Origin, development and chemistry of silica-alumina rock coatings from the semi-arid regions of the island of Hawaii

Abstract

Exposed rock surfaces in the semi-arid regions of the Island of Hawaii are characterized by an x-ray amorphous silica-alumina surface coating, ranging in thickness from a few to several hundred micrometers. These coatings are composed of clear to translucent white lamellae of amorphous silica with a minor component of some amorphous aluminum phase, and in some cases, interlayered fine-grained clastic material. The coatings are derived from exogenous material. Both the amorphous silica-alumina component and the clastic component of the coating are derived locally. Description of the chemical composition, mineralogy and the morphology of the coating, along with an evaluation of possible sources, enables formulation of a model which describes their development. Because the composition and mineralogy of the coatings is dissimilar from all likely source materials, direct incorporation of the source material could not account for the coating. Instead fine-grained clastic material, locally derived from the soil, is blown onto the rock surface; during a later non-violent wetting event, such as fog or dew, some of this material goes into solution. As this solution is evaporated poorly ordered mineral phases precipitate onto the rock surface forming the coating. Some of the clastic material is incorporated into the coating during the precipitation. Late stage precipitates are removed during later more dynamic wetting events, such as a heavy rainfall, which wash across the rock surface.