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The paths of feldspar crystallization in volcanic acid liquids can be illustrated by the relationship of feldspar phenocrysts to groundmass feldspar in porphyritic pitchstones and rhyolites. In this way, a concept of the undetermined system CaAl2Si2O8-NaAlSi3O8-KAlSi3O8-SiO2 can be obtained. In this account the various sub-solidus transformations of alkali feldspar and plagioclase are neglected, the emphasis being rather on feldspar-liquid equilibria. It is shown that the equilibrium crystallization of acid liquids similar in composition to the Icelandic rhyolites is characterized by a single feldspar, namely a representative of the plagioclase-anorthoclase solid-solution series. In the more potassic acid liquids of the British Tertiary and the San Juan region, Colorado, phenocrysts of alkali feldspar (sanidine) and plagioclase are commonly found, plagioclase being subsequently resorbed under certain conditions during equilibrium crystallization.
The paths of well-described tholeiitic and alkaline volcanic series in the tetrahedron CaAl2Si2O8-NaAlSi3O8-KAlSi3O8-SiO2 indicate that under conditions of fractionation the later salic liquids may not become sufficiently enriched in potassium for two feldspars to crystallize; instead, a representative of the plagioclase-anorthoclase series appears. In the orogenic volcanic series, where modification of basaltic magma by assimilation is believed by many to play a dominant role in the production of andesite, the associated acid liquids typically give rise to two-feldspar assemblages. It is therefore suggested that in areas where intermediate rocks are largely absent, two-feldspar rhyolites may result directly from fusion, a conclusion not at variance with the available evidence from the British Tertiary province.
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  R. C. R. Robertson and I. Parsons The Loch Loyal syenites Scottish Journal of Geology, 1974; 10: 129 - 146. [Abstract] [PDF]
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