A muscovite that contains iron almost entirely in the ferric form has been studied by M6ssbauer spectroscopy. It is established that Fe3+ ions in the sites with cis hydroxyl groups have similar parameters in both muscovite and biotite. By assuming that Fe2+ ions will also have similar parameters for these sites in both minerals the M6ssbauer results of other workers have been reinterpreted. This new interpretation, which is in complete agreement with the X-ray structural analysis of Radoslovich (1960), indicates that in muscovite the sites with cis hydroxyl groups are filled in preference to those with trans hydroxyls. These sites are occupied primarily by trivalent ions. There is possibly a preference for divalent ions by the unique sites with trans hydroxyl groups, but occupation of these sites appears to occur only when the other sites have been filled.

The occurrence of cookeite, a di/trioctahedral chlorite of composition (Al3.95Fe0.02Mg0.01Li0.81)(Al0.74Si3.26)O10(OH)8, in the Back Creek pyrophyllite deposit near Pambula is described. The mineral is restricted to a relatively narrow zone near the centre of the deposit and although diaspore is also present in this zone the two have not been found in intimate association, it is concluded that the introduction of lithium has been the principal factor controlling development of the cookeite.

The effect of heat treatment in air at temperatures from 200°C to 1000°C of two Cornish biotites has been studied using the M6ssbauer effect in 57Fe. One of the micas was also studied after heating in vacuo.

Progressive changes in the Mössbauer spectra of the materials after heat treatment can be correlated with the following processes: Oxidation of Fe2+ ions in isolated octahedral sites to give Fe3+ in the co-ordination Fe3+(OsOH); oxidation of Fe2+ ions in adjacent a-octahedral (cis OH−) sites to give Fe3+(O6); and oxidation of Fe z+ in adjacent a and b (trans OH−) sites to give either Fe3+(O5OH) or Fe3+(O6) depending on whether a shared hydroxyl group remains intact or otherwise (these processes occur at temperatures below 500°C Also, at higher temperatures, dehydroxylation of Fe3+(O5OH) to give more Fe3+ in the Fe3+(O6) configuration. Finally, in the temperature range 900 to 1000°C structural breakdown yielding α-Fe2O3 as the iron-containing phase.

The results and interpretation are in good agreement with thermogravimetric data and with a previous infra-red study. The mechanism of charge diffusion in the lattice during oxidation is discussed in the light of the results.

Two clay samples originating in the kaolinized granite area near St. Austell, Cornwall, have been examined by Mössbauer spectroscopy. The iron-containing impurity in the sample with a red discoloration (sample A) has, by measurement of the hyperfine magnetic field, been identified as hematite. In the case of sample B, which was yellow in colour, preliminary Mössbauer work indicated β-FeOOH as the impurity, but detailed measurements of hyperfine field and Nöel temperature, when compared with similar results for a synthetic β-FeOOH sample, suggested otherwise. X-ray and chemical work suggested that the iron-containing phase in sample B is goethite, α-FeOOH in which some substitution of Al+3 for Fe3+ has occurred; the Mössbauer data are consistent with this conclusion. Both clay samples exhibited superparamagnetic behaviour and this has been utilized to obtain rough estimates of the mean radius of the particles of the iron-containing impurity. These were 117 Å for the α-Fe2O3 in sample A and 270 Å for aluminian α-FeOOH in sample B.

The lattice types of omphacite-diopside series clinopyroxenes were determined for 9 Japanese, 2 Norwegian, and 1 South African samples. The boundary between the compositional ranges of P and C lattice clinopyroxenes lies at about 14·5 wt. % CaO in Japanese and Norwegian samples, and the compositional gap between them is very small, if it exists. The South African omphacite has C-lattice in the compositional range of P lattice omphacite in metamorphic eclogites. These data combined with the reported electron microscope studies suggest that the omphacite—diopside series clinopyroxene has a C lattice structure above 3 to 400°C and very slow cooling after regional metamorphism is responsible for the transition to the ordered structure with P lattice symmetry.

A simple and easy-to-use miniphotometer has been designed; the expense involved amounts to only a fraction of that required for large commercially available microphotometers.

The instrument can be employed for routine investigations and for teaching reflected light microscopy in conjunction with standard reflected light microscopes.

The reflectance of grains down to 50 μm size can be measured in white and monochromatic light employing continuous band and line interference filters. The systematic error does not exceed 1.5 % of the values measured. The only aspect that distinguishes the miniphotometer from larger instruments is the limitation of spot size. A comprehensive test programme including comparative sets of measurements on a Zeiss MPM spectral microphotometer has shown that the results obtained by both instruments coincide well.

From field and petrographic evidence, some scoriaceous rock from the upper ignimbrite horizon of the Mount Rivers Volcanic Member of the Carboniferous Dyrring Formation in the Mirannie-Mount-Rivers District, New South Wales, Australia, is believed to have a recent origin.

By heating unaltered dacitic ignimbrite in a furnace it was possible to produce in the temperature range 600-1230°C changes in the rock similar to those observed in specimens of scoria collected in the field. Thus a temperature range for the alterations was established. Two possible sources of heat axe suggested as being responsible for the production of the scoriaceous rock, namely a bush fire and a lightning strike, with a preference for the former.