Tubular and spheroidal halloysite in pyroclastic deposits in the area of the Roccamonfina volcano (Southern Italy)

Abstract

Halloysite-bearing weathered pyroclastic deposits of the Roccamonfina volcanic area (Southern Italy), consisting generally of light grey clay masses richer in slightly weathered pumice grains, mottled patches of whitish clayey material and yellowish-brown veins along vertical root channels, were characterised by chemical analysis, X-ray diffraction (XRD), infrared spectroscopy (FT-IR), optical (OM), scanning (SEM) and transmission (TEM) microscopy, electron diffraction (ED) and energy dispersive X-ray analysis (EDXRA). Both tubular and spheroidal forms of halloysite were observed, different parts of the deposits being characterised by a particular morphology. No consistent differences in terms of the chemical and mineralogical composition of the deposit materials were found. However, the chemical composition of the different particle-size clay fractions indicated that less silica, with concomitant relative enrichment in aluminium, and loss of bases characterised the <0.5 μm compared with the 0.5–2.0 μm fraction reflecting their differences in the content of primary minerals. The amounts of Fe, Al and Si extracted by dithionite–citrate–bicarbonate (DCB) and oxalate reagents suggested the presence of short-range-order aluminosilicates but this could not be proved by other physical techniques. X-ray analysis revealed the presence of hydrated (1 nm) and dehydrated (0.7 nm) halloysite, mica, sanidine and plagioclase minerals. TEM/EDXRA analysis of the <0.5 μm clay fraction revealed an amorphous Si-rich phase in the sample containing spheroidal halloysite. The differential X-ray diffraction analysis (DXRD) from untreated and DCB treated yellowish-brown material along with the electron diffraction (ED) of the <0.5 μm clay fraction showed the presence of goethite and ferrihydrite. The various weathered pyroclastic materials showed a major difference in terms of their physical characteristics, particularly in terms of their compactness and porosity/permeability as revealed by microscopic and sub microscopic observations. It is suggested that the different morphological forms of halloysite may result from conditions of growth in the different microenvironments related to variability in physical conditions.