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The alkali feldspars from microsyenitic dylces of southern Greenland

Published online by Cambridge University Press:  14 March 2018

H. G. Scharbert
H. G. Scharbert*
Affiliation:
Institute of Mineralogy and Petrography, University of Vienna, Austria
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Summary

26 b-axis oscillation photographs of alkali feldspars of microsyenitic dykes of Southern Greenland are presented. The crystals are rather restricted in composition (Or42·4 to Or35·3) but are rather rich in Ca (up to 14 % An). The range of variation of the optic axial angles, 2 V 45° to 71° (exceptions up to 87°), puts the crystals between sanidine-anorthoclase cryptoperthite and orthoclase microperthite, as classified by Tuttle (1952).

Since the syenitic magma cooled down rather rapidly some peculiarities in the structural state of the alkali feldspars are to be expected. The potassium-rich phase is (nearly) monoclinic. According to the values of the reciprocal lattice angles α* and γ* the sodium-rich phases can be divided into three groups: high-temperature pericline-twinned and high-temperature albite-twinned phases, high-temperature pericline-twinned and low-temperature albite-twinned phases, and high-temperature pericline-twinned and low-temperature albite-twinned phases with nearly identical values of the reciprocal lattice angle γ*.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 35 , Issue 275 , September 1966 , pp. 903 - 919
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1966

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