The origin and stability of lunar goethite, hematite and magnetite

EARTH AND PLANETARY SCIENCE LETTERS 17 (1972) 84-88. NORTH-HOLLAND PUBLISHINGCOMPANY

THE ORIGIN AND STABILITY OF LUNAR GOETHITE, HEMATITE AND MAGNETITE Richard J. WILLIAMS and Everett K. GIBSON NASA Manned Spacecraft Center, Houston, Texas 77058, USA Received 5 September 1972 Extra-lunar contamination, fumarolic activity, and exposure to oxidizing gases from comet or carbonaceous meteorite impacts have been previously proposed as the causes of magnetite, hematite, and goethite in lunar materials. However, these minerals can occur in the stable low temperature gas-solidequilibrium assemblages of lunar rocks. Below 600°C magnetite is in equilibrium with C - O - H gases with compositions compatible with high temperature equilibrium with metallic iron; below 150°C hematite is stable in these same gases. Goethite is not stable in carbonaceous gases at low total pressure, and thus gases from impacting carbonaceous material cannot have produced it. Goethite is stable at low temperatures and pressures in almost pure H2-H20 gases. Its minimum stability against hematite is 2 bars total pressure at 130°C and 10 -3 bars at 30°C for H2 to H20 ratios compatible with the high temperature redox state of lunar materials. Thus the traces of magnetite, hematite and goethite in lunar materials may be the result of normal low-temperature processes indigenous to the Moon and not special processes.

1. Introduction Agrell et al. [ 1] noted the occurrence of goethite and red staining in some glass fragTnents of Apollo 14 breccias 14307 and 14301. They speculated that the goethite and stain may be products of fumarolic activity or cometary impact. Other investigators [ 2 - 7 ] have commented on the presence of minute quantities of magnetite, hematite, and goethite in lunar matedals. The occurrence of these oxidized iron compounds can be shown to be compatible with the highly reducing conditions deduced from the presence of native iron and other phases in lunar igneous rocks [8, 9] and breccias [10] if one considers the redox state of the fluid phase associated with lunar rocks at temperatures below 600°C. The occurrence of magnetite may be explained by reference to fig. 1. For C - O gas-controlled equilibria the graphite surface (curve g in fig. 1) represents the minimum possible redox conditions. The surface is depressed somewhat by hydrogen in the gas, as indicated by curve gh in fig. 1, but moved to higher oxygen fugacities by increase of total pressure as illustrated by curve g' (fig. I). Because with falling temperatures these graphite buffer curves become more oxidizing [1 I] than the i r o n - o x y g e n buffer curves [ 12], magnetite is ex-

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Fig. 1. Log fo -T relations for iron-oxygen buffers and for gasses in equ~2brium with graphite. Symbols: H=Hematite, M=MagnetJte, W=Wustite,I=Iron, g=graphite-gas buffer in C-O gas at 1 bar total pressure, g'=graphite~gas buffer in C - O gas at 2000 belts total pressure, gh =graphite-gas buffer in C - O - H gas (H/O = 2) at 1 bar total pressure. (Data from [111, [12] and [17])~

R. J. Williams. E. K. Gibson, Lunar goethite, hematite and magnetite

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Fig. 2. Stability of iron oxides and hydroxides as a function of hydrogen pressure and water pressure at 930°C (dashed line) and 130°C (solid line). Hachured line marks the stability limit of goethite at 130°C. Curves labelled 1, 10, 100, etc., indicates total pressure equal to 1 bar, 10 bars, 100 bars, etc respectively. Symbols: l=Iron, M=Magnetite, W=Wustite, H=Hematite, and G=Goethite. The dotted curve H'G' boundls the lower stability of goethite at 30°C. The square marks a 930°C redox state which would lead successively to goethite and hematite upon cooling at equilibrium to 130°C (arrows). The circle marks a 930°C redox state which would produce goethite, then hematite upon cooling at equilibrium to 30°C. (Data from [12], [14],and [171)

pected to occur in a rock controlled by a C - O - H gas phase even under the most reducing conditions possible. Referring to the one atmosphere graphite surface fig. 1 shows that magnetite becomes a potential phase at about 600°C. At very low temperatures (less than 150°C), the graphite surface intersects the h e m a t i t e magnetite buffer curve, and hematite become stable. Since a C - O - H gas phase probably predominates the redox-sensitive gases in lunar rocks [ 13], magnetite and hematite should occur in them. Goethite (c~-FeOOH) is more difficult to stabilize because of the significant water pressures necessary. However, fig. 2 illustrates a possible mode of formation. In this figure, the i r o n - o x y g e n buffer reactions [12] and the goethite stability data [14] in the sys-

