When amosite (fibrous grunerite, Fe5·5Mg1·5Si8O22(OH)2), is heated in argon or nitrogen, physically combined water is lost up to 500–700° C. Above 500° C (static) or 700° C (dynamic), dehydroxylation occurs endothermically, giving a pyroxene as the main product. Under dynamic heating conditions, part of the hydroxyl water is lost as hydrogen, with concurrent oxidation of the iron. At about 1000° C the pyroxene is decomposed to olivine and cristobalite; at about 1100° C melting begins. In oxygen or air, physically combined water is again lost below 500–700° C. At 350–1200° C a sequence of overlapping dehydrogenation, oxygen absorption, and dehydroxylation reactions occurs, which gives rise to a broad exotherm on the d.t.a. curve. The main products (for static heating conditions) are an oxyamphibole at 350–800° C, and a spinel, hematite, a pyroxene, and X-ray amorphous material at 800–1100° C. Silica crystallizes as cristobalite at 1100–1350° C, and as tridymite at 1450° C. Most of the products in either neutral or oxidizing atmospheres are formed topotactically. The mechanisms and rates of the reactions are discussed, and the problem of determining the chemically combined water in amosite and other minerals of similar composition is considered.

Chemical and optical data are presented for the orthopyroxenes, clinopyroxenes, garnets, plagioclases, and opaque oxides in high-grade regionally metamorphosed basic rocks and their garnetiferous associates from the Willyama Complex. Exsolution phenomena in the pyroxenes have been studied by X-ray diffraction methods and with the electron probe micro-analyser. The (001) lamellae in one lime-rich clinopyroxene are clinohypersthene. Comparison of the Willyama and other granulite facies clinopyroxenes with those of slowly cooled basic igneous rocks suggests that their chemical compositions may be used as guides to temperature conditions during formation. Willyama plagioclases are complexly twinned and possess intermediate structural states.

A unique meteorite, the existence of which has been known since 1941 but which had been lost, has now been relocated in the form of fragments in two separate collections. Though material available is limited to small fragments, it is sufficient to reveal the principal characteristics of the parent mass, which was a variable body composed of stony material not dissimilar from that of the unbrecciated enstatite-achondrite of Shallowater, Texas, U.S.A., and nickel-iron not dissimilar from the ‘pseudo-octahedrite’ of Horse Creek, Colorado, U.S.A. There is evidence suggesting that it was a stony-iron of a hitherto unrecorded type, and that silicate material predominated.

Part of a meteorite was seen to fall on a road (52° 33′ 55″ N., 1° 20′ 23″ W.) in the village of Barwell, Leicestershire, at about 16.20 hrs G.M.T. on 24 December 1965. Fragments have been found over an area some ¾ mile long by ½ mile across, and appear to have been part of a single stone, of which 44 Kg (97 lb) have been recovered. The distribution, impact effects, and crustal morphology of the recovered fragments are described. Barwell is a moderately metamorphosed white olivine-hypersthene chondrite; a chemical analysis, with the derived Wahl norm, and a modal analysis are given, together with optical data and electron-probe analyses of the principal constituent minerals.

Data on the contents of individual rare earths are summarized for 24 members of the aeschynite-priorite series and 3 lyndochites. It is shown that the data are in accord with the assignment of lyndochite to the aeschynite-priorite series. The available X-ray data are not definitive, but indicate at least strong similarity of lyndochite to aeschynite ; the classification of lyndochite as a variety of euxenite is excluded.

The various (P, T) stability fields of iron-free zoisite have been deduced for systems containing excess silica and water with Ca : Al ratios ranging from 1 : 2 (anorthite) to 3:2 (grossular). Zoisite is a possible phase in all systems with Ca:Al lower than 3:2, attaining its maximum stability at the zoisite (Ca:Al = 2:3) and prehnite (Ca:Al = 2:2) compositions.

