Mineralogical and chemical properties of weathered magnetite grains from lateritic saprolite

Magnetite grains in lateritic saprolite have altered in situ to porous hematite (martite) grains. Hematite occurs as relatively large (～ 1 μm) platy crystals, triangular arrays of small (0.01–0.05 μm) lathlike crystals and very small (< 100 Å) platy anhedral particles. Titanium increased in altered grains and may substitute for Fe³⁺ in hematite. Mn, V, Cr and Zn remained approximately constant whereas the Ni content decreased. Halloysite crystallized from soil solution within pores resulting in a marked increase in the Si and Al contents of grains.     

Iron oxides in lateritic soils from Western Australia

Lateritic soils developed from dolerite contained larger amounts of goethite and haematite than those developed from granite. The goethite/(goethite + haematite) ratio in granitic soils ranged from 0.55 to 1 and from 0.29 to 0.83 in doleritic soils. Maghemite ranged in abundance from 0 to 10% and it only occurred in duricrust. Mole % Al substitution ranged from 16 to 33% in goethite and from 2.5 to 10% in haematite and was similar for both granitic and doleritic soils. Al substitution in maghemite was <5%. A significant, positive correlation (P<0.01) occurred between Al substitution in goethite and the amount of gibbsite in the soil. The dehydroxylation temperature of goethite ranged from 292 to 334°C and was positively correlated with the mole % Al substitution. Goethite crystals varied in size from 16 to 26 nm and haematite crystals from 18 to 59 nm. Goethite and haematite crystals occurred as aggregates of subrounded platy crystals.
Iron oxides obtained by NaOH treatment contained much of the minor element contents of the soils; mean concentrations (μg g⁻¹) were: Zn 19.9, Cu 31, Mn 68, Ni 140, Co 32, Cr 394 and V 696. These minor elements were most abundant in iron oxides derived from dolerite, but were not preferentially associated with goethite or haematite.