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The solubility of potassium-feldspar in calcic plagioclase : experimental study and application to natural occurrences

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Ai, Y (1987) The solubility of potassium-feldspar in calcic plagioclase : experimental study and application to natural occurrences. Coursework Master thesis, University of Tasmania.

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Abstract

Unusual feldspar intergrowths were found in two metamorphosed basalt
dykes from Departure Rocks, Antarctica. They are composed of host calcic
plagioclase (Kf 0 _3 An7 5_9 1 Ab9_25) and potassium-rich alkali feldspar
(Kf90 _96 Ano_5 Ab0_8) lamellae. Their bulk composition is estimated to be
Kf An 21 65 Ab14. The complex and yet regular lamella textures of these
intergrowths suggest that they are exsolution products. This requires the
existence of a formerly homogeneous plagioclase, compositionally equivalent
to the estimated bulk composition of these intergrowths. Experimental
studies in the binary system anorthite K-feldspar and in the albite-poor
ternery feldspar system have proven the existence of such homogeneous
plagioclase, which has been synthesized at 1150°C and 10 [(bar under dry
conditions. The unusual feldspar intergrowths were homogenized under these
same conditions, further supporting the hypothesized exsolution origin of
these intergrowths. In an experiment of plagioclase crystallization in a
synthetic rock system, a K-rich plagioclase (Kf 19.6 An70 Ab10 . 4) was
obtained from a K-rich melt at 1140°C and 10 kbar, indicating that the
proposed parental plagioclase for the unusual feldspar intergrowths can
be formed in a complex (rock) system. Such K-rich melt could be generated
through interaction of the dyke magma and its surrounding charnockitic
magma.
It is concluded that limits of solid solutions in feldspar systems
are increased at high pressures due to the increase of feldspar melting
temperatures. Pressure may have an effect on the extend of feldspar solid
solutions, but it is considered to be very small.

Item Type: Thesis (Coursework Master)
Keywords: Dikes (Geology), Feldspar, Plagioclase
Copyright Holders: The Author
Copyright Information:

Copyright 1987 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M. Sc.)--University of Tasmania, 1988. Bibliography: p. 144-150

Date Deposited: 25 Nov 2014 00:44
Last Modified: 23 Aug 2017 05:03
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