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Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. [2] [3] Both the BGS and the IUGS use gneissose to describe rocks with the texture of gneiss, [2] [3] though gneissic also remains in common use. [5]
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe) 3 AlSi 3 O 10 (F,OH) 2. It is primarily a solid-solution series between the iron - endmember annite , and the magnesium -endmember phlogopite ; more aluminous end-members include siderophyllite and eastonite .
The white minerals are plagioclase; the light grey ones are quartz; the dark, greenish ones are biotite and hornblende, which developed foliation. Archean TTG rocks appear to be strongly deformed grey gneiss, showing banding, lineation, and other metamorphic structures, whose protoliths were intrusive rocks. [4]
Biotite is generally of low content and the main mafic minerals are clinopyroxene (±) and amphibole (±). The macroscopic colour is grey, being little darker than granite. There is high-grade metamorphic rock originated from nepheline syenite that is characterized by gneiss texture of very rare occurrence.
The most common high color index mineral in an S-type granite is biotite. [1] [4] Figure 7. Cross-polarized light photomicrograph of sample CV-114 from the S-type Strathbogie Granite with quartz and feldspar displaying a granophyric texture.
Both muscovite and biotite may be present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a rudimentary schistosity, and they may be aggregated into bands in which case the granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the ...
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 ...
The Lilesville Granite is composed of megacrysts up to 4 cm of pink potassium-rich feldspar surrounded by gray plagioclase feldspar, clear quartz, and biotite. Small amounts of muscovite and magnetite are also present. The texture of the rock ranges from medium-grained (~4-5 mm) to large, coarse grains (~4 cm). [3] [4]