A manganese pan near Birness contains grains of an optically uniaxial negative mineral near (Na0·7Ca0·3)Mn7O14·2·8H2O, giving an X-ray powder pattern similar to that of synthetic materials described as ‘manganous manganite’ and δ-MnO2. Material giving a similar pattern has been described from a natural occurrence in Canada, but no mineral name was assigned; the name birnessite is now proposed. The mineral is probably formed by air-oxidation of manganous oxides under alkaline conditions.

Four new occurrences of vanadium minerals are described. New X-ray powder data are given for descloizite and mottramite, and show appreciable differences. Evidence is brought that the original occurrence of mottramite was not at Mottram St. Andrew, Cheshire, but Pim Hill, Shropshire, and that most if not all specimens labelled Mottram St. Andrew or Cheshire really came from Pim Hill.

Cosalite, aikinite, zinckenite, jamesonite, and boulangerite occur at Grainsgill; the first three minerals have not hitherto been recorded from Britain. The specimens probably come from a series of low-temperature quartz-carbonate veins, later than the original mineralization and also distinet from the east west copper-lead veins.

Previous work by Donnay on the basis of the geometrical theory of twinning related the width of albite twin lamellae to the composition of the plagioclase. The curve derived by Donnay has been recalculated for both high- and low-temperature felspars from X-ray data; the two curves differ markedly in the region between 0 and 20 % An. Some experimental evidence in support of the deductions that can be made is given.

Previous work has established that natural ‘low-temperature’ intermediate plagioclases show single-crystal diffraction patterns in which the subsidiary reflections are split into two; the separation of these split reflections appears to be dependent on the composition of the felspar. Several of these specimens have been subjected to varying heat treatments and their diffraction patterns examined.

It is found that over the whole composition range the split subsidiary reflections have disappeared after treatment at high temperatures, and only the principal felspar reflections, which are characteristic of an albite-like structure, remain. Natural specimens initially showing anomalous patterns can also be homogenized in this way by suitable heat treatments. A careful study of the mode of disappearance shows that the separation of the subsidiary reflections is unchanged as long as they remain visible.

The structural and petrological implications of this work are discussed.

Authigenic pyrite crystals or their pseudomorphs found in heavy residues from sedimentary rocks are cubes, octahedra, or pyritohedra. Combinations of these forms are common; trapezohedron and rhombic dodecahedron faces are rare. The cube is most often recorded in the literature, but the octahedron is dominant in some districts and may be favoured by calcareous environment. X-ray tests have been applied and goethite and hematite detected as alteration products.

Analytical data on a garnet-chlorite rock and the contained garnet with spectrographic determinations of the trace elements are presented, and the principles determining the distribution of these elements between the garnet and the chlorite are discussed.

Sepiolite has been identified in the Keuper Marl at a Midlands locality by X-ray diffraction data, electron microscopy, differential thermal analysis, and chemical analysis. It has been calculated that a sample representing 30 ft. thickness of clay may contain as much as 39 % sepiolite. Apart from sepiolite, the Keuper Marl in the area examined consists mainly of mica, quartz, calcite, and ferric oxide.

Quartz from a granite pegmatite near Kalkovo, Bulgaria, has undergone partial replacement by biotite along cracks parallel to (01;0); the biotite is in regular orientation relative to the quartz, and appears to constitute an instance of epitaxic replacement, defined as a replacement in which the replacing mineral develops in a regular, structurally controlled orientation relative to the mineral replaced.

Sixteen samples of Middle and Upper Chalk from the Chilterns yielded what appear to be fragments of fish scales, spines, and bones, which consist of collophane. X-ray examination by the powder method shows that the material has an apatite-type structure with cell dimensions close to those previously reported for francolite. Examination of single grains shows that the optical anisotropy is due to preferred orientation of the c-axes of the crystallites and this direction corresponds to the faster vibration direction. Such grains are found to be pseudouniaxial negative. Preferred orientation could not be detected in isotropic grains.