Mineralogical and chemical characteristics of five nontronites and Fe-rich smectites

Nontronites are the ferric members of dioctahedral smectites, most of which are structural counterparts of beidellite (Brindley, 1980). The existence of a cont inuous b e i d e l l i t e n o n t r o n i t e series was suggested on the basis of chemical analyses by Ross & Hendricks (1945). However, a nontronite with montmorillonitic structure has been described by KSster (1982). Minor amounts of Fe oxides and oxyhydroxides, in particular goethite, are commonly associated with nontronite. Opal has often been found in the vicinity of nontronite occurrences. Such ancillary minerals must be removed from nontronite samples prior to analysis if reliable structural formulae are to be calculated and comparison of different nontronites is to be made. Foster (1951) detected exchangeable Mg ions on the surface of montmorillonites and discussed the influence of this on the measured cation exchange capacity (CEC) and the calculated structural formulae of montmorillonitic clays. Unfortunately this early work has been neglected in subsequent studies. In this paper we describe procedures for the processing and analysis of clay minerals, and apply these to the mineralogical and chemical characterization of five nontronites and Fe-rich smectites. The aim of the oxygen isotope study was to determine the effect of Fe on smectite-water fractionation and to constrain the temperatures of Fe-smectite formation. We adopt here the definitions of nontronite (>1,0 octahedral Fe per half unit-cell) and Fe-rich smectite (0.3-1.0 octahedral Fe per half unit-cell) of Gtiven (1988) for greenish Fe-bearing smectites. G E O L O G I C A L S E T T I N G Nontronite from Hoher Hagen near Dransfeld, Lower Saxony, Germany The Hoher Hagen is a mid-Miocene alkali olivine basal t dome located 1 5 km southwes t o f G6ttingen. Nontronite forms thin coatings on sand grains at the contact of Upper Oligocene sands with 9 1999 The Mineralogical Society 580 H.M. K6ster et al. overlying basalt (Bolter, 1961; Schneiderh6hn, 1965). Nontronite from Manito near Spokane, Washington, USA Nontronite from a railroad cutting 1.5 km west of Manito, Spokane County, Washington, USA, (Reference Clay Mineral No 33b of API Research Project No. 49) occurs in deeply weathered zones within lava flows of the mid-Miocene Columbia River basalts (Allen & Scheid, 1946; Kerr & Kulp, 1949; Hosterman, 1969). Nontronite from Olberg near Hundsangen, Rhineland-Palatinate, Germany The Olberg, located ~10 km northwest of Limburg/Lahn, was one of the basaltic eruption centres of late Oligocene to Pliocene volcanism in the Westerwald area (Ahrens, 1938, 1960). The greenish nontronite is found in strongly altered peridotite nodules with relics of orthopyroxene and picotite (K6ster, 1960) which are embedded in otherwise fresh-looking basalt. Fe-rich smectite from Oberpullendorf Burgenland, Austria The late M i o c e n e basa l t s tock nea r Oberpullendorf is located at the western border of the Pannonian basin (Embey-Isztin et al., 1993). Iron-rich smectite is a ubiquitous weathering mineral found in vesicles of the basalt and in tuffaceous material (Schwaighofer & Miiller, 1979). Associated minerals are sphaerosiderite, aragonite and frequently opal (Schroll et al., 1965). Fe-rich smectite from Sauteloup, Dordogne, France The Sauteloup quarry is located 3 km south of the village Buisson near Les Eyzies de Tayac. A palaeokarst sinkhole in Maastrichtian limestones is filled with halloysite-bearing kaolinitic sands of early Tertiary age, which are topped by Eocene ferricretes (Millot, 1970; Simon-Coinqon et aL, 1996). Green Fe-rich smectite occurs in layers several cm thick or as small veinlets within the ferricretes, close to opal-CT-bearing silcretes (Simon-Coingon et al., 1996). C L A Y P R O C E S S I N G A N D A N A L Y T I C A L T E C H N I Q U E S Clay processing techniques Crystal chemical studies of smectites require extraction and purification of the minerals of interest from the raw clay samples. Firstly, carbonates and Fe oxides must be removed and cations naturally adsorbed on the clay surface exchanged for a defined cation, e.g. Na. The Na clay is easily dispersible in distilled water. During particle size fractionation, smectites are concentrated almost quantitatively in the finest (<0.2 p.m) clay fraction, whereas all other mineral components and accessories of the clay samples will be concentrated in the coarser fractions. The clay samples were first treated with a 0.1 M EDTA solution at pH 4.5 and 60~ to decompose carbonates. If this procedure is repeated twice, all Ca adsorbed on the clay is exchanged for Na. Most adsorbed Mg, however, remains on the clay surface and can be exchanged for Na using a weakly alkaline 0.1 M EDTA solution at pH 8.0 (K6ster, 1982; K6ster et al., 1973). Iron-rich clay minerals often show a mottling with Fe oxides. Iron oxides, with the exception of magnetite and other Fe-rich minerals of the spinel group and ilmenite, can be removed from clay samples by a reduction with Na dithionite in a Na citrate solution buffered to pH 7.3 with Na bicarbonate (Mehra & Jackson, 1960). Only minor loss of Fe and changes in the CEC on treating a nontronite in this manner led Mehra & Jackson (1960) to state that the dithionite-citratebicarbonate system "has almost no destructive effect on iron silicate clay minerals". A later study by Russell et al. (1979) indicated significant loss of tetrahedral Fe 3+ from nontronites following dithionite reduction, but the reduction in that study had been carried out in an unbuffered acid environment. The proportion of Fe (5.9%) extracted from a sample of Garfield nontronite associated with goethite by a dithionite treatment in a buffered solution, in contrast, did not differ greatly from that attributed to goethite in the same sample by M6ssbauer spectroscopy (4.6%), indicating that the nontronite may have suffered, at most, minor attack by the dithionite treatment (Murad, 1987). We believe that purification of the samples with Na dithionite provided it is carried out in a properly buffered (slightly alkaline) solution is preferable, for the characterization Characteristics of nontronite and smectite 581 of nontronites, to an assertion of sample purity without this treatment. Particle-size fractions were isolated by sieving (>63 ~m), settling (63-20, 20-6, 6 -2 and <2 ~tm), and centrifugation (2-0.6, 0.6 0.2 and <0.2 gm) of the Na clays suspended in distilled water. Generally the finest clay size-fractions (0.6 0.2 and <0.2 ~tm) are free from quartz, feldspar, mica and other accessory minerals. The finest clay fractions contain the smectites and minor amounts of opal or amorphous silica, which can be determined by the method of Hashimoto & Jackson (1960). Particlesize distributions of the processed smectites are shown in Table 1.