The consequences of varying the Al : Fe ratio are next examined. Zoisite with 4 % pistacite is stable to 525° at 2 kilobars, compared with ∼585° for clinozoisite and 620 to 630° for epidote (Ps35) at the same pressure. Increasing iron content also stabilizes epidote minerals relative to their low-temperature and high-pressure decomposition products.

Examples of natural zoisite-clinozoisite, zoisite-epidote, and clinozoisite-epidote assemblages are described. It is concluded that zoisite-epidote mixtures result from the disproportionation of clinozoisite outside its own stability field, but within those of zoisite and epidote. The assemblage zoisite-clinozoisite is probably not stable, but further evidence is needed on this point. The assemblage clinozoisite-epidote is stable below 550° C, at which temperature the solvus in the Al-Fe series closes.

A full-matrix, three-dimensional refinement of kalsilite, KAlSi04 (hexagonal, a 5·16, c 8.69 Å, P6a), shows that the silicon and aluminium atoms are ordered. The respective tetrahedral distances of 1·61 and 1·74 Å agree with values of 1·61 and 1·75 Å taken to be typical of framework structures. As in nepheline, an oxygen atom is statistically distributed over three sites displaced 0·25 Å from the ideal position on a triad axis. This decreases the bond angle from 180° to 163° in conformity with observations on some other crystal structures. The potassiumoxygen distances of 2·77, 2·93, and 2·99 Å are consistent with the wide range normally found for this weakly bonded atom.

Electron-diffraction photographs of some natural and synthetic mullites and a typical sillimanite have been obtained and are interpreted using diffraction data due to Agrell and Smith. The position and diffuseness of mullite reflections (h0l) with l = ½ have been re-studied. In such reciprocal lattice sections maxima are symmetrically disposed in pairs about the positions of sillimanite reflections with l odd. They lie on a* rows and show different separations in the specimens studied. These diffraction conditions are closely analogous to those observed in the intermediate plagioclases (paired ‘e’ maxima) and in nepheline, and a similar explanation is envisaged in terms of antiphase domain structure. Intimate association of a mullite-type phase and sillimanite is shown to result from the breakdown of muscovite in a thermal metamorphic aureole, and a chemical analysis of a natural Fe,Ti-bearing mullite from a spinel-mullite buchite in the same aureole, is presented.

The course of all crystalline structural changes undergone by the mineral beryl, its melt and glass, has been followed as a function of widely differing but accurately known thermal history. Non-equilibrium ordering processes have been characterized by new techniques in high-temperature microscopy.

The relative influence that beryllium and aluminium ions exert on the phase changes has been established by studies on synthetic melts and glasses representing hypothetical beryls of differing ratios of Be2+:Al3+ ions.

The dynamic changes between the oxide and orthosilicate structures into which the systems reconstitute include the formation of a metastable hybrid beryllium aluminium silicate, which is viewed as a beryllium-containing mullite. The crystallochemical changes have been interpreted in terms of a structural model of the melts and glasses that is shown to be consistent with the ordering effects predicted from the field strengths of the cations.

26 b-axis oscillation photographs of alkali feldspars of microsyenitic dykes of Southern Greenland are presented. The crystals are rather restricted in composition (Or42·4 to Or35·3) but are rather rich in Ca (up to 14 % An). The range of variation of the optic axial angles, 2 V 45° to 71° (exceptions up to 87°), puts the crystals between sanidine-anorthoclase cryptoperthite and orthoclase microperthite, as classified by Tuttle (1952).

Since the syenitic magma cooled down rather rapidly some peculiarities in the structural state of the alkali feldspars are to be expected. The potassium-rich phase is (nearly) monoclinic. According to the values of the reciprocal lattice angles α* and γ* the sodium-rich phases can be divided into three groups: high-temperature pericline-twinned and high-temperature albite-twinned phases, high-temperature pericline-twinned and low-temperature albite-twinned phases, and high-temperature pericline-twinned and low-temperature albite-twinned phases with nearly identical values of the reciprocal lattice angle γ*.

