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Halloysite Formation Through in Situ Weathering of Volcanic Glass From Ciay Minerals (1988) 23, 423-431 mineralogy. Phys. ition of Mössbauer L HALLOYSITE FORMATION THROUGH Ih' SITU shaviour? J. Mag. WEATHERING OF VOLCANIC GLASS FROM 3n analysis of two TRACHYTIC PUMICES, VICO'S VOLCANO, ITALY d.X, 29-31. ir Conímbriga and P. QUANTIN, J. GAUTHEYROU AND P. LORENZONI* ts correlation with .central Portugal). ORSTOM, 70 route d'Aulnay. 93143 Bondy Cedex, France, and *ISSDS, Piazza d'Azeglio 30, Firenze, Italy ibrico da cerâmica (Received October 1987; revised 5 April 1988) of dolomites, clays ABSTRACT: The weathering of a trachytic pumice within a pyroclastic flow underlying an ., MADWCKA.G., andic-brown soil on the volcano Vico has been studied. The main mineral formed is a spherical 10 A halloysite which has been shown by SEM and in situ microprobe analysis to have formed norphology on the directly from the glass. The major mineralogical characteristics as determined by XRD, IR, DTA, TEM and microdiffraction are typical of 10 A halloysite. However, some minor mineralogical properties and the high Fe and K contents, suggest that it is an interstratification of 74% halloysite and 26% illite-smectite. The calculated formula of the hypothetical 2:l minerals reveals an Fe- and K-rich clay, with high tetrahedral substitution, like an Fe-rich vermiculite, but the detailed structure of this mineral remains uncertain. This study deals with the weathering of trachytic pumices to a white clay which seems to be derived directly from glass, without change in texture. This clay is a well crystallized 10 A halloysite, and although nearly white in colour, has an unusual composition being rich in Fe and Ti, and having a high K content. The aims of the study are to show the transformation of glass into halloysite, to discuss the structural formula of this clay mineral, and to suggest the hypothesis of a composite building of 1 :1/2 :1 phyllosilicates. MATERIALS AND METHODS The weathering of trachytic pumices has been observed in the fourth pyroclastic flow of Vico, in the Latium, Italy (Fig. 1). This flow dates back to about 140,000 BP (Sollevanti, 1983) and is covered by an andic-brown soil (andic Dystrochrept) under which the coarse pumices are almost completely weathered to a white gel whereas the flow matrix is weakly altered. As the alteration occurs only at the top of the flow, near the soil, it is probably due to weathering. The reference soil profile, related to the weathered pumice sample, is Vico 29 (Lorenzoni et al., 1986; Bidini et al., 1987). An unaltered pumice from the bottom of the flow, and a strongly weathered pumice from the top, near the soil, have been analysed. The petrography of the unaltered pumice was studied in thin section by optical microscopy. The mineralogy of the powdered rock was determined by X-ray diffraction (XRD) and the chemical composition was obtained by a Li metaborate fusion method. The weathered pumice was studied in situ along a transect (Fig. 2), from the weakly weathered greyish core, through a semi-altered transition zone, to the completely weathered white cortex, first by optical microscopy, and then by scanning electron microscopy (SEM) 0 1988 The Mineralogical Society ORSTOM Fa..& QocuneBtaire No ; -30.049 .@yl 424 P. Quantin et al. and microprobc powdered rock the < 0-2 pm f VITERBO XRD was p dehydrated at 1 at IOO’C, and 2 electron micros at 100°C and 1 (TGA). The < elements were (ORSTOM Lat fraction. The ci ;ampling s and pH 7 by paracrystalline (Parfitt & Heni 1ROMA The ambient cl - 1400 mm. Th FIG.1. Location of Vico’s volcano. soil through thc obvious illuviat pumices are ver the cement ma. alteration of th glasses. There “argillified” py! 1978). The poor -. vertical drainag WHITE VERY ALTERED PUMICE Bidini et al., 19, +=+- Ca’+, 24 mEq/l soil has a pH o ,TRANSITION TO VERY ALTERED GREY SEMI ALTERED PUMICE The grey unaltei The pumice is augite, biotite ai chemical comp (A1203 24%, 1 The white weath In situ observa FIG. 2. Transect across altered pumice. original fibrous -, . .. .j/ - - .-.---*(_Y- ---------~-rp.-T>7*.