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Weathering of Basalt: Formation of Iddingsite

Published online by Cambridge University Press:  02 April 2024

Katherine L. Smith ,
Anthony R. Milnes  and
Richard A. Eggleton
Katherine L. Smith
Affiliation:
Department of Physics, New South Wales Institute of Technology, Sydney, New South Wales, Australia
Anthony R. Milnes
Affiliation:
CSIRO Division of Soils, Glen Osmond, South Australia, Australia
Richard A. Eggleton
Affiliation:
Department of Geology, Australian National University, G.P.O. Box 4, Canberra City 2601, Australia
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Abstract

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The formation of iddingsite by the oxidative weathering of Fo80 olivine begins by solution of Mg from planar fissures, 20 Å wide and spaced 200 Å apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks < 50 Å in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20–50 Å wide and 1–7 layers thick, form bridges 50–100 Å long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 × 100 × 200 Å (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

Keywords

GoethiteIddingsiteIronOlivineScanning electron microscopyTransmission electron microscopyWeathering
Type
Research Article
Information
Clays and Clay Minerals , Volume 35 , Issue 6 , December 1987 , pp. 418 - 428
Copyright
Copyright © 1987, The Clay Minerals Society

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