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tem H 2 - H 2 0 have been drawn for 130°C the maximum lunar surface temperature [ 15] and 930°C (the temperature of metamorphism of medium-grade Apollo 14 breccias, as interpreted by Williams [16]). The heavy lines mark constraints of Ptotal= PH~O + PH2, the square indicates a possible redox state of lunar rocks at 930°C, as indicated by the presence of metallic iron in them, and the hachured lines bound the stability fields of goethite at 130°C. Referring to the redox state indicated by the square in fig. 2, it is clear that if the H 2 0 / H 2 ratio at 930°C can be held constant or increased on cooling to 130°C and if total pressures of at least 2 bars are maintained, goethite will be stable, as indicated by the arrow emanating from the square. Note that the arrow is drawn from 10 bars total pressure for the 930°C state to 2.5 bars total pressure at 130°C to allow for goethite formation while accounting for the isochoric pressure decrease of the gas with cooling. The square represents a lower limit of total pressure if goethite is to be stabilized under the assumed conditions. If significant loss of hydrogen by diffusion occurs, starting pressures would have to be greater to maintain pressure high enough for formation of goethite. Dilution of the fluid by small amounts of other components could be tolerated, but unless pressures of ten to hundreds of bars are evisione d, major amounts of these could not be present. Carbon cannot have been present, even in relatively small amounts, because its reactions to form oxides and hydrides are so extensive [ 17] that the required water pressures would never be realized. Thus unlike magnetite, goethite could form only from an almost pure H 2 - H 2 0 fluid, a constraint which makes it unlikely that the goethite in lunar rocks resulted from the impact of a comet or carbonaceous chondrite. A few meters below the surface the maximum temperature is 100°C lower than the surficial maximum [5] and goethite will be stable there to much lower PH O( an approximate minimum P a~v .c~ of 1 0 - 3 a t 30°C ~s obtained by extrapolating Scfirnaltz' data [ 14]). Thus, if adequate insulation from lunar surface temperatures is assumed, goethite has a much larger stability range than the hachured boundary in fig. 2 indicates, and starting pressures of no more than 0.02 bars at 930°C suffice to stabilize goethite on cooling to 30°C. The dotted line on fig.2 represents the lower stability limit of goethite against hematite at 30°C and the circle with attached arrow illustrates the lower limiting total pres-

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R. J. Williams, E. K. Gibson, Lunar goethite hematite and magnetite

Fig. 3. Photograph of iron-goethite relations in rock 14301.19, kindly supplied by S.O. Agrell. Symbols: I=Iron, G=Goethite, H=Hematite, T=Troilite.

sure for goethite formation from metallic iron. Be.cause the formation of hydrated iron oxides will lower PH O, the equilibria will move to the goethite-hematite ~oundary where the system would be buffered in PH20 (short horizontal arrows of fig. 2). The red

stains around the goethite (fig. 3) are possible hematite resulting from this shift of equilibrium. Compared with the other oxides and silicates involved in the redox reactions in lunar rocks, goethite is extremely rare. Two factors probably cause this rarity.

R. J. Williams, E. K. Gibson, Lunar goethite, hematite and magnetite

First, to be stable goethite requires high water pressure, relative to the lunar vacuum. Thus it can occur only in systems which have an almost pure H 2 - H 2 0 fluid phase and which have never been exposed to the lunar vacuum. Second, because the production of goetbite from metallic iron or magnetite requires both oxidation and hydration, its formation may be kinetically inhibited. The importance of the kinetic factor is probably large because of the low temperatures involved. These factors restrict goethite to systems which both attain the proper physical conditions and maintain these conditions for some extended period of time. In view of this, the rarity of goethite at the lunar surface is not surprising. In a model process, a glass containing H 2 as its only volatile component is heated to about 930°C, where reaction (1), FeO(glass)+ H 2 ~ Fe(metal) + H 2 0 , takes place. An iron-bearing lunar glass particle with 100 ppm of solar wind hydrogen [18] could yield about 0.3% by weight of metal by this reaction, and thus the availability of reducing agent should not limit the process. Upon cooling below 130°C, the stability field of goethite is entered and the reaction (2), Fe + 2 H 2 0 FeOOH + 3/2H2, takes place. The decrease in pressure drives the system to the g o e t h i t e - h e m a t i t e phase boundary. It is apparent that if reaction (1) is the sole source of H 2 0 in the system then reaction (2) cannot consume the iron produced at high temperatures. Thus the result of this process should be an iron particle surrounded by a rim of goethite which in turn is surrounded by a rim of hematite as is observed (fig. 3). In addition to goethite, akaganeite (/3-FeOOH) and lepidocrocite (7-FeOOH) can occur. Although their stabilities in terms of water pressure should be similar to goethite, they are known to result only from the hydrolysis of iron salts (akaganeite has been prepared only by hydrolysis of iron chlorides [ 19] ). Chloride and sulfate are present in minor amounts in lunar goethite [ 1 ]. These anions may have stabilized polymorphs of FeOOH other than goethite, or, because large ions and molecules can be trapped in hydrated iron oxide structures [20], the sulfate and cloride ions may simply be trapped within the goethite without having stabilized either akaganeite or lepidocrocite.