A hydrated barium pyrochlore (pandaite) has been found at Mrima Hill (Kenya) in a residuum consisting mainly of goethite/limonite and gorceixite. The pandaite occurs as pale yellow or white octahedral crystals, which have a micro-crystalline interior. It is isotropic, the refractive index varies from 2·07 to 2·11. The calculated specific gravity is 4·07, and the unit cell dimension is 10·59 Å. After deduction of impurities the following formula was obtained (Na0·03 K0·02 Mg0·05 Ca0·09 Ba0·36 Ln0·05 Th0·01 Pb0·004) (Ti0·21 Nb1·78 Ta0·01) (H2O)1·64 O5·52.

Comparison of the calculated formula, essentially A0·614B2 (H2O)1·64 O5·52 with the ideal pyrochlore formula A2B2O6F indicates important deficiencies in the structure. Differential thermal analysis shows a large endothermic peak at 340° C and moderate exothermic peaks at 480 and 780° C. In its physical, chemical, X-ray, and thermal properties the Mrima pandaite is similar to pandaites described from other localities.

The nature of the alteration of primary pyrochlore to pandaite has been studied using an electron probe.

Two small fragments of unpromising-looking ferruginous material, recovered from inside the Dalgaranga crater, near Mount Magnet, Western Australia, have been sectioned and, while one is largely composed of iron oxides and adds little to the record of this crater, the second fragment is one of the rare, predominantly stony ones reported by Nininger and Huss, and is sufficiently fresh for a thin section to be prepared. As far as is known no thin sections have been described of material from this crater. Microscopic study with transmitted light shows the stony material to be achondritic and to have affinities with the mesosiderites, and, in spite of the low nickel-iron content of this fragment, it seems preferable to regard it as representative of an iron-poor area within a mesosiderite than as true stony meteorite material. Although closely related to the howardites it cannot be classified with any known achondrite type because of the prominent olivine phenocrysts and it bears no resemblance at all to chondritic material. Mineralogical and petrographical details are described and illustrated, and the significance of this new evidence discussed. The origin of the crater is reconsidered.

Several cordierites from pegmatites were found by emission spectroscopy to contain as much as 0·7 wt. % of BeO. Structural crystallographic considerations suggest that the beryl molecule, Be3Al2Si6O18, may have appreciable solubility in the structure of cordierite. Some hydrothermal experiments carried out on this assumption indicated about the same amount of BeO substitution in the magmatic temperature range as is shown by the pegmatite cordierites. The natural BeO-bearing cordierites have distinctive infra-red absorption properties, which are reproduced in the synthetic solid solutions.

Leucite and pseudoleucite invariably have lower K/Rb ratios and higher K/Sr and K/Ba ratios than the groundmasses of the rocks in which they occur. The distribution of Rb, Sr, and Ba between K-rich minerals (e.g. alkali feldspars) and the liquids from which they crystallize is generally such that Rb is impoverished and Sr and Ba enriched, relative to K, in the crystallizing phase. Leucite shows the opposite relationship and this is attributed to the leucite structure being sensitive to the size and valency of the ions replacing K.

Reyerite is found in close association with gyrolite in the metamorphic aureole of the volcanic plug of 'S Airde Beinn, northern Mull, and has apparently been formed by metamorphism of the gyrolite. The gyrolite, with a composition close to Ca16Si24O60(OH)3.14H2O, closely approaches trigonal symmetry with a 9·76±0·02 Å and c 3 × 22·26±0·1 Å; biaxial with a small 2V and α 1·537, β 1·547, γ 1·549. The reyerite is trigonal with a 9·76±0·02 Å, c 19·10±0·04 Å and space group Pω 1·568 and ε 1·563, and the unit cell contains about 14 atoms of Ca, 24 of Si (with some substitution by 4(OH)−), and 14H2O as (OH)− and H2O.

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.