Techniques associated with temperature control and measurement in quench furnaces are described and a study made of calibration problems. Systematic temperature errors as great as 30°C in published results from separate laboratories are explicable. A comprehensive redetermination of temperatures of invariant equilibria in systems containing CaO, MgO, Al2O3, SiO2, Na2O, Fe-O2, has been initiated and the results to date are presented, including approximate determinations of three invariant equilibria in the system CaO-MgO-Al2O3-SiO2 involving liquid, monticellite, spinel, and two of merwinite, periclase, and forsterite, which are important in the melting of magnesia refractories, and precise determinations of the three newly recognized invariant equilibria involving liquid, spinel, diopside, and two of forsterite, melilite, and anorthite.

The position of the 01 X-ray reflection in albite varies with the degree of Al/Si ordering. This is also true for binary plagioclases but the 01 reflection for high-temperature plagioclases is constant and therefore independent of anorthite content. For plagioclases of ternary composition grown in a hydrous silicate melt, the position of the 01 reflection gives a good estimate of the orthoclase content when a determinative curve is used that was originally drawn from data on high-temperature synthetic binary alkali feldspars.

Data are presented from cobaltite (CoAsS), gersdorffite (NiAsS), and ullmannite (NiSbS) for cell size, compositional zoning, mineralogical association, X-ray diffraction powder intensities, and optical anisotropy. The presence of powder X-ray diffraction reflections 001 and 011 are related to the crystal structure type. The amount of distortion from cubic symmetry is related to the 001 reflection intensity and also to the optical anisotropism strength. These properties are related approximately to the thermal stability of the distorted crystal structure. This thermal stability increases with the substitution of cobalt for nickel, which decreases the cell size. An order-disorder change occurs before the distortion release in non-cubic gersdorffite (P1) with a large cell size, whereas only a distortion release is observed in cobaltite (Pca21) and non-cubic gersdorffite (P1) with a small cell size. Glaucodot (Co,Fe)AsS converted to cobaltite at 620 C. No evidence was found to relate optical anisotropism of pyrite to deviations from cubic symmetry.

The principal data are collected about the fall and the distribution of the fragments of the Siena, Italy, meteorite (11·6°E., 43·1°N.). A complete individual, weighing 110·55 g, is described in some detail. Crust morphology, mineralogical composition, and structure were studied. Optical data were established by microscopical analysis and both thin and polished sections were observed. Compared with available electron-probe analysis they are found in good agreement. The chemical analysis is Fe 12·93, Ni 1·39, Co 0·09, FeS 5·46, SiO2 37·10, TiO2 0·14, Al2O3, 3·91 FeO 11·46, MnO 0·35, MgO 23·81, CaO 1·63, Na2O 0·90, K2O 0·16, P2O5 0·44, H2O+ 0·10, H2O− 0·00, Cr2O3 0·56, total 100·43. Siena is an ordinary chondrite, with relatively ambiguous characteristics and evident features of recrystallization and metamorphism.

Chemical analyses, optical properties, and specific gravities are presented for nine axinites. A reciprocal substitution relationship is demonstrated between Mg and Mn2+, and a linear variation diagram is constructed between refractive indices and the (Fe2++Fe3++Mn2++Ti) ions calculated on the basis of 16(O,OH). The composition of these axinites is independent of the composition of their host rocks; their colour appears to be independent of the Fe2+/Mn2+ ratio for the range of iron and manganese values of the specimens investigated.

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.

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.

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.

If the true values of a number (n) of variables are exactly related by one or more (m) equations but all experimental values are liable to experimental error then, provided the errors of measurement can be assumed to be symmetrically distributed about a zero mean, provided the relations are linear, and provided the assessed probable errors of measurement of the several variables remain in constant ratio from one set of measurements to another, the best estimate of the relations derived from N sets of measurements will be given by the latent vectors corresponding to the m smallest latent roots of the n-square matrix whose terms are Σwi ξij ξik (summed from (summed from i = 1 to i = N) for all j and k from 1 to n, where wi is the weight of the ith set of measurements and ξij is the standardized value of the ith measurement of the jth variable measured from its weighted mean.

It is also shown that the alleged inconsistency of the maximum-likelihood estimate of the variance in such a case is simply a confusion of the root-mean-square residual with the root-mean-square normal residual.