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2. Conclusion Ill conclusion, it is apparent that small amounts of oxidized materials in lunar samples are readily explainable in terms of changes in redox states which result from equilibrium cooling in the presence of a plausible lunar fluid phase. Magnetite and hematite can occur rather generally in equilibria involving C - O - H gas, whereas goethite can occur only in H 2 - H 2 0 gases. Although these materials could be products t3f lunar fumarolic activity or extra-lunar hydration and o x i d a t i o n neither special geological processes nor addition of extra-lunar material need be postulated to account for their occurrence.

Acknowledgements The authors wish to thank S. Agrell for providing them with a photomocrograph of the g o e t h i t e - m e t a l glass relations and Robin Brett for pointing out several references to them.

References [ 11 S. O., Agrell, J. H. Scoon, J. V. P. Long and J. N. Coles, The occurrence of goethite in a microbreccia from the Fra Mauro Formation, Lunar Science-Ill, LSI Contr. 88 (1972) 7 (abstr.). [2] S. K. Runcorn, D. W. Collinson, W. O'Reilly, D. Stephenson, M. H. Dattey, A. J. Manson, P.W. Readman, Magnetic properties of Apollo 12 lunar samples, Proc. R. Soc. Lond. A. 325 (1971) 157. [3] R. A. Weeks, J. L. Kolopus and D. Kline, Magnetic phases in lunar material and their electron magnetic resonance spectra: Apollo 14, Luna Science-Ill, LSI Contr. 88 (1972) 698 (abstr.). [4] J. Jedwab, A. Herbosch, R. Wollast, G. Naessen and N. van Geen-Peers, Search for magnetite in lunar rocks and fines, Science 167 (1970) 618. [5] A. P. Vinogradov, Preliminary data on lunar ground brought to earth by automatic probe "Luna-16", Proc. 2nd Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 2 1 (1972) 1. [6] P. Ramdohr, Zur Mineralogie des Mondes nach Ergebnissen der Untersuchung der bei der Apollo 11- und Apollo 12-Landingausgesammelten Gesteine, Fortschr. Mineral. 48, (1970) 31. [7] P. Ramdohr and A. E1 Goresy, Mikroskopisde Untersuchung der Mondproben des Apollo 11- Unternahmen, Nature 57 (1970) 98.

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R.J. Williams, lz\ K. Gibson, Lunar goethite, hematite and magnetite

[8] R. Brett, P. Buter Jr.,C. Meyer Jr., A. M. Reid, H. Takeda and R. Williams, Apollo 12 igneous rocks 12004, 12008, 12009, and 12022: A mineralogical and petrological study, Proc. 2nd Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 2, 1 (1971) 301. [9] S. E. Haggerty, Subsolidus reduction of lunar spinels, Nature Physical Sciences 234 (1971) 113. [10] W. A. Gose, G. W. Pearce, D. W. Strangway and E. E. Larson, Magnetic properties of Apollo 14 breccias and their correlation with metamorphism, Proc. 3rd Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 3, 3 (1972) in press. [ 11 ] B.M. French and H. P. Eugster, Experimental control of oxygen fugacities by graphite-gas equilibrium, J. Geophys. Res. 70 (1965) 1529. [ 12[ H. P. Eugster and D. R. Wones, Stability relations of the ferrogious biotite, annite, 1. Petrol. 3, (1962) 82. [ 13] T. WeUman, Gaseous species in equilibrium with the Apollo 11 holocrystalline rocks during their crystallization, Nature 225 (1970) 716.

[14] R. F. Schmaltz, A note on the system Fe~O3-H20 , J. Geophys. Res. 64 (1959) 575. [ 15 ] M. G. Langseth Jr., S. P. Clark Jr., J. L. Chute Jr., S. J. Keihm and A. E. Wechsler, Heat-flow experiment, Apollo 15 Prelim. Sci. Rep., NASA SP-289 (1972) 11-1 to 11-23. [ 16] R. J. Williams, Petrogenesis of lunar breccias, Earth Planet. Sci Letters (1972) in press. [ 17] B. M. French, Some geological implications of equilibrium between graphite and a C - O - H gas phase at high temperatures and pressures, Rev. Geophys. 4 (1966) 223. [18] S. Epstein and H.P. Taylor Jr. (1971) O18/O 16 , S~3°/ Si 28 , D/H, and C 13/C12 ratios in lunar samples, Proc. 2nd Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 2, 2 (1971) 1421. [ 19] H. B. Weiser and W. O. Milligan, (1939) The constitution of colloidal systems of the hydrous oxides, Chem. Rev. 25 (1939) 1. [20] F. J. Ewing, The crystal structure of diaspore-goethite J. Chem. Phys. 3 (1935) 203.