A COMPARATIVE STUDY OF THE MORPHOLOGY, CHEMICAL COMPOSITION AND WEATHERING OF RHYOLIT IC AND ANDESITIC GLASS

ABSTRACT: The morphology ofvolcanic glass particles in rhyolitic and andesitic tephra of central North Island and Taranaki areas of New Zealand has been studied by scanning electron microscopy. Electron probe analyses ofthe glasses are compared with those of the clays to which they weather. Loss of silica characterizes the weathering of both glasses. The rapid rate of weathering ofandesitic glass is attributed toits occurrence asfine, soft microlites and extensive substitution fAl for Si in the structure. Rhyolitic glass weathers more slowly because it occurs as hard and brittle particles containing relatively ittle ahunina. It is suggested that the structure, chemical composition and chemical ctivity of allophane isgoverned largely by the chemical composition a d bonding characteristics of the parent glass. Recent investigations by Kirkman (1975, 1980) have shown that rhyolitic and andesitic volcanic glasses of central North Island and Taranaki areas of New Zealand weather to allophane at widely different rates. Whereas glass in the < 1.0 #m clay fractions of rhyolitic tephra alters to allophane after about 3000 years, very little glass remains in the clay fractions of andesitic tephra which are only 300 years old. After a further 7000 years, rhyolitic allophane begins to crystallize to halloysite, but andesitic allophane persists fo