myrmekite

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myrmekite

irregular, wormy penetration by quartz in plagioclase feldspar; these wartlike, wormlike, or fingerlike bodies may develop during the late stages of crystallization of igneous rocks if the two minerals (quartz and feldspar) grow simultaneously in the presence of a volatile phase. Myrmekite also occurs after the rock crystallizes by replacement of the plagioclase during metasomatism or hydrothermal alteration. In rare cases the plagioclase may be almost completely obliterated by the quartz intergrowths. Although the origin is not always clear, it is evidently a late development in the rock. Felsic rocks, such as granite, most commonly display this feature, especially if the plagioclase feldspar is adjacent to potash feldspar; rocks displaying this feature are said to have a myrmekitic texture

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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