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Volume 38 - December 1972

Contents

Page 6 of 11

Short Communications

Research Article

Short Communications

Research Article

Short Communications

Research Article

  • The transformation manganite → pyrolusite

  • P. E. Champness
  • Published online by Cambridge University Press:
    05 July 2018, pp. 245-248

  • During the transformation of manganite to pyrolusite the b-axis must contract about 15 %. There is evidence that the shrinkage takes place partly by contraction of the whole crystal, but electron-microscope investigations of natural pyrolusites and of manganite oxidized in the laboratory show that the reaction also involves the formation of lamellar pores about 85 Å apart, parallel to (010) of the original manganite. Thus, although pyrolusite is tetragonal, the a- and b-axes are not microscopically identical, a fact which could explain why, when natural pyrolusite is treated hydrothermally at 200 and 300 °C and 1000 bars, only one orientation of manganite is formed (Dent Glasser and Smith, 1968). The pores and the accompanying distortion of the structure act as a ‘memory’ of the parent phase.

Short Communications

Research Article

  • The composition of anomalous plagioclase glass and coexisting plagioclase from Mistastin Lake, Labrador, Canada

  • K. L. Currie
  • Published online by Cambridge University Press:
    05 July 2018, pp. 511-517

  • 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.

Short Communications

Book Reviews

Short Communications

Page 6 of 11