Cordierite commonly alters to chlorite-sericite, called pinite. Besides pinite alteration, cordierite from the Fishtail Lake shows textural evidence indicating retrogression to kyanite, andalusite, and coarse flakes of chlorite. This is consistent with the experimental data on the stability of cordierite and may be expected to be limited to potash-deficient rocks, such as the cordierite-garnetgedrite bearing gneisses of the area. In common potash-bearing pelitic rocks, retrogression of cordierite results in the formation of sericite and chlorite. In gneisses containing sillimanite and cordierite, the breakdown of cordierite in this way results in the association of the three aluminosilicates (kyanite, andalusite, and sillimanite) in the same rock. The association of kyanite-andalusite in equilibrium probably suggests that the retrogression of cordierite took place close to the univariant line of kyanite-andalusite.

A method of preparing compositions suitable for phase equilibrium studies in systems containing CaO-MgO-Al2O3-SiO2-Fe-O2-Na2O employing a gelling technique is described. The subsolidus assemblages obtained from the recrystallization of some of these gels and from the recrystallization of devitrified glasses of the same composition are compared. It is not yet clear whether either starting material has any advantage as the gels may retain early formed metastable phases and the glasses may fail to nucleate stable phases.

The recently described electrolysis of molten basalt (Oppenheim, 1968) gave indefinite results with regard to the behaviour of iron. Repeating the experiments in an inert atmosphere has shown that ferric iron moves towards the anode, and ferrous iron to the cathode. All metallic ions of valency three and more are now seen to be concentrated towards the anode, with relative enrichments in decreasing order: Fe3+, P, Ti, Si, and Al; and oxygen gas is released at the anode. Metals of valency one and two are concentrated towards the cathode with relative enrichments in decreasing order: Na, Fe2+, K, Ca, Mn, and Mg.

The series expected to be differentiated within a magma consequent upon its electrolysis is redefined. The absence of natural analogues suggests either the non-operation of suitable electric currents within magmas, or the destruction of their effects by convection, fractional crystallization, or both. An electrolytic process is feasible at the mantle-core boundary, leading mainly to deposition of iron on the core and release of oxygen upwards.

Chemical analyses and optical properties of ten orthopyroxenes, two clinopyroxenes, and six biotites (partial analyses) are presented from basic to intermediate charnockites of the Garbham area. The orthopyroxene is hypersthene to ferrohypersthene and the clinopyroxene is sahlite. Biotite has secondarily formed from pyroxene, particularly orthopyroxene, as a result of injection of potassic fluid in the basic charnockite. By combining the data of Srikakulam with those from other areas of the world it is shown that during the transformation of pyroxene to biotite in charnockitic rocks, in general, Mg and Fe 2+ attain near-equilibrium distribution between orthopyroxene and biotite as well as between clinopyroxene and biotite. The magnesium distribution coefficient (KD) of coexisting orthopyroxene and biotite is found to increase with the increase of temperature of biotite formation. A tentative geothermometric scale has been proposed plotting temperature of biotite formation against magnesium distribution coefficient of coexisting orthopyroxene and biotite. According to this scale, biotite in basic and semibasic charnockites from Garbham has formed between 285 and 465 °C.

Mineral data and a brief outline of the structure for zussmanite (ideal formula

KFe13Si17AlO42(OH)14)

have been published previously. This paper describes the determination of the crystal structure and presents and discusses the results. The structure consists of rhombohedrally stacked layers of Fe-(O,OH) octahedra to each side of which are attached rings of six (Si,Al)-O tetrahedra. These composite layers are linked to one another by rings of three (Si,Al)-O tetrahedra, and by potassium atoms, the latter in positions analogous to those in a mica. There is some evidence of disorder of the layer stacking. Interatomic distances and angles, and valency balance, are discussed.

A model has been constructed that establishes the geometry of inclusion fabrics to be expected in near-spherical crystals that grow continuously while either the crystals rotate in a static matrix or a matrix fabric rotates about them. Garnets in certain metamorphic rocks contain inclusion fabrics whose geometry is very similar to that of the model. Such syntectonic, snowball, or rotational garnets, as they have previously been termed, can be studied in three dimensions. A method is outlined whereby the rotation axes for individual crystals viewed in thick sections can be located in certain planes of cut. The significance of the geometry of included rotational fabrics is discussed in relation to previous work and to possible mechanisms of rotation.

A glass-bearing dolerite, which consists of orthopyroxene phenocrysts (γ 1·689, α 1·678, β 1·680, 2Vα 81°, S.G. 3·35) and a glassy mesostasis (n 1·568, S.G. 3·25), is described petrologically. Three new chemical analyses are given. From the petrological and chemical data it can be concluded that the orthopyroxene formed under intratelluric conditions and probably under high pressure. According to the recalculated chemical analysis of the orthopyroxene 0·22 Al3+ is in the Y position and much less Al3+ in the Z position. The chemical analysis of the residual glass is compared with similar residual glasses, and it shows that iron enrichment characterizes the earlier stages of crystallization differentiation, whereas the later stages are marked by an increase in Si4+, K+, and Na+.

