Zonal myrmekitic and granophyric intergrowths in plagioclase and K-feldspar phenocrysts are described. There are two such zones in the feldspars of the Late Granite, G2, but only one in the Early Granite, G1. The zonal granophyric intergrowths are the end-product of the replacement of myrmekitic plagioclases by K-feldspar. Transitional stages of this replacement, which took place in the solid state, are represented in numerous examples. The zones of myrmekite, which also represent growth in the solid state, occur between zones of magmatic plagioclase, which is oscillatory-zoned. It is concluded that the granites evolved at depth under the influence of at least three phases of volcanic activity. Each of these phases resulted in the partial melting of the country rocks with subsequent magmatic crystallization, followed further, under the slow cooling conditions, by extensive reaction in the solid state. During each of the final two phases, some of the granite was intruded as a crystal mush to form G1 and G2, in which the phenocrysts retain evidence of their evolution at depth whilst the ground-mass gives evidence of the final magmatic crystallization.

The alkali feldspars in the Ardara pluton are microcline- and orthoclase-microperthites. Their compositions have been determined by the 20 X-ray method, both before and after homogenization. The bulk compositions are very rich in potassium as a result of postmagmatic recrystallization of the alkali feldspar. There is no significant variation in either the bulk composition or the composition of the potassic phase in different parts of the intrusion. The structural state of the feldspars has been studied by measurement of 2V and obliquity of the potassic phases, and the reciprocal lattice angles α* and γ* of the potassic and sodic phases in selected crystals. Most of the specimens contain near-maximum microcline, but there is a decrease in the obliquity of the potassic phase at the eastern end of the intrusion. This can be related to the subsequent intrusion of the Main Donegal Granite.

The values of 2θ 01 for some homogenized lime-rich fine microperthites from the Kangerdlugssuaq alkaline intrusion, East Greenland, are investigated and found to be consistently high when compared with the values used in the standard curves for estimating the Or content of alkali feldspars. This deviation, which is approximately proportional to anorthite content, results in anomalously low Or percentages being obtained and is ascribed to a distortion of the structure due to the unusually high calcium content of the microperthites.

The necessary conditions for optical pleochroism in orthopyroxenes are: ordering of Fe2+ ions, with preferential occupation of M2 positions; presence of small, high-valence cations (Al3+, Fe3+, Ti4+) in M1 positions; replacement of Si by Al; and a sufficient Fe2+ content. These factors favour an intensification of the charge-transfer absorption bands in the ultra-violet that is more marked for the α vibration direction than for β and γ. This leads to absorption in the blue and a red colour in transmitted α-polarized light, while β and γ, remain green.

In this paper we give data for An and Sr in plagioclase to demonstrate the presence of numerous amphibolite xenoliths within parts of a basic igneous mass where, after initial reconnaissance mapping, none was suspected. The data also show that the concentration of Sr in plagioclase from the margin of some amphibolite xenoliths is appreciably higher than in plagioclase from either the cores of the xenoliths or the enclosing metagabbro even though the modes and An contents of the plagioclase are similar. An explanation is offered, which implies that some of the xenoliths were amphibolites rather than anhydrous equivalents prior to their enclosure in the basic magma.

An investigation of the system Cu-Fe-O has established the existence of a phase near 3Cu2O.Fe3O4 and the non-existence of Cu2O.Fe2O3 a in the system over the range of oxygen pressures investigated.

The X-ray diffraction patterns obtained from four samples of natural delafossite have been found to be similar to that obtained from a synthetic sample of the 3Cu2O.Fe3O4 phase. It has also been shown that, when heated under equilibrium conditions in the thermobalance in air, a sample of natural delafossite and the synthetic 3Cu2O.Fe3O4 behaved similarly.

The subsolidus phase equilibrium of the lithiophilite-tephroite system was studied in the temperature range 500 to 900° C. The solubility of tephroite in lithiophilite is limited and increases with temperature. The reaction is very sluggish and has peculiar kinetic properties. No reverse solubility was observed. Similar asymmetry was found in the LiMgPO4-forsterite system of which a preliminary study was made. Apparent absences of silicate phosphate olivine solid solutions in nature are related to our experimental findings and to geological factors.

A test of the linearity of a photomultiplier has been made using crossed polars as an optical attenuator. An expression is derived that allows for the measurable transmission of a polarizing filter in directions other than the permitted vibration direction. The linearity is shown to be adequate for accurate reflectivity measurement.

Four orthopyroxene: calcic-pyroxene pairs from pyroxene-hornblende- and pyroxene-granulites from the Precambrians around Saltora, eastern India, have been chemically analysed. The distribution coefficients with respect to Mg-Fe (KD) are found to be 0·56, 0·57, 0·60, and 0·61, a set of values well within the limits characteristic of metamorphic parentage.

A comparison has been made of the relative speed, accuracy, and precision of several methods of measuring large optic axial angles with the universal stage. It is concluded that a method based on the direct location of a single optic axis and the application of the Biot-Fresnel law will frequently be the most satisfactory when only the standard, low refractive index centre plate for the stage is available. If a centre plate of high refractive index is employed, however, good results can normally be obtained by using the more rapid method based on the direct location of both optic axes.

The mineralogy of the green, yellow, and brown secondary constituents occurring as amygdales, irregular infillings, and replacements in the Rajmahal basalts of eastern India and commonly described under the ill-defined names palagonite and chlorophaeite, has been revised on the basis of optical, X-ray, and thermal studies. The results indicate that these coloured constituents, which have been chiefly derived by the alteration of pyroxenes, iron ores, and primary glass, belong to the clay minerals, particularly smectite and vermiculite. This brings in question the validity of the terms palagonite and chlorophaeite which, though widely current in basaltic literature, have no definite mineralogical identity. It is suggested here that they are but forms of clay.