Abstract--Feldspars in granitic saprolite in southwestern Australia have altered to halloysite, kaolinite, and gibbsite with no evidence of noncrystalline material. The secondary minerals are commonly present as intimate mixtures within altered feldspar grains, but discrete zones of gibbsite or halloysite-kaolinite also are present. Variations in the chemical microenvironment within micrometer-size zones in grains apparently controlled the type and distribution of secondary minerals. The alteration of both plagioclase and alkali feldspars involved congruent dissolution by soil solution and crystallization of halloysite, kaolinite, and gibbsite from this solution. Highly altered feldspar grains consist of etched feldspar fragments embedded within a highly porous framework of subhedral to euhedral platy crystals of kaolinite and gibbsite, or of spherical and felted aggregates of halloysite.

Maghemite in Darling Range lateritic duricrust ranges from 2 to 10% in abundance, contains less than 5 mole % Al and occurs as aggregates of very small ~ 150 A subrounded platy crystals. Corundum is also present, ranging from 1 to 3.5% in abundance, and it contains less than 3 mole % Fe and occurs as rounded (~250 A) crystals. The maghemite and corundum contents are positively correlated (P < 0.01), which may indicate a common origin due to dehydroxylation of Fe and Al oxyhydroxides in bush fires.

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Abstract--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.

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