To classify volcanic rocks by this method requires a microscope and thin sections. Hundreds of mineral grains are identified and carefully counted before using this diagram.
- 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.
Many volcanic rocks aren't suited for this classification method:
- Aphanitic rocks are classified by chemical, not mineral content
- Rocks with isolated larger grains (phenocrysts) may yield distorted results
- Rare rocks including carbonatite, lamproite, keratophyre and others that are "off the chart"