BGS Rock Classification Scheme
Fine-grained normal crystalline rock - In the Rock Classification Scheme, a crystalline rock is fine-grained if its average crystal size is less than 0.25 mm. This classification is used for fine-grained, very fine-grained, and glassy crystalline igneous rocks. The class includes felsite, mafite, glassy-igneous-rock, and porphyry. This group corresponds to the volcanic division of the IUGS scheme. British Geological Survey Research Report RR/99/06, section 1.3 and figs. 1, 3.
| Code | FGNR |
|---|---|
| Description | Fine-grained normal crystalline rock - In the Rock Classification Scheme, a crystalline rock is fine-grained if its average crystal size is less than 0.25 mm. This classification is used for fine-grained, very fine-grained, and glassy crystalline igneous rocks. The class includes felsite, mafite, glassy-igneous-rock, and porphyry. This group corresponds to the volcanic division of the IUGS scheme. British Geological Survey Research Report RR/99/06, section 1.3 and figs. 1, 3. |
| Qualifiers | No qualifiers currently exist. |
| Definition | Chemically/mineralogically normal rock with >75% crystals av. between 0.032-0.25mm; includes most of the volume of fine-grained igneous rocks |
| Linked data | http://data.bgs.ac.uk/id/EarthMaterialClass/RockName/FGNR |
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.