observations of tube failures listed in Table I (and indicated in Fig. 1) it is inferred that the oxygen pressure a t the eutectic situation is approximately 18 atmospheres. An independent check of this result is obtained by extrapolations t o higher temperatures of the data of Darken and G ~ r r y . They ~ determined experimentally oxygen pressures as a function of temperature a t which hematite and magnetite coexist in equilibrium, covering the pressure range from 10-4.18to one atmosphere and the temperature range from 1130 to 1455'. Their experimental data are plotted as solid dots on the diagram in Fig. 2 , and a smooth curve is drawn to pass through these points as closely as possible. The intersection of the extension of this curve (dashed in Fig. 2)
STABILITY RELATIONS OF
Oct., 1960
IRON OXIDES
1453
TABLE I SUMMARY OF DATA Temp. of equilibration run. OC.
Phases presentn
Compn. of mixture, weight % Fat04 Fa901
82.3 17.7 Magn. I588 Magn. 1394 Magn. hem. 1314 58.6 41.4 Liq. magn. 1575 magn. hem. Liy. 1566 Magn. hem. 1557 Magn. hem. 1194 52.1 47.9 Liq. magn. 1573 Liq. magn. 1568 Magri. hem. 1564 42.0 58.0 Liq. 1585 Liq. 1573 hfagrl. hem. 1565 39.5 60.5 Tube split 1572 30.3 69.7 Tube split 1587 Tube split 1571 Liq. hem. 157Ib 20.4 79.6 Tube split 1588 Tube split 1573 Liq. f hem. 1571* hem. blngri. 1466 8.1 91*9 Liq. hem. 1571b Map. hem. 1405 Rlrtg~i. hem. 1231 99 Tube split 1568 HCl~, 1556 Iiem 1543 Hem. 1533 Hem, 1502 Abbreviations used h a m these meanings: magn. = crystals of ms,gncstite; hem. = crystals of hematite; liq. = liquid. * Thc sealed tubes containing these samples were subjected to a n external pressure of approximately 27 atmospheres.
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0 1 2 Log PO, (atm.). Fig. 2.-Extrapolations of data reported by Darken and Gurry3 t o estimate oxygen pressure of gas phase present in equilibrium a t the eutectic in the system FerOd-Fedh Solid dots represent experimentally determined oxygen pressures as a function of temperature for equilibrium coexistence of magnetite and hematite. Open circles represent compositions of magnetite as a function of oxygen pressure a t 1566'. The straight, horizontal line represents composition of magnetite phase 8 s well as temperature at eutectic situation, as explained in text. -1
magnetite at three different oxygen pressures a t 1566O, and a smooth curve has been drawn to pass through these points. The scale for this curve (right-hand vertical axis) has been chosen such that the coniposition of magnetite (40 weight % ' with a straight line representing the eutectic tem- Fe203) in equilibrium with hematite at 1566' perature (lii66') defines the oxygen pressure of the (see Fig. 1) is represented by a point a t the same gas phase a t the eutectic. The value thus deter- height as the 1566' point on the left-hand temperamined is approximately 16 atmospheres (log PO, ture scale. Hence the two extrapolated (dash) curves must intersect at this common value on the = 1.2). Another extrapolation from a set of data obtained by Darken and G ~ r r isy also ~ ~shown vertical scale. (4) A. XIuan, Am. J . Sci., 256, 171 (1958). in Fig. 2. Open circles represent compositions of
