Showing papers in "Mineralogical Magazine in 1965"
TL;DR: In this article, phase-equilibrium relations have been determined at 1000 kg/cm2 water pressure for compositions within the system NaAlSi3O8-KalSi 3O 8-NaAlSiO4-KAlSiOsO4 in the area adjacent to the temperature minimum, which suggest that many undersaturated rocks are derived by fractional crystallization from a trachytic magma.
Abstract: Phase-equilibrium relations have been determined at 1000 kg/cm2 water pressure for compositions within the system NaAlSi3O8-KalSi3O8-NaAlSiO4-KAlSiO4 in the area adjacent to the temperature minimum. The composition and temperature of the minimum are Ne50Ks19Qz31 and 750° ± 7° C respectively. The compositions of 102 plutonic rocks and 122 extrusive rocks, from Washington's tables, that carry 80% or more of normative Ab + Or + Ne have been plotted; the areas of high density show a marked similarity to the positions of the low-temperature regions of the synthetic system and suggest that many undersaturated rocks are derived by fractional crystallization from a trachytic magma.
161 citations
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TL;DR: In this article, a full-matrix, three-dimensional refinement of kalsilite, KAlSi04 (hexagonal, a 5·16, c 8.69 A, P6a), showed that the silicon and aluminium atoms are ordered.
Abstract: A full-matrix, three-dimensional refinement of kalsilite, KAlSi04 (hexagonal, a 5·16, c 8.69 A, P6a), shows that the silicon and aluminium atoms are ordered. The respective tetrahedral distances of 1·61 and 1·74 A agree with values of 1·61 and 1·75 A taken to be typical of framework structures. As in nepheline, an oxygen atom is statistically distributed over three sites displaced 0·25 A 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 A are consistent with the wide range normally found for this weakly bonded atom.
74 citations
TL;DR: In this article, a modification of the rules for the cMeulation of C.I.P.W. norms when no normative quartz is present is suggested, and it is shown that the modified rules can be used for the CMEU.
Abstract: A modification of the rules for the cMeulation of C.I.P.W. norms
when no normative quartz is present is suggested.
73 citations
TL;DR: In this paper, the cathodo-luminescence of minerals in thin sections of rocks has been examined with the aid of an electron-probe microanalyser and also with a simple electron source producing an unfocused beam.
Abstract: The cathodo-luminescence of minerals in thin sections of rocks has been examined with the aid of an electron-probe microanalyser and also with a simple electron source producing an unfocused beam. Experimental details of the technique are briefly described and colour photographs, taken at a primary magnification of 30 times, are included. The observed variations in colour and intensity of luminescence are interpreted in the light of electron-probe and spectrochemical determinations of trace elements. Applications of the technique are discussed.
73 citations
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TL;DR: In this article, an alternative mechanism is proposed by which all or nearly all the Mg ions together with some of the Si ions liberated in the reaction zones where water is formed migrate into regions where forsterite is formed.
Abstract: Forsterite formed from serpentine powders at various temperatures up to 800° C has been measured quantitatively by X-ray diffraction. The results are considered in relation to a reaction mechanism proposed by Ball and Taylor and are shown to be inconsistent with the details of this mechanism. An alternative mechanism is proposed by which all or nearly all the Mg ions together with some of the Si ions liberated in the reaction zones where water is formed migrate into regions where forsterite is formed. This mechanism is in closer accord with the experimental results. The retarded development of enstatite until temperatures above 1000° C are used is attributed to the nature of the reaction by which it is formed, namely a reaction between forsterite and silica.
62 citations
TL;DR: In this paper, a tentative hypothesis of fractional melting of hydrated upper mantle material is proposed, which appears to be a possible explanation of the range of composition encountered in dredged glass samples.
