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Wikipedia says for QAPF diagrams:

"A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition."

Why don't pyroxenites and peridotites appear on this diagram?

Aren't they igneous rocks?

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Why don't pyroxenites and peridotites appear on this diagram?

Because they do not have any quartz, feldspars, or felspathoids.

The same Wikipedia page also says this:

QAPF diagrams are also not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites).

Instead, we use a different diagram, that has olivine, orthopyroxene, and clinopyroxene at the vertices. Like this one right here.

Aren't they igneous rocks?

Not necessarily, actually. But that's a completely different question.

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  • $\begingroup$ I guess some peridotites and pyroxenites reach metamorphism and so they are not igneous. $\endgroup$ – user12525 Mar 30 at 16:29

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