Igneous Rock Types Simplified
The official classification of igneous rocks fills an entire book. This page is a simplified description of how to classify igneous rocks with visible mineral grains—rocks with phaneritic texture—according to their content of feldspars and quartz. (about igneous rocks)
This procedure works best in plutonic rocks, in which all of the minerals have crystallized into visible grains. Volcanic rocks, both intrusive and extrusive, usually have extremely small grains (aphanitic texture) or even none (glassy texture), so for them the procedure usually takes a microscope.
- Determine the percentage, called the mode, of quartz (Q), alkali feldspar (A), plagioclase feldspar (P), and mafic minerals (M). The modes should add up to 100.
- Discard M and recalculate Q, A and P so that they add up to 100—that is, normalize them. For example, if Q/A/P/M are 25/20/25/30, Q/A/P normalizes to 36/28/36.
- Draw a line on the ternary diagram below to mark the value of Q, zero at the bottom and 100 at the top. Measure along one of the sides, then draw a horizontal line at that point.
- Do the same for P. That will be a line parallel to the left side.
- The point where the lines for Q and P meet is your rock. Read its name from the field in the diagram. (Naturally, the number for A will also be there.)
- Notice that the lines that fan downward from the Q vertex are based on values, expressed as percentage, of the expression P/(A + P), meaning that each point on the line, regardless of the quartz content, has the same proportions of A to P. That's the official definition of the fields, and you can calculate your rock's position that way too.
(c) 2004 Andrew Alden, licensed to About.com, Inc.
Notice that the rock names at the P vertex are ambiguous. Which name to use depends on the value of M. For plutonic rocks, gabbro has M above 35 (that is, it is dark) and diorite has M below 35 (light). In volcanic rocks, basalt and andesite lie above and below M = 35.
A large proportion of igneous rocks aren't suited for this classification method:
- Aphanitic rocks—these are classified by chemical, not mineral content
- Rocks without enough silica to yield quartz—these instead contain feldspathoid minerals and have their own ternary diagram (F/A/P) if they are phaneritic
- Rocks with M above 90, ultramafic rocks, which have their own ternary diagram with three modes (olivine/pyroxene/hornblende)
- Gabbros, which can be further classified according to three modes (P/olivine/pyx+hbde)
- Rocks with isolated larger grains (phenocrysts) may yield distorted results
- Rare rocks including carbonatite, lamproite, keratophyre and others that are "off the chart"
