Three new analyses of kalsilite are given, with optical and X-ray data; two (K/(K + Na) = 0·95. 0·98) are from rocks where it is closely associated with nepheline. An analysis of one of the nephelines is also given (K/(K + Na) = 0·31). The cell dimensions vary appreciably with sodium content, and the composition of kalsilite may be determined from the equation:

100 K/(K + Na) = 100−227·27(Δ 6·264),

where Δ is the difference, in degrees, between the diffraction angles 2ϑ for 102 and 101, using Cu-Kα radiation.

An experimental and theoretical study has been made in order to determine the number and the structure of the possible polymorphs and to determine the structural relations between them. The simplest structures are 1M, 2M1, 2M2, 3T, 20, and 6H polymorphs, and more complicated types can be developed. Some of the previously described polymorphs were not contained in the theoretical list and were re-examined. The 6M structure was found to be a 2M2 polymorph, the 6-layer triclinic type was found to be a 2M1 polymorph, and the 3M structure was shown to be a 3T type. The 24-layer triclinic structure could be described on a simpler 8-layer cell. This type together with a new 12-layer monoclinic structure, as well as other structures of higher periodicity, presumably consists of complex stacking and results from spiral-growth mechanism. Two extreme types of layer-disordered crystals may be built and a disorder of individual ions may also occur. Single stacking faults result in twinned crystals. A new twin relation (180° rotation about the [100] axis) has been recognized. Twenty specimens from extreme geological environments have been examined in order to evaluate the control of environment on the stacking. The type of stacking could not be attributed solely to the influence of pressure and temperature. Composition appears to play a dominant role in the type of stacking, and semi-quantitative structural arguments appear to support this contention. The influence of growth mechanism is discussed. A scheme for the identification of the mica polymorphs by X-ray powder and single-crystal methods is given.

Blanfordite (a manganiferous aegirine), rhodonite, alurgite, manganophyllite, and piemontite are deseribed from a pegmatite that cuts a manganese ore band in the Sitasaongi mine, Chikla, Bhandara district, India. In addition, rhodonite and piemontite are described from gneiss and schist areas. Chemical analyses and optical properties of all these minerals are given and their possible origin suggested.

The Eskdalemuir tholeiite has the following modal composition: glass 42·2 %, felspar 24·4 %, pyroxene 30·6 %), magnetite 2 %, chlorite 1%. The glass has been separated and analysed. It has 70% SiO2 and is of granitic composition. On a selected triangular diagram the plots of the Eskdalemuir rock, its residual glass, and its crystallinc portion are spread along a narrow belt marking the compositions of the Tertiary tholeiites. Because the differences in the chemical compositions of the phrases of the Eskdalemuir tholeiite are produced by crystallization, it is concluded that the variety of composition of the other Tertiary tholeiites is consistent with a genesis controlled by crystallization differentiation.

Phase relationships in the section CaSiO3-MgSiO3-Al2O3 through the quaternary system CaO-MgO-Al2O3-SiO2 have been determined. The bearing of the results on the genesis of aluminous pyroxenes and associated minerals in metamorphic rocks is discussed.

This zeolite, hitherto recorded only from Chaux de Bergonne, France, has been found as zoned spherules with thomsonite (var. faroelite), on specimens from Aci Castello and Aci Trezza, Sicily, and Capo di Bove, Rome, from Weilberg, Rhineland, and from Klöch, Styria. Three chemical analyses confirm previous results; optical data suggest some variation in composition within a spherule. There is a striking resemblance between X-ray fibre photographs of gonnardite and of thomsonite, and between X-ray powder photographs of gonnardire and of natrolite. The Klöch occurrence, where the gonnardite is accompanied by phillipsite, is described in detail.

When using many patterns of integrating stage the depth of focus is so great that it is not possible to distinguish between the upper and lower surfaces of a thin section; this may introduce serious errors into micrometric analyses. A procedure to meet this difficulty is suggested.

Thin-section analysis is essentially an areal measurement, the measurement area usually being the upper surface of the section. If transmitted light is used for the measurement, the apparent areas of opaque grains will in general be somewhat larger than their true areas on the measurement surface. For strictly spherical opaque particles in a transparent matrix the expected excess of apparent over true area is shown to be (πr2k)/(2r + k) where r is the spherical radius and κ is the thickness of the thin section. A table shows the relation between true and apparent area as a function of r/k.