The close interrelation between sintering and grain growth is discussed critically, and the necessity for control, but not suppression, of grain growth is established. A distinction between discontinuous and exaggerated grain growth is drawn, and the superiority of grain boundary films as grain-growth control agents is demonstrated. Work on the establishment of tentative criteria for the selection of grain-growth control additives for alumina is reviewed and discussed, and the extension of these ideas to the selection of NaF as a successful aid for magnesia is described.

Authigenic envelopes up to 0·18 mm thick on clastic potassium feldspar grains from arkoses and subgreywackes of the Devonian Nannyarra Greywacke have been examined with the four-axis universal stage. The plane of the optic axes is parallel to (010) in some segments of individual envelopes and normal to (010) in others, and the range of 2Vα over all envelopes is from close to 0° to 65°. The optics are ascribed to modifications of potassium feldspar corresponding to high-and low-sanidine and orthoclase.

The associations hornblende and anthophyllite or cummingtonite are dealt with briefly in the light of experimental data and naturally occurring assemblages. On the basis of the distribution of Mg and Fe2+ in coexisting amphiboles and on the calculated and experimental slopes of a reaction in which the amphiboles hornblende and anthophyllite are formed a tentative P-T-x diagram is suggested for these parageneses.

Russellite Bi2O3. WO3 occurs in a small pegmatite near Poona, Western Australia. The fine-grained yellow to pale green material is an inseparable mixture of russellite, bismite, koechlinite, and bismutite. X-ray powder diffraction, physical, and chemical data agree in general with the original description of the mineral from Cornwall, England. The original analyses made on micro quantities are now supplemented by analyses on macro quantities.

The manganiferous amphibole juddite has been reported by a number of workers from metamorphosed manganese formations of India. The mineral is developed at the contact of pegmatite and gondite, and associated characteristically with blanfordite (manganoan aegirine-augite) and winchite (manganoan richterite-tremolite). The range of the refractive indices of the mineral and the orientation of the optic axial plane normal to (010) suggest the mineral to be of magnesioriebeckite composition. The 100 Mn2+:(Mn2++Mg+Fe2+) and 100 Fe3+:(Fe3++Alvi+Ti) ratios plotted in the glaucophane-magnesioriebeckite diagram show that eight out of nine analyses (eight new) are grouped under magnesioriebeckite composition range. It may thus be concluded that the juddites generally refer to (manganoan) magnesioriebeckite compositions. In some cases, however, the composition of juddite shows considerable departure from ideal magnesioriebeckite composition and the juddite and winchite (manganoan tremolite-richterite) show evidences of compositional transition, indicating partial miscibility. The juddites have been formed by the interaction of pegmatitic fluid on manganese ore, gondite, and other country rocks such as mica schists and diopside-forsterite marble.

This paper is a report on the various features of the crystallogenesis of calcite as shown in specimens collected from iron-ore skarn deposits. Although there are several forms, each showing a variety of habit, a detailed study shows that there is a unified sequence of successive habit substitution. A structural analysis of calcite based on P.B.C. (Periodic Bond Chain) vectors is then considered.

It is argued on crystal-chemical grounds that Si deficiency is compensated by the entering of Al, Fe3+, or Ti4+, in this order of preference, into tetrahedral sites. It is pointed out that Ti4+ occurs in tetrahedral coordination in exceptional circumstances only, while Fe3+ relatively easily takes a tetrahedral coordination.

Unit cell dimensions and refractive indices have been determined for synthetic pyroxenes in the system CaMgSi2O6-CaFeSi2O6-NaFeSi2O6, which have been crystallized and annealed on the nickel-bunsenite (Ni-NiO) oxygen buffer curve at 700 °C and a total pressure of 2 kb. The data presented confirms the many postulates that a complete solid solution series does exist in the subsolidus region of the ‘ternary’ system. The unit cell dimensions show an almost linear change with composition between the end members. The determined cell parameters for a series of analysed natural alkali pyroxenes have been compared with the data for the synthetic pyroxenes and close agreement was found between both sets of data. The variation of the unit cell dimensions and refractive indices has been examined in relation to the possible ionic substitutions taking place in the pyroxenes.

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.

The densification of pure magnesia was examined along with mixtures containing 0·5 and 1·0 mol % of calcium oxide and forsterite. The sintering behaviour at temperatures between 1400 and 1800 °C was studied and measurements made of changes in shrinkage, porosity, bulk density, and grain size for sintering times of up to 8 h at sintering temperature. It was found that both the 0·5 and 1·0 mol % additions enhanced the sintering of pure magnesia in the temperature range 1500–1700 °C Increased grain growth was found for mixtures containing forsterite but little change occurred for mixtures containing lime. Examination of the grain growth data showed that the mixtures obeyed a relationship: Time at temperature × Constant = (grain size)n, where n = 2 for the grain growth of pure magnesia and mixtures containing lime, but n = 3 for mixtures of magnesia with additions of a silicate such as forsterite. A value of 78 kcal/mole was obtained for the grain growth of pure magnesia, which agrees with other researches.

The extent of the solubility of lime in periclase for each lime-magnesia mixture at each temperature was also examined using electron probe analysis and related to the sintering behaviour observed for these mixtures.

Forty-three natural alkali feldspars have been homogenized and used to correlate with wt % K2O. The regression of Or on is Or (wt %) = 465·5 (Å)-1877·5, and is comparable to that obtained on an exchanged orthoclase (Wright, 1968). A deviation from the Orville (1963, 1967) determinative line for synthetic sanidine-albite is greatest at the high potassium end of the series. The discrepancy is not caused by failure to homogenize or incomplete disorder. The presence of cations other than potassium and sodium, or cation deficiency, or both cannot be discounted as a cause.

