Here's the QAPF diamond - and at no extra charge, two more triangles of rocks!

All the rocks in the QAPF diamond are called 'felsic', meaning they contain lots of minerals with silicon, like feldspar and quartz. But here you also see a triangle of 'mafic' rocks. These have less silicon and more magnesium and iron. And there's also a triangle of 'ultramafic' rocks, with even less silicon.

I am not ready to understand mafic and ultramafic rocks. I'm still curious about why Q and F - that is, quartz and feldspathoids - don't coexist! I looked it up, and here's the story:

Feldspathoids are really just like feldspars with less silica (silicon dioxide). In the presence of silica, any feldspathoid will react with it to produce a feldspar. The two key reactions are

NaAlSiO₄ + 2 SiO₂ → NaAlSi₃O₈ (nepheline + quartz → albite)
KAlSi₂O₆ + SiO₂ → KAlSi₃O₈ (leucite + quartz → K-feldspar)

Both reactions run strongly to the right under high pressures and temperatures, like deep underground. So a rock is either silica-oversaturated and contains quartz, or silica-undersaturated (not enough SiO₂ to put all the Na or K into feldspathoids) and contains feldspathoids. Not both!

It's nice when things kinda make sense.

Next, a clearer version of the QAP triangle.

(2/n, n = 2)