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

doi:10.1016/S0016-7061(00)00076-8

Geoderma

Volume 99, Issues 3–4, February 2001, Pages 295-316

https://doi.org/10.1016/S0016-7061(00)00076-8 Get rights and content

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.

Introduction

Studies on formations of volcanic ash and pumice deposits aimed at characterising their clay mineralogy and weathering sequences have been extensively carried out all over the world Nagasawa, 1978, Sudo and Shimoda, 1978, Dixon, 1989, Wada, 1989. As a consequence of localised variations, either in the composition of the volcanic materials or in the intensity of decomposition processes, it has been found that different weathering products may be detected within the same rock mass (Aomine and Wada, 1962). Halloysite is a well-known weathering product of pyroclastic materials (Dixon, 1989) and the morphology of this 1:1 layer mineral has been shown to be closely related to its genesis Nagasawa and Miyazaki, 1976, Quantin et al., 1988, Bailey, 1990. Most commonly, halloysites formed by alteration of volcanic glass and pumice have the shape of balls or scrolls, sometimes associated with short-tubes, whereas those formed by weathering of feldspar in granitic rocks and those formed by deep weathering of sands have the shape of long tubes (Nagasawa, 1978). There is evidence that the total SiO₂, Al₂O₃ and Fe₂O₃ contents of halloysite vary with its morphology Tazaki, 1982, Noro, 1986. The dissolution rate and the solid/water ratio of the starting material also seem to have a strong influence on the precipitation processes thereby producing morphological variations Nagasawa, 1978, Tomura et al., 1985.

Halloysite-bearing pyroclastic deposits are present in the Roccamonfina volcanic area of Southern Italy. White halloysite formed from deeply weathered volcanic materials has been reported to be present in the vicinity of Ailano and other places in the neighbouring valleys (Caserta province, Campania Region) on the slopes of hills made up by Mesozoic limestone–dolomitic rocks. It has been suggested that, probably, the process of formation involves: (i) dispersion and deposition of erupted pyroclastic materials all over the volcanic area, (ii) water erosion and accumulation in the valleys of the volcanic products (iii) in situ attack by water containing dissolved carbon dioxide Sinno, 1966, Sinno, 1967, De Gennaro et al., 1973.

During studies of weathered in situ pyroclastic deposits near Ailano, it was found that both tubular and spheroidal halloysite occurred within the same outcrop. It was considered that this represented a good opportunity to compare the chemistry and mineralogy of these two morphological forms and from this to draw inferences that might clarify their conditions of formation.

Section snippets

Description of the area

The Roccamonfina volcanic area is located in Northern Campania about 60 km Northwest of Naples and 140 km Southeast of Rome. It is characterised by a Quaternary volcano standing out from a wide tectonic depression surrounded by Mesozoic limestone–dolomite structures (Mts. Aurunci NW, Mt. Massico SW, Mt. Maggiore SE and Mt. Camino N).

The Roccamonfina stratovolcano is part of the alkali-potassic magmatic Campano-Roman Province and the beginning of its activity dates back 1.5–2.0 Ma. The volcano

Materials and methods

The halloysite-bearing pyroclastic deposits studied are located south-east of Ailano, in the western part of the Roccamonfina volcanic area (latitude 41°23′25″N, longitude 14°12′08″E), covering in discontinuous an area of about 5 km². According to Sinno, 1966, Sinno, 1967 and De Gennaro et al. (1973) the deposits consist of in situ weathered volcanic materials mainly eroded by water and accumulated in the valley. The outcrops range in thickness from a maximum of 6 m to less than 1 m. The

OM and EM/EDXRA observations

Optical observations of thin sections showed that all the pyroclastic materials were mainly formed of scoriae, vitreous, vesicular and highly porous pumices, xenoliths of lava and phenocrysts of sanidine, labrodoritic plagioclases and biotite embedded in a microgranular groundmass formed of glassy fragments. The vitro-clastic texture of the grey mass was loosely compacted compared with that of the whitish mottled material. The pumices often were characterised by a fibrous appearance with

Discussion

The above results show that the weathered pyroclastic deposits to the south east of Ailano in the Roccamonfina volcanic area consist largely of fully hydrated 1-nm halloysite together with dehydrated 0.7-nm halloysite, mica, sanidine and plagioclase feldspar. Halloysite exhibited both tubular and spheroidal morphology in different areas of an outcrop of the pyroclastic material, with the light grey clay mass (sample 1) consisting of tubular particles and the whitish mottles (sample 2) of

Conclusions

Tubular and spheroidal halloysite have been found in different parts of the same outcrop of weathered pyroclastic material originating from the Roccamonfina volcano in southern Italy. Comparison of the chemical and mineralogical characteristics of the weathered material with which the different forms of halloysite are associated revealed few substantial points of difference that could explain the different morphologies of the clay mineral. However, micromorphological observations show that

Acknowledgments

The authors are grateful to Professor Koji Wada of Kyushu University (Japan) and to Professor R.J. Gilkes of Western Australia University for their suggestions which improved the presentation of the results and for the helpful review of the manuscript. Thanks are due to Mr. Gennaro Cafiero (CIRUB-Napoli) for his valuable assistance with the electron microscopes and to Mr. Maurizio Clumez for his skilful technical assistance in the electronic preparation of the figures. This research was aided

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

Abstract

Introduction

Section snippets

Description of the area

Materials and methods

OM and EM/EDXRA observations

Discussion

Conclusions

Acknowledgments

Geoderma

Geochim. Cosmochim. Acta

Geoderma

Differential weathering of volcanic ash and pumice resulting in the formation of hydrated halloysite

Am. Mineral.

Halloysite — a critical assessment

Quantification of Allophane and Imogolite

L'halloysite come prodotto di alterazione nelle vulcaniti campane e laziali

Atti Acc. Sc. Fis. e Mat., Napoli, s. 3

Kaolin and serpentine group minerals

Silica in soils: quartz and disordered silica polymorphs

Semiquantitative chemical analysis of asbestos fibers and clay minerals with an analytical electron microscope

Clays Clay Miner.

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Electron diffraction and the study of ferrihydrite coatings on kaolinite

Clay Miner.

Iron oxide removal from soils and clays by dithionite–citrate systems buffered with sodium bicarbonate