Weathering of Ilmenite in a Lateritic Pallid Zone

-In a lateritic pallid zone ilmenite crystals alter via pseudorutile to porous leucoxene grains composed of randomly oriented aggregates of ~0.06/~m anatase crystals. This style of alteration differs from that in beach sands where parallel oriented futile crystals develop from pseudorutile. Increased Si and A1 in the altered grains is due to the crystallization from soil solution of halloysite, kaolinite and gibbsite within pores rather than to the incorporation of these elements into anatase crystals. Manganese was a significant constituent (3.2% Mn203) of the original ilmenite but was not retained by the leucoxene grains. The minor constituents Ni, Zn, Cu, Mg, Co and Ca were also lost, but Cr and V were retained. Key Words--Anatase, Halloysite, Ilmenite, Kaolinite, Laterite, Leucoxene, Pseudorutile, Ruffle, Weathering. I N T R O D U C T I O N The weathering of i lmenite is not well understood and deserves further investigation. I t has been reported to alter to various products including anatase, ruffle, a noncrystalline iron titanate (Bailey et al., 1956); arizonite (Palmer, 1909; Karkhanavala, 1959); hydroi lmenite (Hinter, 1959); proarizonite (Bykov, 1964), and pseudorutile (Teufer and Temple, 1966; Grey and Reid, 1975). The relationship of the weathering environment and the nature of the alteration products of i lmenite is also not known. The alteration of i lmenite in beach sand ore bodies has been extensively studied due to its economic importance, but relatively little attention (Hartman, 1959; Van Houten, 1968) has been given to the stability of this mineral in other soil environments. Har tman (1959) considered that the alteration mechanism of i lmenite during bauxit izat ion is different from that in weathered beach sands and that three distinct types of alteration occurred in bauxites: (1) direct alteration to leucoxene; (2) direct alteration to optically identifiable anatase; (3) alteration to "patchy" ilmenite. The weathering of i lmenite in soils is of considerable interest to geologists because this mineral is a major pr imary source of the Ti in soils. Furthermore, the form of Ti in soils is to some extent determined by the alteration processes occurring within the confines of a single i lmenite grain rather than in the general soil matrix. I lmenite commonly contains significant substitutions o fMn, Mg, V, Cr (Deer et al., 1962), so that its mode of alteration will also affect the concentration and form of these elements in some soils. These considerations suggest that there is clearly a need for a better understanding of weathering of i lmeni te in soils. This paper deals with mineralogical and chemical aspects of i lmenite weathering in the pallid zone of a laterite developed from dolerite in southwestern Australia. Copyright 9 1984, The Clay Minerals Society MATERIALS A N D METHODS Samples from a laterite pallid zone developed from metamorphosed quartz dolerite were collected from a railway cutting at Jarrahdale, 45 km southeast of Perth, Western Australia. Detai led profile descriptions of the deeply weathered lateritic soil profile at this site were given by Sadleir and Gilkes (1976). The profile contains a distinct sequence of horizons, i.e., parent material (deeper than 10 m and not exposed), mottled, red-white pall id-zone clay (depth interval, 6-10 m), irregular, brown, nodular concretions (4-6 m), loose, brown, nodular hard cap (2-4 m), and pisolitic hard cap (0-2 m). Altered i lmenite grains were separated from the bulk samples which had been disaggregated by gentle shaking in 0.1 M NaOH solution. Partially and highly altered (leucoxene) grains remained intact and were next separated from other mineral grains in a Frantz isodynamic magnetic separator. X-ray powder diffraction (XRD) patterns o f bulk samples of altered grains separated from the pallid zone were obtained with a Philips diffractometer with Fefiltered CoKa radiation. For the measurement of the unit-cell parameters a CaF2 internal standard was added, and the powdered sample was scanned at a rate of 0.25*20/rain. Single-grain X-ray diffraction patterns were made using a Gandolfi single crystal camera (Gandolfi, 1967). Semiquanti tat ive analysis of the minerals in the pallid-zone clay was made by thermogravimetric analysis (TGA). Simultaneous T G A and differential thermogravimetric (DTGA) curves for 10-mg samples were obtained in flowing air at a heating rate of 10~ min using a Perkin Elmer T G 52 instrument. Dehydrat ion maxima at 60, 280, and 500~ produced by halloysite(10/k), gibbsite, and halloysite(7Ik) + kaolinite, respectively, were used to estimate the concentrat ion of these minerals. Polished sections of bulk pallid-zone clay samples and i lmenite concentrates were made for petrographic