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Gneiss, displaying characteristic darker (melanocratic) and lighter (leucocratic) bands. Color index, as a geological term, is a measure of the ratio between generally dark mafic minerals and generally light felsic minerals in an igneous rock.
Augen gneiss, from the German: Augen, meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains (porphyroclasts), normally feldspar, surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this ...
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The color index of S-type granites can vary from melanocratic to leucocratic. Higher color indices correlate with higher plagioclase to alkali feldspar ratios. [ 7 ] The most common high color index mineral in an S-type granite is biotite.
The hornblende gneiss member is similar to the layered gneiss member, but with hornblende-bearing dark gneiss accounting for about half of the outcrop area. [1] The Slaughterhouse Gneiss weathers to a pinkish to orange color, and consists of relatively uniform, medium-grained microcline-quartz-plagioclase gneiss with muscovite, biotite, or both ...
It ranges in color from salmon-pink to light pinkish-gray and in texture from fine- to coarse-grained. Foliation is lenticular and trends northeast. The modal composition is 16 to 32 percent microcline , 29 to 36 percent plagioclase (An27-33), 25 to 30 percent quartz , 2 to 9 percent biotite , and with accessory opaque minerals, muscovite ...
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Morton gneiss, also known as rainbow gneiss, is an Archean-age gneiss found in the Minnesota River Valley of southwestern Minnesota, United States. It is one of the oldest stones on Earth, at about 3.5 billion years old. Along with the nearby Montevideo Gneiss, it is the oldest intact continental crust rock in the United States. [1]