It is usually assumed that the oxidation state of the small proportion of Mn sometimes present in micas is +2, although there is evidence from electronic spectroscopy (Burns, 1970) for at least the occasional occurrence of Mn(III) in manganophyllite. We describe here X-ray photoelectron spectroscopic (XPS) measurements on the Mn in a Norwegian lepidolite which was the subject of a concurrent structural study by X-ray photoelectron diffraction (Evans & Raftery, 1982). To establish the Mn oxidation state we have compared the Mn2p core-electron binding energies (BE), the Mn2P3/2-O ls BE differences, and the Mn2p XPS peak profiles from the four common oxides of manganese (MnO, Mn3O4, Mn2O3 and MnO2) with those from the lepidolite. A re-examination of these oxides was undertaken because the agreement between reports in the literature was unsatisfactory, and uncertainty existed concerning the integrity of some of the surfaces previously examined.

It is generally agreed that the English mid-Jurassic and Lower Cretaceous fuller's earths were derived from volcanic ash. In the Lower Cretaceous fuller's earth of Woburn, Kerr (1932) recognized partially decomposed relics of shards in a matrix of montmorillonite, and Grim (1933, 1935) described montmorillonite pseudomorphs after glass fragments in the Bath fuller's earth of Bathonian age. Jeans et al. (1977) published twenty-two SEM pictures of pyroclasts, including sanidine, sphene, trachytic pumice, and a basaltic glass fragment. Photomicrographs of the Lower Cretaceous fuller's earth show shard relicts ranging in length from 0·8-1·26 mm (median ∼1·12 mm) and in thickness from 5-15µm (Jeans et al., 1977, fig. 14a). However, much remains obscure about the shape, size and mode of packing of the argillized vitric particles which make up the bulk of fuller's earths. This note describes the texture of an English fuller's earth which was freeze-fractured (Tessier, 1978), prior to examination with a scanning electron microscope.

The explosive ejection of material from a volcanic vent produces pyroclastics. These are stratigraphically important because of their isochronous nature, lateral extent and potential for absolute age determinations. The pyroclastics also provide information on the nature of the volcanic activity, which is possibly the only record because of non-preservation in other areas. The recognition of pyroclastics is therefore important, but often difficult,because the high-temperature igneous material is susceptible to alteration in the diagenetic environment. In Quaternary sediments the composition of glass shards is an important aspect of tephrochronology (Westgate & Briggs, 1980) but in older sediments glass shards are usually altered to clay minerals. Exceptionally, shards are preserved unaltered, as in the case of early carbonate and silica cementation (Francis et al., 1968; Jeans et al., 1977). According to Ross (1928), features that are diagnostic for the recognition of a volcanic clay (bentonite is the term used by Ross) are, in order of apparent reliability: (i) presence of glass relict structure; (ii) presence of minerals and crystal forms characteristic of volcanic rocks, including biotite, feldspar, pyroxenes and hornblende; (iii) absence of minerals that do not have a volcanic origin, including microcline, garnet, muscovite, kyanite and andalusite; (iv) waxy lustre; (v) reaction to water; (vi) optical properties; (vii) chemical properties.

Precise identification of clay minerals found in granular rocks has always posed a great problem to the clay petrographer. Even if it is possible to locate the position of authigenic clay mineral formation in a thin section, subsequent identification of this same material by X-ray diffractometry is usually very difficult. Attempts have been made using selected-area radiation of thin sections (Pawluck & Dumanski, 1973; Wicks & Zussman, 1975; Wilson & Clark, 1978) but the area analysed remains relatively large, i.e. of the order of several mm2. The other solution is micro-picking of material from a thin section and subsequent identification by Debye-Scherrer camera methods (Wallace, 1955; Rickwood, 1977). This method, however, does not allow preferred orientation, and thus precise identification, of many clay species. The ideal method is to combine micro-picking from thin sections from areas of several hundreds of square microns with an oriented sample preparation, which can then be treated in the traditional way (glycolation, heating, etc.) for characterization by X-ray diffractometry.

Traditionally, adsorption from binary solutions is determined as the difference between the adsorbate concentration in the solution before and after contacting with the solid adsorbent e.g. hexadecylpyridinium-clay. this is essentially a static method. The bulk composition can be measured by means of a differential liquid interferometer (Dékány et al., 1978b) or by gas chromatography (Nikulitchev et al., 1979). The specific excess adsorption of the mixtures, n1°, is then calculated as (Kipling, 1965):

1

where n0 is the total amount of the liquid mixture referred to unit mass of the adsorbent (mmol/g adsorbent), xl,0 is the mole fraction of the ith component before adsorption, and xi is the mole fraction of the ith component in the equilibrium homogeneous liquid phase.

In the laboratories of the Geological Survey of Sweden we have for many years used an acid-resistant, stainless steel filtering funnel to prepare ‘oriented’ clay mounts for X-ray diffractometry. The funnel consists of three parts (left and centre foreground of Fig. 1). The circular plate is porous and made of sintered stainless steel and serves as a support for a 47 mm diameter membrane filter. The cylinder is manufactured in two different sizes, 250 ml and 600 ml. The funnel is generally mounted on a suction flask but when the filtrate has to be analysed a suction cloche is preferable.

When many mounts have to be prepared the use of a manifold is practical and time saving. We have adapted the Sartorius six-branch manifold and added a transition tube of stainless steel (right foreground of Fig. 1), threaded to fit the manifold outlet and tightened through a gasket. The funnel itself is mounted on the tube through a rubber stopper. Suction is by a small pump (25 l/min, 0·15 kW). A suction flask is placed between the filter stand and the pump to collect the combined filtrates (Fig. 2).