Based on nearly 1500 published amphibole analyses the maximum possible Alvi in hornblendes is shown to increase with increase of Aliv. New analyses of hornblendes from amphibole-corundum rocks, with and without anorthite, are given and after critical examination of the available data it is concluded that the maximum verified Alvi-rich calciferous amphibole that approaches the closest to hypothetical tschermakite comes from a kyanite-bearing aluminous high-pressure-crystallized schist from Lukmanier, Switzerland. Pure natural edenite or ferroedenite is unknown, but a new analysis of the nearest known natural edenite, from Mysore, India, agrees with the postulated view that extraordinarily low temperatures are needed for edenite-ferroedenite crystallization, much below that possible in magmas and only rarely achieved in metamorphic rocks containing amphiboles. The limit of the approach of igneous hornblendes to edenite-ferroedenite and tremolite-ferroactinolite is outlined.

At least 1100 °C is required for complete expulsion of water from some amphiboles.

1666 moldavites from 19 localities in Bohemia and 617 moldavites from 10 localities in Moravia were measured to yield parameters L (length), B (breadth), and T (thickness). A digital computer was programmed to calculate values T/L, (L-B)/(L-T), and according to Sneed and Folk (1958) and values q = B/L, p = T/B, and F = p/q according to Zingg (1935). Averages of large numbers of values characterize Bohemian and Moravian localities. Moldavites from most localities in Bohemia are often drop-like, flat or elongate with average values of between 0·53 and 0·66 and F between 0·68 and 0·91. Moldavites from Moravia are frequently massive to spheroidal with average in the range 0·60 to 0·75 and F between 0·87 and 1·08. Among the localities in Bohemia, four have moldavites with morphological features of Moravian moldavites. The data obtained permit an interpretation that localities of moldavites with low average sphericity contain less heated material. Less heated and thus more viscous glass could have flown along shorter trajectories (Bohemia). More heated glass was ejected to geographically more distant places (Moravia) or travelled along relatively long steeper trajectories (several localities in Bohemia).

A second occurrence of the mineral bolivarite has been found in the Kobokobo pegmatite, Kivu, Republic of Congo (Kinshasa). This amorphous mineral was previously considered to be evansite. Its physical properties are: weak birefringence; biaxial to uniaxial positive; n = 1·50–1·51; specific gravity 1·97–2·05; hardness 3 to 3½.

Three chemical analyses have been performed on the Congolese material. The general formula of the mineral is Al2(PO4)(OH)3.4–5 H2O. Both known bolivarites from Spain and Congo contain abnormal amounts of U. The bright green UV fluorescence is due to the presence of U. The DTA curve of the Congolese bolivarite is given. Its genesis is discussed. This uranium-bearing mineral may be an indicator of uranium mineralization in Spain.

Spherulitic devitrification is a post-eruptive process affecting many rhyolitic lavas of the Taupo region. The spherulites consist of cryptocrystalline intergrowths of α-cristobalite and alkali feldspar (calcic anorthoclase), with minute granules of magnetite, hematite, and secondary (?)goethite. The effects of chemical fractionation occurring during progressive spherulite growth has been studied from a suite of samples from the Aratiatia rhyolite. The most significant effect is the progressive enrichment of both bulk spherulite compositions and the coexisting residual glass in potash with increasing spherulite development. This effect is due both to the very low potash in the earliest formed spherulites and to the consistently higher Na/K ratios of the spherulites relative to the total rock compositions. These differences progressively decrease with increasing spherulite crystallization. The bulk rock compositions, however, evidently remain essentially constant. The degree of potash enrichment in the residual glasses during advanced stages of devitrification is greater than expected by reference to the ternary feldspar and quartz-feldspar systems. This post-eruptive alkali fractionation during spherulite formation is superimposed on the pre-eruptive phenocryst-liquid fractionation.

The assemblage aluminous pyroxenes+ spinel forms reaction zones between olivine-rich and plagioclase-rich layers in the Jotunheimen complex. These zones have probably formed by cooling from high T at moderate P, passing through the ol + plag → pyroxenes + spinel reaction. The absence of garnet, which would be expected to form upon further cooling, is attributed to interruption of the cooling history by rapid uplift. This uplift is demonstrated by symplectitic breakdown of aluminous pyroxenes, with expulsion of Ts and Acm, to yield low-Al pyroxenes, spinel, and plagioclase. The uplift must be Precambrian, and suggests that the Jotun rocks were involved in rapid vertical movements long before the Caledonian thrusting took place.

Most of the granitic sheets and veins in the Loch Coire migmatite complex do not have chemical compositions in accordance with a formation by local anatexis. Regional metasomatism by fluids from outside the exposed confines of the complex was an important factor in the development of the sheets and veins, although there was probably some overlap with magmatic conditions.

