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Volume 37 - March 1970

Contents

Page 6 of 11

Short Communications

Research Article

  • The mineralogy of Egyptian serpentinites and talc-earbonates

  • E. Z. Basta, Z. Abdel Kader
  • Published online by Cambridge University Press:
    05 July 2018, pp. 394-408

  • Optical, d.t.a., X-ray, and chemical analysis investigations were carried out on serpenttinites from various Egyptian localities and on their associated talc-carbonates. The serpentinites were proved to consist of pure antigorite, lizardite, or clinochrysotile, or of mixtures of any pair of these serpentine minerals. Lizardite, which is reported for the first time in Egypt, is found to be of common occurrence. Talc, tremolite, carbonates, chlorite, olivine, biotite, chromite, magnetite, goethite, and stevensite are the main admixtures. The talc-carbonates consist mainly of talc and magnesite, or magnesite, dolomite, and talc together with minor amounts of antigorite, magnetite, goethite, and occasionally a little chlorite or calcite. Spectrographic analyses of some of the magnetically separated serpentines have been carried out and the origin of the Egyptian serpentinites and talc-carbonates is briefly discussed.

Short Communications

Research Article

  • An X-ray investigation of the thermal decomposition of portlandite

  • N. H. Brett
  • Published online by Cambridge University Press:
    05 July 2018, pp. 244-249

  • The thermal decomposition of portlandite, Ca(OH)2, has been studied in air and in vacuum using X-ray single-crystal techniques. In air, the crystals were decomposed in situ on the goniometer arcs whilst X-ray reflections were simultaneously recorded. The transformation to CaO was not accompanied by topotaxy; this is attributed to the high nucleation rate of CaO crystallites in air. When Ca(OH)2 single crystals were decomposed under vacuum (and subsequently exposed to X-rays), some orientation of the CaO crystallites occurred. These results are compared with those of previous workers using electron-diffraction techniques. Decomposition of Ca(OH)2 single crystals commences at ∼ 450 °C in air and at ∼ 230 °C in a vacuum of 10−6 mmHg; reaction commences at crystal edges and surface defects, the reaction boundary moving inwards to the centre of the crystal. This observation is consistent with a homogeneous mechanism of decomposition.

Short Communications

Research Article

  • The preparation of fused samples in X-ray fluorescence analysis

  • P. R. Hooper, Lynne Atkins
  • Published online by Cambridge University Press:
    05 July 2018, pp. 409-413

  • A rapid fusion technique is described in which the Li2B4O7 bead is polished, not crushed. The reliability of the technique is illustrated by a series of precision tests and analyses of standard rocks, using a 2:1 Li2B4O7 : rock-powder dilution. It is also pointed out that such a low-dilution fusion method may be used for Na2O and trace element analyses with only a small loss in sensitivity relative to the unfused-powder method. Any fusion method has the advantages of greatly reducing absorption effects and eliminating particle-size effects.

Short Communications

Research Article

  • Study of minerals from the pegmatites of the Nellore mica-belt, Andhra Pradesh, India Part I. Microcline perthites

  • V. R. R. M. Babu
  • Published online by Cambridge University Press:
    05 July 2018, pp. 135-139

  • 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.

Book Reviews

Short Communications

Research Article

  • Synthesis of aenigmatite

  • R. N. Thompson, J. E. Chisholm
  • Published online by Cambridge University Press:
    05 July 2018, pp. 253-255

  • Aenigmatite with the formula Na4[Fe10 2+Ti2]Si12O14 has been synthesized hydrothermally, at 700 °C and 1000 bars P H2O, from an oxide starting material. The oxygen fugacity was controlled by means of an iron-wüstite buffer. An indexed X-ray powder pattern of synthetic aenigmatite is given, together with comparative data on two natural samples. One of the aenigmatites, from Kola, U.S.S.R., is known to be triclinic (Kelsey and McKie, 1964). The powder patterns of this and the synthetic aenigmatite are so similar that it is concluded that the latter is also triclinic, at least at room temperature.

Short Communications

Research Article

Short Communications

Book Reviews

Research Article

  • On the X-ray identification of amblygonite and montebrasite

  • A. A. Moss, E. E. Fejer, P. G. Embrey
  • Published online by Cambridge University Press:
    05 July 2018, pp. 414-422

  • Previous investigations have been confined to individual members or to small compositional ranges of the amblygonite-montebrasite series. The present study is a survey based on twenty-two specimens in the range Amb14Mon86 to Amb92Mon8 (1·8–11·8 % fluorine), and indicates that the fluorine content may be related both qualitatively and quantitatively to differences in the X-ray powder patterns. Fully indexed powder data to d = 1·68 Å and derived cell parameters are given for four members of the series. There is no evidence of a break in the continuity of the series. The first British occurrence of both amblygonite and montebrasite is reported from a pegmatite vein in aplite at Meldon, Okehampton, Devonshire.

Page 6 of 11