BGS Rock Classification Scheme
Andesite - A type of andesitic-rock. In the Rock Classification Scheme, this name is used for a fine-grained crystalline igneous rock when the modal mineralogy is available, and the mineral mode plots in fields 9 and 10 of the quartz - alkali feldspar - plagioclase - feldspathoid (QAPF) diagram. Andesite and basalt may be separated chemically using SiO2 content: if wt% SiO2 is greater than 52% the rock is andesite. British Geological Survey Research Report RR/99/06, section 5.2 and figs. 17a, 19, 21.
| Code | AND |
|---|---|
| Description | Andesite - A type of andesitic-rock. In the Rock Classification Scheme, this name is used for a fine-grained crystalline igneous rock when the modal mineralogy is available, and the mineral mode plots in fields 9 and 10 of the quartz - alkali feldspar - plagioclase - feldspathoid (QAPF) diagram. Andesite and basalt may be separated chemically using SiO2 content: if wt% SiO2 is greater than 52% the rock is andesite. British Geological Survey Research Report RR/99/06, section 5.2 and figs. 17a, 19, 21. |
| Qualifiers | No qualifiers currently exist. |
| Definition | Fine-grained, av. <0.25mm normal crystalline extrusive/intrusive rock; CI <35vol%, <40wt%; quartz <20% or felspathoid <10% of QAPF, feldspar >65% plagioclase; TAS-SiO2 57-63wt%, Na2O+K2O <7wt% |
| Linked data | http://data.bgs.ac.uk/id/EarthMaterialClass/RockName/AND |
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.