- -~jc- Halloysite formation from volcanic glass 425 and microprobe analysis for the major elements. The < 2 pm fraction was separated from the powdered rock after cation exchange with NH,f and dispersion by ultrasonic agitation, and the < 0.2 pn fraction further separated by ultracentrifugation. XRD was performed on the whole powder, on the > 2 pm fraction (wet, and also dehydrated at 100°C) and on the oriented < 2 pn fraction (wet, glycerol solvated, dehydrated at 100°C, and calcined at 470°C). The < 2 prn fraction was also studied by transmission electron microscopy (TEM), infrared spectroscopy (IR), (on the sample wet, and dehydrated at 100°C and 1SOOC), differential thermal analysis (DTA) and thermo gravimetric analysis (TGA). The <Oe2 pn fraction was examined by TEM. Chemical analysis of the major <.- .elemen& were “Óbtained oñtlíe “triacid” extract of the whole weathered pumice powder P - __-- (ORSTOM Laboratory of Chemistry method), and by microprobe analysis on the < 2 pn fraction. The cation exchange capacity (CEC) of the whole powder was determined at pH 4 and pH 7 by NH, oxalate-oxalic acid extraction (Quantin, 1982), and allophane and paracrystalline oxides were determined by the NH, oxalate (pH 3, in the dark) method (Parfitt & Henmi, 1982). THE WEATHERING CONDITIONS The ambient climate is near to wet-temperate (Lulli et al., 1988) with an annual rainfall of - 1400 mm. The weathering of pumices is due to the slow infiltration of rainwater from the soil through the weakly-permeable matrix of the volcanic tuff. However, there is not any obvious illuviation of humic acids or mineral products from the soil. The soaked white pumices are very weathered and very hydrated (HzO- = 65%) whereas, around the pumices, the cement matrix and the fragments of massive lava remain weakly altered. -This rapid alteration of the pumice is due to its high porosity as well as to the fibrous texture of the glasses. There has not been any alteration prior to pedogenesis as occurred with the “argillified” pyroclastic tuffs of Mt. Sabatini, near Bracciano in Italy (Lenzi & Mattias, 1978). The poor permeability of the tuff results in a waterlogged environment with very slow vertical drainage. At the base of the soil, the weathered tuff (ref. samples 29C and 29R1, ‘UMICE Bidini et al., 1987) has a pH of 7.2, and a CEC of 70 mEq/100 g composed of 40 mEq/100 g Ca2+,24 mEq/100 g K+ and 4 mEq/100 g Mg*+.However, the weathered pumice under the soil has a pH of 6.2, revealing a slight cation-lixiviation in the pumice itself. i .TERED s RESULTS MICE The grey unaltered pumice The pumice is composed of hair-like vesicular glass (> 80%) and crystals of sanihine, green augite, biotite and titano-magnetite, as well as a few relics of zeolitized leucite (Table 1). The chemical composition is similar to that of an alumina-rich alkaline-potassic trachyte (Alzo3= 24%, K20= 7*1%,NazO = 2.6%) poor in Fe, Ti, Mg and Ca (Table 2). The white weathered pumice - In situ observations. When examined by optical microscopy, the pumice is seen to retain its original fibrous vesicular texture, even in the completely weathered white cortex (Fig. 3a). 426 P. Quanrin et al. TABLE1. Mineralogical composition. White Pumice2 Grey Pumice’ Fraction >2 pm <2 pm Glass + + + + 10 A Halloysite ++++ 8 Sanidine ++ Mica; 10-14 Sm ++ 1 Augite + Zeolites + < 0.5 Biotite + Sanidine ++ - Ti-Magnetite + Augite + - Zeolites tr Ti-Magnetite + < 0.5 I Grey = very weakly altered; 2White = strongly altered. + + + + >60%; + + 10-20%; + 5-10%; tr trace. TABLE2. Chemical composition. in siru microprobe analysis Whole Whole White unaltered grey -t transition + white cortex altered <2 pm Oxides pumice core ring a b pumice fraction normalised % (1) (2) (2) (2) (2) (3) (4) SiQz 57.79 56.81 52.29 51.63 49.94 49.66 50.84 A1203 24.26 34.48 35.89 38.18 38.40 37.58 38.62 3.88 3.99 6.07 6.57 7.03 7.30 7-25 Tio2 0.52 0.64 0.84 0.84 044 0.98 1.00 MnO 0.15 0.06 0.00 0.06 0.00 0.07 0.13 M’go 0.68 022 0.24 0.24 0.30 0.49 0.29 Ca0 2.88 0.88 1.18 0.78 0.62 1.76 0.66 K2O 7.12 2.07 2.17 1.36 1.67 1.69 0.97 Na20 2.59 0.32 0.28 0.17 0.28 0.29 0.0 1 p20, 0.10 0.22 0.14 0.18 0.19 0.15 0.22 so3 n.d. 0.30 047 - 0.74 - - Mol. Si02/A1203 4.05 240 2.48 2.30 2.2 1 2.24 2.24 Mol. SiO,/R,O, 3.67 2.61 2.24 2.07 1.99 1.99 2.00 (1) Whole pumice, by fusion method; (2) by microprobe: a-windows at the surface of fibres, and b-spots on single particles; (3) whole white pumice, by “Tri-acid extraction”; (4) <2 pm powder, by microprobe. I I Phenocrysts of sanidine, augite, biotite and magnetite remain unaltered and some very small opaque crystals of magnetite can be seen along the fibres. There is no product of illuviation. The fibrous texture can also be seen by SEM in its original form in the core as well as in the k cortex (Fig. 3b), but the glass surface looks more granular in the white cortex than in the grey core (Fig.
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