A 250 µm olivine crystal in Sharps contains 1 to 5 µm insets identified by microprobe analysis as ferroan monticellite, spinel, and fassaite with average compositions: olivine (host), (Fe0·44Mg1·51Ca0·05) (Si0·99Al0·01)O4; ferroan monticellite, (Ca0·89Fe0·41Mg0·66) (Si1·00Al0·02)O4; spinel, (Fe0·41Mg0·59)(Fe0·063+Cr0·23Al1·69Si0·01Ti0·01)O4; and fassaite, Ca0.96(Fe0·01Mg0·59)(Al0·18Ti0·07Fe0·163+)(Si1·53Al0·47)O6. Textural and experimental data suggest early crystallization of spinel from a calcic olivine melt, exsolution of ferroan monticellite from the host olivine, and reaction of spinel and ferroan monticellite to form fassaite. The severe depletion of alkalies and silica in this crystal suggests it is a residue from vapour fractionation.

The cause of coloration of Blue John fluorite from Castleton, Derbyshire, and blue banded fluorites from Ashover, Derbyshire, and Weardale, Co. Durham, has been investigated by a number of techniques, including mass spectrometry, optical spectroscopy, and paramagnetic resonance measurements on natural and irradiated samples. In all respects Blue John is indistinguishable from the other blue banded fluorites. Although traces of hydrocarbons were found in all the natural fluorites including Blue John, they are shown not to be the cause of the colouration. The optical spectra and bleaching behaviour are consistent with colouration by colloidal calcium rather than F-centres. The causes of colour banding are discussed.

The phase equilibrium diagrams of CaMgSi2O6-CaTiAl2O6 with 10, 20, and 30 wt % Ca2MgSi2O7 in the join CaMgSi2O6-Ca2MgSi2O7-CaTiAl2O6 were determined. Six three-phase assemblages, six four-phase assemblages, and two five-phase assemblages (isobaric trivariant, divariant, and univariant) were confirmed. The univariant assemblages are diopsidess+forsterite+åkermanitess + perovskite + liquid and diopsidess+åkermanitess+perovskite+spinel + liquid. Because of the complex solid solutions of diopside and åkermanite the crystallization ceases before invariant assemblages are reached and the final phase assemblage in the diopside-rich region is diopsidess+åkermanitess+perovskite, in the diopside-poor region diopsidess+åkermanitess+perovskite + spinel. Two invariant points, diopsidess+forsterite+åkermanitess+perovskite+spinel+liquid and diopsidess+åkermanitess+anorthite+perovskite+spinel+liquid in the silica-poor region of the system CaO-MgO-Al2O3-TiO2-SiO2 are estimated. The diopside solid solution in this join is discussed with its bearing on the natural titanpyroxenes from alkalic rocks including melilite.

A precise and accurate X-ray diffraction method has been developed whereby the weight percentages of aragonite and low- and high-magnesium calcite are determined from the integrated peak areas of spiked and unspiked samples. The spike mixture was prepared from organisms extracted from the samples to be analysed. Use of a spiking method also avoided the preparation of working curves from artificial mixtures of carbonate minerals, which may not have the same diffraction behaviour as the unknowns. A test of the precision of the method indicates the following coefficients of variation: aragonite, 1·4 %; low-magnesium calcite, 1·5 %; high-magnesium calcite, 7·8 %. A test of the accuracy of the method indicates no significant bias in any of the carbonate results, except in samples where high-magnesium calcite values are below 10 %. Quartz may also be determined by this method (coefficient of variation 23·9 %; positive bias in values greater than 10 %).

Two vanadates, tangeïte and volborthite, new to Britain, their association with vanadiferous nodules, and their geological environment are described. A percentage composition graph for vanadium taken across a vanadiferous nodule, by scanning electron probe, is given.

Detailed methods are described for the laboratory preparation of the manganese minerals manganosite, hausmannite, manganite, partridgeite, birnessite, cryptomelane, pyrolusite, and todorokite. New data are presented on the conversion of birnessite to cryptomelane, and on the exchange properties of potassium in these two minerals. It was found that the upper limit of potassium in cryptomelane is about 7 % K, while the lower limit lies between 0·25 % and 2·2 % K.

Alkalic rhyolites with peralkaline affinities, bearing quartz and potassic feldspar phenocrysts, are described from Iceland for the first time. Evidence from electron-probe analyses of feldspar phenocrysts indicates crystallization in or near the thermal valley of the system SiO2-Or-Ab. Icelandic acid volcanic rocks are subdivided into alkalic rhyolites, belonging to transitional and alkalic basalt lineages, and the mildly calc-alkaline rhyolites of tholeiite lineages.

Anorthosite country rocks near the fissure vents of an andesitic resurgent caldera contain coexisting crystalline plagioclase, pseudomorphous plagioclase glass, and anomalously dense, massive plagioclase glass. Glass pseudomorphous after labradorite (An 53) has the composition of oligoclase (An 17). Large masses of dense plagioclase glass have compositions near An 53, but relict crystalline plagioclase within them has compositions near An 80. Devitrification products of this glass have potassium-rich compositions. These compositions are compatible with partial thermal melting in a high-temperature, moderate-pressure pulse. Such a pulse might be asscciated with a confined chemical explosion.