Abstract: Basalts dredged from the floor of the deep ocean show general tholeiitic affinities. Some samples are rich in aluminium and of very similar composition to the Warner high-alumina basalt from California. Both olivine tholeiite and high-alumina basalt have been found in the form of glass in dredgings from the Mid-Atlantic Ridge, indicating that liquid magmas of both compositions have been erupted on to the sea bed in this area. In explanation of this association a tentative hypothesis of fractional melting of hydrated upper mantle material is proposed. It is suggested that under the mid-ocean ridges the ‘basaltic fraction’ of the mantle is locally in the form of an amphibole. Fractional incongruent melting of this amphibole appears to be a possible explanation of the range of composition encountered in dredged glass samples.
56 citations
TL;DR: In this paper, it is suggested that the reaction staurolite+quartz → garnet+kyanite is unlikely to be a common reaction in nature and that its use to subdivide the amphibolite facies is unjustified.
Abstract: Mineral data from Glen Clova, Angus, suggest that the development of kyanite in graphite-bearing staurolite mica-schists, treated in terms of a Ca-free system, involved decrease of staurolite and a decrease in biotite MgO:FeO ratio. A kyanite isograd may be defined in Glen Clova as the locality where kyanite joins the assemblage staurolite-garnet-biotite-muscovite-plagioclase-quartz-graphite, the biotite having mol. , although the persistence of this identical assemblage over some 600 metres introduces some uncertainty of interpretation. It is suggested that the reaction staurolite+quartz → garnet+kyanite is unlikely to be a common reaction in nature and that its use to subdivide the amphibolite facies is unjustified.
TL;DR: In this paper, chemical and optical data are presented for the orthopyroxenes, clinopyroxene, garnets, plagioclases, and opaque oxides in high-grade regionally metamorphosed basic rocks and their garnetiferous associates from the Willyama Complex.
Abstract: 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.
TL;DR: In this paper, 25 new analyses of 10 spilitic pillows from the British Isles have been carried out and the authors found evidence of considerable variability in distribution of components, notably Si, Na, Mg, and water.
Abstract: Twenty-five new analyses of 10 spilitic pillows from the British Isles offer evidence of considerable variability in distribution of components, notably Si, Na, Mg.Fe2+, Ca, Fe3+, and water. Selvedges of pillows have compositions strongly contrasted with but essentially complementary to those of cores. Redistribution of components is believed to have occurred during devitrification. Geosynclinal spilites are regarded as basalts which have suffered such redistribution under conditions of diagenesis or low-grade metamorphism.
TL;DR: The mode of occurrence of uraninite, pitchblende and coffinite at five localities in south-west England is described in this article, where chemical and isotopic analyses of concentrates containing these minerals provide the following 206Pb/238U ages.
Abstract: The mode of occurrence of uraninite, pitchblende and coffinite at five localities in south-west England is described. Chemical and isotopic analyses of concentrates containing these minerals provide the following 206Pb/238U ages (My.): uraninite, South Crofty, Cornwall, 277 ± 10; uraninite, Geevor, Cornwall, 223 ± 5; pitcbblende-coffinite, King's Wood, Devon, 206 ± 5; pitchblende, Wheal Bray, Cornwall, 165 ± 4; pitchblende, South Terras, Cornwall, 474 ± 2; coffinite, Geevor, Cornwall, c. 45 (chemical analysis only). These results are compatible with geological and mineralogical evidence. Consideration of these and other results indicates that there were at least three periods of uranium mineralization in south-west England, at c. 290 My., c. 225 My., and c. 50 My. Other intermediate ages may also be valid but further work is required to substantiate these.
TL;DR: A number of authors have attempted to use chemical composition as a means of distinguishing amphibolites derived by isochemical metamorphism of basic igneous intrusives or flows from those of sedimentary parentage as discussed by the authors.
Abstract: Numerous authors have attempted to use chemical composition as a means of distinguishing amphibolites derived by isochemical metamorphism of basic igneous intrusives or flows from those of sedimentary parentage. In recent years contributions have come from Engel and Engel (1951, 1953, 1962), Lapadu-Hargues (1953, 1958), Poldervaart (1953), Eckelmann and Poldervaart (1957), Wilcox and Poldervaart (1958), Evans and Leake (1960), and Walker, Joplin, Lovering, and Green (1960).