Confusion over the status of crichtonite is due to the fact that de Bournon included two different minerals in his species: thin hexagonal plates are indeed ilmenite, as has been generally assumed, but the much rarer steep rhombohedra with basal plane are a separate species, with a distinctive X-ray powder pattern (three strongest lines: 3·39 Å,s; 2·875,s; 2·131,S) and a composition (Fe2+,Fe3+,Ti)1·71O3 with Fe2+:Fe3+:Ti near 8:7:33. There is a rhombohedral pseudo-cell containing three oxygens, with α 23° 19′, a 7·117 Å but there is some evidence of twinning, and the true symmetry is probably orthorhombic or lower.

The effects of hydrothermal treatments on gel mixtures and synthetic minerals in the C3S2-M3S2 and C2S-M2S regions of the CaO-MgO-SiO2 system have been investigated. The products formed in the temperature range 180-350°C at pressures up to 220 N/mm2 were identified. The reactants generally behaved as simple mixtures of binary silicates and their products agreed well with existing data for reactions in the CaO-SiO2-H2O and MgO-SiO2-H2O systems. An unidentified phase that resulted from the hydration of åkermanite could be a new compound in the quaternary CaO-MgO-SiO2-H2O system. There was no evidence to indicate that a magnesium analogue of kilchoanite can be formed from materials that are isostructural with γ-Ca2SiO4.

In an attempt to correlate the chemical compositions and cell sizes of omphacites and related pyroxenes, the cell dimensions of fifty-five analysed pyroxenes have been determined, or taken from the literature. Twenty-two of the chemical analyses are new, nineteen of them being done by electron microprobe. Approximately two-thirds of the total number of analyses may be considered first class, the remainder are of doubtful or unknown quality. Cell parameters, determined by X-ray powder diffraction methods, have errors of 0·1 % for the majority of samples, although for some samples taken from the literature errors are unknown.

The majority of methods of recalculating omphacite analyses into their end-member molecules are unsuitable for correlation of cell constants with chemistry, mainly due to the impossibility of graphical representation of more than three end-member molecules, and to the non-stoichiometry of these molecules. Using a modification of Tröger's (1962) method of recalculating chloromelanite analyses the present analyses have been recalculated into the diopside-jadeite-acmite and diopside-jadeite-hedenbergite molecules and compared with their determined cell parameters. Because of the gradations in all parameters between these end-member molecules, determination of compositions based on the cell parameters (a, b, c, vol, or β) can only be made within wide limits. However, using a method of projection of compositions from the acmite and hedenbergite apices to the diopside-jadeite join the ratios of diopside to jadeite can be determined for most samples to within ±5 mol%. As there are the most important constituents of most omphacites, this method permits an approximate estimation of omphacite compositions. From a knowledge of the cell sizes of the omphacite a rough indication of the conditions of formation of its host rock may also be obtained.

Surface areas of ‘amorphous’-type opals measured by nitrogen-absorption techniques were not consistent either with the surface area of the spheres of about 2000 Å diameter observable by electron microscopy or with the thermal dehydration data. This apparent inconsistency can be explained in terms of a smaller particulate structure of the order of 100–200 Å shown by higher resolution electron micrographs. This structure also accounts for a marked shrinkage shown in dilatometer curves of this type of opal.

Hydrothermal almost pure potassium feldspar (adularia) forms incrustations on fissured wall rocks in an active geothermal environment. Measured max. temperature, 265 °C, and concentration of K+ and Na+ cations in the geothermal fluid require K-feldspar and Na-feldspar to be deposited under equilibrium conditions. On fissure walls only K-feldspar is precipitated, whereas andesine phenocrysts of host rocks are replaced by both K-feldspar and Na-feldspar, and groundmass of host rock is replaced by K-feldspar.

Optical data indicate that K-feldspar forming incrustations is partly monoclinic, partly triclinic. The triclinic material displays polysynthetic and ‘microcline’-like twinning, though X-ray diffractograms indicate only monoclinic structure. A primary origin for the triclinic adularia is postulated. Wairakei hydrothermal K-feldspar, max. 0·5 Myr old, is compared with Arkansas adularia 287 or 214 Myr old, also containing both monoclinic and triclinic material. The comparison suggests that, after crystallization, the structural state of adularia changes little, if at all, during geological time.

The occurrence of coexisting paragonite (containing 0·70 % CaO) and muscovite (containing 0·16 % CaO) in a staurolite-grade paragonite-muscovite-hornblende-garnet-biotite schist from Sulitjelma enables an estimate to be made of the temperature of recrystallization, using an extension, based on crystal chemistry considerations and available analyses of muscovites and paragonites from metamorphic rocks, of the experimentally determined solvus in the K-Na white mica system into the Ca-K-Na system. This temperature of recrystallization is approximately 550–70 °C and is consistent with the experimental data on the coexistence of staurolite and quartz in the system Fe-A1-Si-O-H (Richardson, 1968).

