Geochemical characteristics and expansion properties of a highly potassic perlitic rhyolite from Lopburi, Thailand

Abstract

Miocene volcanic rocks from northeast of Lopburi, Thailand, include a suite of highly potassic (<6.3% K2O) perlitic rhyolites that are similar chemically to rhyolites from Sardinia (which are also perlitic) and Vulcano. Electron microprobe analysis using a defocussed beam has shown that glass compositions contain up to 12.9% K2 O (0.9% Na2O), and within the same samples spherulites are enriched in Na2O (<6.2%) and depleted in K2O (2.1%). The perlitic rhyolites contain up to 4% water of meteoric origin; from Fourier transform infra-red spectroscopy data, expansion on heating involves a reduction in both hydroxyl and molecular water contents. When expanded using a laboratory test furnace, specific gravities as low as 26 kg/m 3 have been achieved for a 0.15-0.3-mm feed size fraction (reference perlite ores from Milos and Sardinia gave 47 and 33 kg/m3, respectively), and 10 kg/m3 for a 0.5-1-mm fraction separated after expansion. Expansion involves the loss of fluorine from the perlite, presumably as HF; unexpanded perlite contains 90-550 mg/kg F, which is reduced on expansion by 4-65%. Water-soluble fluorine after expansion is increased by a factor of 10 up to 3.3 mg/kg. Expansion also increases the availability of K to plants by a factor of up to 6, with a maximum available K content of 1245 mg K/kg perlite. This contrasts markedly with the chemically inert character of other, commercial, perlites, suggesting that the distinctive K-enrichment of the perlitic rhyolite has given a raw material capable of generating products with nutritional functionality for plant growers. © 2007 The Authors Journal compilation.