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. Notice that the rock names at the P vertex are ambiguous. Which name to use depends on the composition of the plagioclase. For plutonic rocks, gabbro and diorite have plagioclase with a calcium percentage (anorthite or An number) above and below 50, respectively. The middle three plutonic rock types - granite, granodiorite and tonalite - are together called granitoids. Aphanitic rocks: These are classified by chemical, not mineral content.Ī large proportion of igneous rocks aren't suited for this classification method: The corresponding volcanic rock types are called rhyolitoids, but not very often.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.
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