Many syntheses have been carried out in order to establish the possibility of replacing manganese in spessartine, Mn3Al2(SiO4)3, by various elements (Be, Mg, Ca, Sr, Ba, Pb, Cu, Ni, Co, Fe2+, Sc, Yt, Zn, Cd) as well as aluminium by Sc, Yt, Zr, Ti, Cr, Fe3+ by a solid state reaction. The syntheses were done by sintering pellets of the desired composition at about 1100 °C for 5–10 hours under normal pressure. It has been found that an inert atmosphere creates more favourable conditions for the syntheses of spessartine analogues, compared to the results obtained in the open-air reaction, where often spinel was obtained. All the reaction products were submitted to X-ray and chemical analysis and to IR spectroscopical examination.

Chemical analyses, optical properties, specific gravities, and cell dimensions are presented for six topazes. A linear relationship is confirmed between the b cell dimension and the OH/(OH+F) ratio.

Olivine microphenocrysts in a large microporphyritic chondrule in the Parnallee chondrite show a linear preferred orientation of the long (β = c) axes, which is thought to reflect flowage of a partly crystalline parent magma. The orientation pattern is somewhat disturbed by post-crystallization fracturing.

Sandstones from Mull and Morvern, most of which are from the Greensand formation, have been shown by X-ray diffraction to contain minerals of the plumbogummite group in very small amounts in the < 1·4 µm fraction. The minerals were concentrated by HF digestion of the clay minerals. X-ray spectrographic traces show concentrations of Sr, La, Ce, Yt, and Ba, and a semiquantitative spectrochemical analysis also shows a concentration of Ca and Pb and the presence of numerous rare-earth elements. Individual minerals, which have a complex composition and can not be assigned to any one named species, are disseminated throughout the rocks as particles with an estimated size of between 0·1 and 0·25 µm.

Wardite, NaAl3(OH)4(PO4)2. 2H2O, has a = 7·03 Å, c = 19·04 Å; space group P41212 or P43212. Its crystal structure was solved by a three-dimensional Patterson function computed using intensity data photographically collected by the Weissenberg method, and refined by successive Fourier maps and least-squares cycles to a R index 0·062 for 316 independent observed reflections.

The wardite structure is formed by layers of Al and Na coordination polyhedra sharing vertices and edges. These sheets, parallel to the a axes, are connected to each other in the c direction by PO4 tetrahedra and H-bonds. This structural feature accounts for the perfect {001} cleavage of wardite and explains the change that occurs in lattice parameters when Al is substituted by Fe in avelinoite. The relationships with the minerals of the trigonal families of crandallite, woodhouseite, and jarosite are also discussed.

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.

A trachyte plug near Jos contains a suite of inclusions, commonly rounded and of large size, including olivine nodules, pyroxenites, and a variety of basic to intermediate plutonic rocks, as well as albite megacrysts.

The olivine nodules are typical of those found elsewhere in alkaline basic lavas and widely interpreted as upper mantle fragments. Most of the other inclusions contain evidence of crystallization at high pressures, even the syenites, which are interpreted as plutonic equivalents of the trachyte; for the feldspar in them is sanidine-microperthite.

Discovery of this inclusion assemblage in a trachyte provides further support for the thesis that alkaline salic lavas can be generated at sub-crustal depths, by partial melting beneath domically uplifted continental shield areas (Bailey, 1964).

The Nigerian Cenozoic volcanic province lies in an area of regional uplift, and is characterized by scattered small volcanic centres rather than by major eruptive foci. High pressure inclusions are being found at an increasing proportion of these centres, suggesting that the magmas rose directly and rapidly to the surface from the upper mantle levels at which they were all (including the trachyte) generated.

It is now well established that naturally—occurring sulphides—particularly galena—are frequently found in the deformed state and that at least part of this deformation can be eliminated by annealing in the laboratory.

The present contribution is concerned principally with galena, for which it is indicated: that tectonic deformation leads to natural work hardening, which may be reduced by simple heating in vacuo; that the rate of softening is related to the annealing temperature; that softening is related to recovery at low temperatures and to both recovery and recrystallization above 250–300 °C that single crystals of galena exhibiting minor slip and kinking recrystallize yielding new, unstrained grains elongated parallel to the old slips and kinks, i.e. a foliation involving unstrained grains may form under a regime devoid of compressive stress; and that where deformed galena and deformed sphalerite coexist, the galena may be made to recrystallize completely (e.g. at 325 °C in 50 days) while leaving the deformation in the sphalerite entirely intact; apparently the coexistence of a strained component A and an unstrained component B in a deformed ore or other metamorphic rock does not necessarily indicate differences in age or deformational history—it may simply indicate that post-deformational temperatures have exceeded the recrystallization temperature of one component but not of the other.

An unusual occurrence of porphyritic plagioclase-hornblende-pyroxene granulite (porphyritic basic charnockite) has been observed within the ‘Charnockite region’ of the Eastern Ghats of the Precambrian formations of India. From the X-ray and chemical data the phenocrysts are considered to be augite phenocrysts. Based on petrographic and chemical evidence it is inferred that the basic charnockites of Chipurupalli area in Visakhapatnam district are formed from tholeiitic magma as a gabbroic mass under deep seated plutonic conditions. The gabbroic mass has been recrystallized as plagioclase-hornblende-pyroxene granulite due to later metamorphism, leaving most of the phenocrysts unaffected.