In traditional English and North American usage, a gneiss is a coarse-grained metamorphic rock showing compositional banding (gneissic banding) but poorly developed schistosity and indistinct cleavage.

Apr 23, 2023 · Gneiss is a foliated metamorphic rock that is a common distribute type of rock high-grade regional metamorphic approaches from pre-current formations that have been initially both igneous or sedimentary rocks. It has a glorious banding which is apparent on microscopic scale and hand specimen.

Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying mineral composition. Some of these bands (or lenses) contain granular minerals that are bound together in an interlocking texture. Other bands contain platy or elongated minerals that show a preferred orientation that parallels the overall banding in the rock.

Although a gneiss is commonly feldspar - and quartz -rich, the mineral composition is not an essential factor in its definition. Varieties are distinguished by texture (e.g., augen gneiss), characteristic minerals (e.g., hornblende gneiss), or general composition and/or origins (e.g., granite gneiss, orthogneiss, paragneiss).

Granitic Gneiss: This type is rich in quartz and feldspar, giving it a light-colored appearance similar to granite. Micaceous Gneiss: Characterized by a high concentration of mica minerals like biotite or muscovite, resulting in a sparkly or flaky texture.

Precambrian : ~4600 Ma to 541 ± 1.0 Ma - based on 6 recorded ages. https://www.mindat.org/min-50739.html Please feel free to link to this page. A gneiss of granitic composition.

Aug 28, 2022 · More specific names abound – for example, pelitic gneisses form by metamorphism of originally clay-rich sedimentary rocks, granitic gneisses (such as the one shown in Figure 8.31) form by metamorphism of granites, and mafic gneisses form by metamorphism of mafic igneous rocks. Sometimes key minerals are often included in rock names.