An Autocode computer programme has been written to carry out the necessary operations for correlating the physical properties and chemical properties of an isomorphous series of minerals using this procedure.

Cell dimensions have been measured from correctly indexed powder patterns of thirty natural and two synthetic orthopyroxenes. The natural specimens (mostly metamorphic) contain exsolved phases largely avoided during microprobe chemical analysis. Linear regression analysis between the cell dimensions and Mg, Al, Ca contents (ignoring minor elements) was quite unsatisfactory until a term in Mg2 was added. Two separate regression analyses for the ranges Mg 0–0·5 and 0·5–1 using only Mg, Al, and Ca were satisfactory. Olivine yielded satisfactory regressions for the whole range without a term in Mg2 (Louisnathan and Smith, 1968). The difference between olivine and pyroxene results from absence of site preference by Mg and Fe in olivine compared to strong preference in pyroxene revealed by electron density and Mössbauer studies (Bancroft, Burns, and Howie, 1967; Ghose and Hafner, 1967).

The data were recalculated by means of a new best-fit procedure developed by Hey in which the errors in the chemical and physical parameters are considered simultaneously.

Earlier measurements of a and b for plutonic pyroxenes are consistent within possible experimental uncertainties with those given here; however a and b data obtained by Kuno and by Hess on volcanic specimens are considerably higher by variable amounts. Although there are uncertainties in the Ca content and its effect on a and b, the larger dimensions probably result from lower site preference as indicated by Mössbauer studies.

Prediction of Mg, Ca, and Al from just the cell dimensions is only moderately accurate even for metamorphic orthopyroxenes.

Many examples of epitaxic relations between coexisting pyroxenes are found in the picritic dykes that cut the Lewisian gneisses of the Lochinver area, Sutherland. In some dykes augite forms oriented overgrowths on the (100) face of orthopyroxene; in others orthopyroxene forms similar overgrowths on augite. A number of unusual variants of the first case are described together with a complex ‘induced’ twinning found in the augite overgrowths. Analyses of the coexisting pyroxenes suggest that the overgrowth relation has been facilitated by increased solid solution between the pyroxenes, so reducing the crystallographic misfit along the c-axis. However, crystallization of the pyroxenes in an ordered sequence rather than simultaneously also seems to have been an important factor.

Specimens of tavistockite fall into two groups: true tavistockite from the George and Charlotte mine, Tavistock, Devon, and wavellite from the Stenna Gwyn mine, St. Austell, Cornwall. Both were sold as tavistockite by the discoverer, Richard Talling.

Tavistockite proper is a fluorapatite, as shown by optical and X-ray examination, and the alumina and water in the original analysis are certainly derived from kaolinite with which the apatite is intimately associated.

The published optical properties attributed to tavistockite were determined by E. S. Larsen on Stenna Gwyn material, and are those of wavellite.

Re-examination of a portion of Buttgenbach's type bialite, which he related to tavistockite on optical grounds, shows it to be wavellite.

The thermal decomposition of alunite and of mixtures of alunite with dickite or quartz or both has been studied, and X-ray determinations of the products at various stages of the decomposition have been made. Between 600 and 700°C the product from alunite is a mixture of anhydrous alum and amorphous alumina; between 700 and 770°C the anhydrous alum breaks down into aluminium sulphate and alkali sulphate; then in the prominent 850°C endothermic reaction three-quarters of the sulphate is lost, leaving a mixture of alkali sulphate and γ-alumina. The area of the 850 °C endothermic peak may be used as a measure of the percentage of alunite in alunitized rocks (alunite-quartz-dickite mixtures).

The obliquity values of microcline perthites are nearer to maximum microcline type, indicating that they have less than 10% Ab in their lattices, apart from exsolved albite. The perthites are classified based on form as stringlet, string, film, vein, and replacement types. It is concluded that stringlet and string perthites are formed by exsolution along contraction cracks, followed by film as a result of decrease in rate of cooling and increase in water vapour content of the pegmatites, whereas vein and replacement perthites are formed last, by replacement when the pegmatites crystallized in a closed system. Based on the chemical composition of microcline perthites, a temperature of 600 °C is arrived at for unmixing of feldspar to give rise to microcline perthite.

The muscovites are 2M1 polymorph type; minimum temperature of crystallization of 435 °C is obtained from the molecular percentage of paragonite in muscovite.