A study of augite in over three hundred thin sections of mainly alkalic rocks permits the distinction of two main types of hour-glass structure. The common ‘swallow-tailed’, sometimes skeletal augite crystals are found in the fine-grained groundmass of many rock types, and it is suggested that rapid crystallization alone accounts for their formation. Hence, this type of hour-glass structure has been called ‘quench hour-glass’. The hour-glass structures of larger augite crystals of porphyritic and coarse-grained rocks are commonly described as hour-glass ‘zoning’, as they result primarily from compositional differences between the different sectors. These were formed under conditions of relatively slower cooling than the ‘quench hour-glass’, and thus cannot be explained in the same way. They are thought to have formed by a process involving adsorption of impurities on a particular crystal face so as to impede growth of these faces, producing an initial skeleton of hour-glass form, which is completed by later crystallization of augite richer in FeO, Na2O, TiO2, and Al2O3. This hypothesis also explains the patchy zoning of other augite crystals, casting doubt on some petrogenetic interpretations of such zones as core zones.

In the aureole of the syn-orogenic Hasvik Gabbro a variety of exsolution phenomena developed during the contact metamorphism. These exsolution phenomena have similar textures to those produced during the age-hardening of metal alloys. In feldspars the exsolution precipitates are closely related to the pattern of excesses over and deficiencies from the formula requirements, particularly in terms of SiO2 and Al2O3. The exsolution products are dependent upon both the temperatures of ageing and the details of the mineral chemistry. Evidence supporting an exsolution theory of origin for myrmekite is provided, and it is shown that precipitation from heterotype feldspar solid solutions during metamorphism may in fact be a complex process involving a number of precipitate phases.

The nucleation of sillimanite as a precipitate phase is discussed and it is shown how sillimanite develops at a number of distinctive sites within feldspar and garnet. From the textural relations of sillimanite precipitates and their host minerals a control exerted by the thermal stress configuration over the nucleation of precipitate phases is suggested. It is argued that the thermal stresses exerted across grain boundaries and within minerals during metamorphism may have considerable relevance to the interpretation of textures in metamorphic rocks.

The relationships between the 2:1 layer silicates are discussed in terms of the charge balance between the ions occupying X, Y, and Z sites. This brings out the relationship of glauconite to the illites, and clarifies the reason for the ideal occupancy of the Y sites in montmorillonite. It not only shows hectorite, saponite, and sauconite to be tri-octahedral smectites, but also shows vermiculite to be a tri-octahedral analogue of the illites. Apparently anomalous differences in permitted compositions of the di-octahedral and tri-octahedral micas are explained in terms of mismatch between the tetrahedral and octahedral layers, and it is shown that ephesite is to be regarded as a tri-octahedral common mica, not an analogue of margarite. Anandite is the nearest known naturally occurring equivalent to a tri-octahedral analogue of margarite, but the true analogue is a barium magnesium mica synthesized by Frondel and Ito.

Phase equilibria involving calcite, apatite, portlandite, liquid, and vapour were determined at 1000 bars. Calcite, apatite, and portlandite melt at 654 °C to form a liquid of composition (wt %), Ca(OH)2 53 %, CaCO3 46 %, Ca3(PO4)2 1 %. With water present melting occurs at 635 °C to form a similar liquid with a low water content. The X-ray diffraction patterns of the equilibrium apatites were not significantly different from synthetic hydroxyapatite.

Chlorine is estimated in nine hornblendes (0·13–1·12%) and nine biotites (0·19–0·62%) from the Kondapalli charnockites for the first time, using the electron microprobe. The data obtained suggest that these Kondapalli minerals are unexpectedly and unusually rich in Cl, which is well equilibrated in the five available coexisting hornblende-biotite pairs.

Fresh and weathered biotites have been studied using Mössbauer, chemical, and X-ray analytical techniques. In the Mössbauer spectra the complete interference of both low-spin ferric lines by one of the high-spin ferrous lines and the existence of high-spin ferric iron in the weathered biotite has been demonstrated. In view of this interference, which appears to be common, allowances must be made in spectral interpretation if chemical analysis reveals ferric iron to be present in detectable amounts. Hypothetical extension of the weathering trends observed suggests that high-spin ferric iron is the only stable iron species in the end product.

During weathering marked loss of ferrous iron occurs, whereas the ferric iron content remains approximately constant. As a result of the instability of ferrous iron in the weathering environment there is a significant rearrangement of the octahedral layer.

Clinoptilolite-heulandite is reported from Jurassic, Cretaceous, and Palaeocene sediments in south-east England. It occurs mainly in 1–20 μm size fractions, and is associated with abundant montmorillonite and often also with glauconite and amorphous silica or low-temperature disordered cristobalite-tridymite. Its mode of occurrence and the associated minerals suggest an authigenic origin, but there is no evidence that the source material was volcanic.