Igneous (modified from IUGS [Streckeisen, 1976])


Definition:Rock formed from molten or partially molten rock along with gases (Dietrich & Skinner, 1979). Igneous rocks can be classified by composition and texture: fine-grained rocks are volcanic and coarse grained rocks are plutonic. Volcanic rocks are fine-grained which makes them difficult to identify in the field. The composition of an igneous rock can be estimated in the field based on the presence and the amount of the minerals, or mineral groups; quartz (Q), plagioclase (P), alkali feldspar (including albite) (A), ferromagnesian minerals (dark colored minerals) and feldspathoids (F) (see diagram below) An igneous rock with greater than 10 % of the lighter colored minerals (quartz, plagioclase, alkali feldspar, and feldspathoids) can be plotted on the diamond shown below to determine the name. Rocks that are composed primarily of dark colored minerals should be plotted on the triangle shown on the lower right of the diamond. Compositions plotted on the diagrams are estimates of the mineral or mineral group in the sample that are normalized to 100 %. Each corner of the triangle represents 100 % of that component and the line opposite is equal to 0 % of that component i.e., Q is equal to 100 % quartz and a line drawn between A and P is equal to 0 % quartz. An igneous rock that has quartz will not have feldspathoids. A detailed classification can be made from samples in a lab that are analyzed for the minerals present and the amounts of each mineral. These results are plotted on the diagrams shown at the bottom of this page. Secondary minerals are used in the name from the least abundant to the most abundant i.e., hornblende biotite granodiorite is a granodiorite (65 - 90 % P, 20 - 60 % Q) with more biotite than hornblende (Blatt & Tracy, 1996).
 
 

              (modified from Blatt & Tracy, 1996)
       
Volcanic                 Silicic: quartz is greater than 20%                         Rhyolitoid                         Trachytoid                 Intermediate: quartz is less than 20%, plagioclase is 60 - 100% and feldspathoids are from ~ 0 - 20%                         Dacitoid                         Andesitoid                 Mafic: quartz is less than 10%, plagioclase is 60 - 100% and feldspathoids are from ~ 0 - 20%                         Basaltoid                 Silica undersaturated: feldspathoids are from 20 - 100 % and quartz is not present                         Foiditoid                         Undifferentiated Volcanic         Plutonic                 Silicic: quartz is greater than 20%                         Granitoid                 Intermediate: quartz is less than 20%, plagioclase is 60 - 100% and feldspathoids are from ~ 0 - 20%                         Dioritoid                 Mafic: quartz is less than 10%, plagioclase is 60 - 100% and feldspathoids are from ~ 0 - 20%                         Gabbroid                 Ultramafic: Q + A + P + F content is less than 10% of the entire rock composition, the rest of the rock is composed of dark colored minerals.                         Peridotite                 Silica undersaturated: feldspathoids are from 20 - 100 % and quartz is not present                         Foidolite                         Undifferentiated Plutonic         Igneous Undifferentiated
                         (modified from Blatt & Tracy, 1996)
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(last modified 11/22/99)