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The Weathering of Olivine Basalt in Hawaii and its Pedogenic Significance¹

ABSTRACT

The examination of weathered boulders of olivine basalt in the solum of a soil belonging to the Molokai family of the Low Humic Latosol revealed a sequence of secondary minerals with the time of exposure to weathering and to the general weathering environment. The rainfall at this site is approximately 20 inches per year. Inside the core of the rock olivine weathered directly to montmorin type of clay mineral. The evidence suggested that montmorin type of clay formed due to presence of appreciable amounts of soluble magnesium in the weathering mineral. At the outer edge of the rock, mica-like crystals were formed after olivine. The identity of this mineral or minerals was not positively established, except that the differential thermal curves indicate the development of kaolin. Outside of the weathering core of the rock, the exfoliated decomposed layers of the rock contain very appreciable amounts of kaolin where drainage was good. A plastic montmorin type of clay was found under the rock where conditions favored the retention of bases. The type of secondary clay mineral formed from a basic olivine basalt is determined by the weathering environment which includes the supply of bases, the release of bases and the rate of leaching. However, there is little difference in the mineral weathering products whether there is a preponderance of soluble sodium or magnesium.

¹ Published with the approval of the Director of the University of Hawaii Agr. Exp. Sta. as Technical Paper No. 353.

² Chemist and Graduate Student, University of Hawaii Agr. Exp. Sta.

Received for publication July 14, 1955.