The Crystal Morphology of Plagioclase Feldspar Produced During Isothermal Supercooling and Constant Rate Cooling Experiments

G. M. Corrigan*
Department of Geology, University of Sheffield, Sheffield S1 3JD
*Present address: Meadow House, Hill House Radd, St. Bees, Cumbria CA27 0BZ.

Abstract: The crystal morphology of plagioclase feldspar in seven mafic rocks has been studied by isothermal supercooling and constant rate cooling experiments at 1 atmosphere using the ‘wire-loop’ suspension method. There is a systematic variation in crystal morphology with supercooling (−ΔT), but the transitions between the different morphologies are gradual, and consecutive forms sometimes occur together. As −ΔT increases, the sequence of plagioclase morphology is generally laths → skeletal forms → fan spherulites. During isothermal supercooling the position of the plagioclase morphological fields is shown to be dependent on −ΔT, superheating (+ΔT), and also on time to a greater or lesser degree. Variations in melt composition and viscosity affect the position and slope of the boundaries of the fields. In the constant rate cooling experiments, the morphological type is dependent on the cooling rate and can be used to a limited extent as an indicator of the range of supercooling over which the first crystals nucleated and grew.

Mineralogical Magazine; December 1982 v. 46; no. 341; p. 433-439; DOI: 10.1180/minmag.1982.046.341.04
© 1982, The Mineralogical Society
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