BGS Rock Classification Scheme
Gneiss - A type of metamorphic rock with unknown protolith based on textural attributes. In the Rock Classification Scheme, it is defined as a medium- to coarse-grained inhomogeneous rock, commonly with a well-developed preferred orientation of constituent minerals, and characterised by a coarse foliation or layering that is more widely spaced, irregular or discontinuous than that in a schist. Adjacent layers generally exhibit contrasting texture, grain size, and mineralogy. However, there is a continuum between schists and gneisses, with factors such as the spacing of the foliation and the degree of contrast between adjacent layers contributing to the assignment of a rock to either category. Gneiss is distinguished from schist where some layers are over 5 mm thick. Gneisses generally occur in areas of middle to upper amphibolite or granulite facies metamorphism and can encompass a wide range of lithologies. Note that orthogneiss indicates a rock that is thought to have an igneous protolith, and paragneiss indicates a rock that is thought to have a sedimentary protolith. British Geological Survey Research Report RR/99/02, sections 3.3, 6.1 and fig. 1.
| Code | GNSS |
|---|---|
| Description | Gneiss - A type of metamorphic rock with unknown protolith based on textural attributes. In the Rock Classification Scheme, it is defined as a medium- to coarse-grained inhomogeneous rock, commonly with a well-developed preferred orientation of constituent minerals, and characterised by a coarse foliation or layering that is more widely spaced, irregular or discontinuous than that in a schist. Adjacent layers generally exhibit contrasting texture, grain size, and mineralogy. However, there is a continuum between schists and gneisses, with factors such as the spacing of the foliation and the degree of contrast between adjacent layers contributing to the assignment of a rock to either category. Gneiss is distinguished from schist where some layers are over 5 mm thick. Gneisses generally occur in areas of middle to upper amphibolite or granulite facies metamorphism and can encompass a wide range of lithologies. Note that orthogneiss indicates a rock that is thought to have an igneous protolith, and paragneiss indicates a rock that is thought to have a sedimentary protolith. British Geological Survey Research Report RR/99/02, sections 3.3, 6.1 and fig. 1. |
| Qualifiers | No qualifiers currently exist. |
| Definition | Medium to coarse-grained (av. 0.25-16mm) crystalloblastic metamorphic rock (protolith undefined) with >5mm irregular/discontinuous layers defined by contrasting texture/grainsize/mineralogy,well developed preferred mineral orientation common |
| Linked data | http://data.bgs.ac.uk/id/EarthMaterialClass/RockName/GNSS |
CAVEAT
"Thumbnail" definitions attached to some of the table entries are an interim attempt to provide simple short descriptions of the "unqualified" rock types presented within the BGS Rock Classification Scheme. Every effort has been made to ensure accuracy and consistency but minor contradictions or omissions might be present. The BGS welcomes any feedback pointing out such minor imperfections and/or providing constructive comment and suggestions for improvement of the definitions. In the first instance such feedback should be directed to Dr Tim McCormick.