augen-gneiss A medium- to coarse-grained, banded, regional metamorphic rock composed mainly of quartz and feldspar with hornblende and mica also present in variable quantities, and characterized by large ovoidal megacrysts of feldspar known as ‘augens’ (derived from the German for ‘eyes’). The banding is due to variations in the modal proportions (see MODAL ANALYSIS) of the mineral phases constituting the rock. The best-developed augen-gneisses are formed by high-grade metamorphism of aluminous sediments. See also GNEISS.
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