TL;DR: In this paper, the electron probe was used to analyse for Fe, Ti, Mg, Al, and Mn in homogeneous magnetite solvices, averaging close to 61% Fe and 8 % TiO2.
Abstract: The electron probe was used to analyse for Fe, Ti, Mg, Al, and Mn. Vanadium could not be determined because of interference from Ti. Calcium was shown to be present only in inclusions of apatite and silicate; it does not oceur in the titanomagnetite structure. Homogeneous grains (magnetite-ulvospinel solid solutions) do not vary greatly in bulk composition, averaging close to 61% Fe and 8 % TiO2. Inhomogeneous grains contain variable amounts of hematite-ilmenite lamellae produced by progressive oxidation of the homogeneous grains. In the early stages there is rapid build-up of titanium in the rhombohedral lamellae, which are initially relatively poor in iron. As oxidation proceeds the almost titanium-free cubic phase becomes strongly enriched in magnesium, aluminium, and manganese, while the iron: titanium ratio in the rhombohedral phase increases. The end-product may be a homogeneous titanhematite, if initial concentrations of Mg, Al, and Mn are low. Otherwise residuals of a relatively iron-poor almost titanium-free (Mg, Al, Mn)-enriched cubic phase remain in the titanhematite. The behaviour of iron and titanium agrees with recent synthetic work in the system. Some of the oxidized grains contain transparent spinel lamellae exsolved from the cubic phase. Exsolution may commence at different stages in the oxidation process, but the reason is not clear—there are no significant differences in the composition of phases with and without spinel lamellae. Analyses and normative compositions are presented for the different phases in each grain, where possible. The compositional changes accompanying oxidation satisfactorily explain the slope of the thermomagnetic curves : there is no need to invoke bulk compositional variations within a given ironsand sample. Samples from an ilmenite and a non-titaniferous magnetite concentrate were also briefly examined.
TL;DR: In this paper, the compositions of the groundmass and of the plagioclase and sanidine phenocrysts of a series of over and under-saturated trachytes indicate that plagiase will precipitate at some stage in the crystallization interval of all calcium-bearing potassic trachyte.
Abstract: The compositions of the groundmass and of the plagioclase and sanidine phenocrysts of a series of overand under-saturated trachytes indicate that plagioclase will precipitate at some stage in the crystallization interval of all calcium-bearing potassic trachytes. Using this data obtained from natural salic liquids, a concept of crystallization within the system CaA12Si~Os-NaA1SiO 4KA1SiO4-SiO2 may be obtained.
TL;DR: A new occurrence of kornerupine in the skarn zone at Mautia Hill, Tanganyika, has yielded unit-cell dimensions a 16-100, b 13.767, c 6'735 A, densi ty 3\"2%, and composition Nao.olMnoo,Tio o2Mg3.69FelII~Al+36Sia uBo.~lO21.7~(OH)o.
Abstract: A new occurrence of kornerupine in the skarn zone at Mautia Hill, Tanganyika, has yielded unit-cell dimensions a 16-100, b 13.767, c 6'735 A, densi ty 3\"2%, and composition Nao.olMnoo,Tio o2Mg3.69FelII~Al+36Sia uBo.~lO21.7~(OH)o.25. The unit-cell clearly contains 88 anions rather than the 86 proposed previously. Grandidierite from Sakatelo, Madagascar, has unit-cell dimensions a 10.978, b 10.335, c 5.760 A, space group Pna2~ or Pnam, densi ty 2.976, and composition
TL;DR: Most circumoceanic basalts contain less than 1·75% of the TiO2 of oceanic basalt as mentioned in this paper, which is the smallest fraction of the global average.
Abstract: Most circumoceanic basalts contain less and most oceanic basalts considerably more than 1·75% of TiO2. The difference of TiO2 averages is large enough to require rejection of the hypothesis that circumoeeanie basalt results from the assimilation of sial by oceanic basalt.