ANALYTICAL ELECTRON-MICROSCOPIC STUDY OF THE NONCRYSTALLINE PRODUCTS FORMED AT EARLY WEATHERING STAGES OF VOLCANIC GLASS

Formation processes of weathering products of pumice collected from a rhyolitic pyroclastic how deposit were investigated by X-ray powder di ffraction (XRD). scanning electron microscopy (SEM), transmission elec tron microscopy (TEM) and energy dispersive X-ray analysis (EDX). SEM clearly showed the presence of some weathering products adhering to th e surface of pumice. XRD showed that the products were composed mainly of noncrystalline materials with a relatively small amount of halloys ite (10 Angstrom). TEM and EDX revealed texture transformation sequenc es from the earliest weathering product to a final product of halloysi te as follows: 1) precipitation of very thin flaky or film-like noncry stalline Al-hydroxide; 2) transformation into irregularly aggregated A l-Fe-Si-rich fibrous material; 3) morphological changes to rounded agg regates; 3) development of halloysite-like curled domains with success ive decrease in Fe content; and 5) further development of curled domai ns in rounded aggregates. These materials must be metastable phases at early reaction stages with kinetics limiting formation of the stable equilibrium phase of kaolinite.