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QAPF diagrams are double ternary diagrams used for magmatic rock classification based on mineralogy.

Quartz and Feldspathoids are opposed on the pointy ends of the diamond, with Alkali feldspar and Plagioclase (feldspar) on the intermediary vertex. That direction indicates from more to less silicate (SiO2) content, leading to formation of one or the other.

The other direction differentiates feldspar types, from Alkali feldspar (Na-K) and Plagioclase (Na-Ca). My problem lies here: Albite (Sodium mineral) is the end of the ranges of BOTH of them, thus I don't know which side it should be considered.

I made some searches on the web but cannot find anything conclusive as some geologists claim one or the other, while most simply don't talk about it. Does anybody have any answer, if possible with reliable sources?

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When it comes to igneous rocks classification, the bible is Igneous Rocks: A Classification and Glossary of Terms (Le Maitre et al., 2002). It says (p.4):

A = alkali feldspar, including orthoclase, microcline, perthite, anorthoclase, sanidine, and albitic plagioclase (An$_0$ to An$_5$)

P = plagioclase (An$_5$ to An$_{100}$) and scapolite

So I guess everyone is partially right: if your albite is An$_{0-5}$, then you should consider it on the A side; if your albite is An$_{5-10}$, then you should consider it on the P side.

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  • $\begingroup$ Thanks. That definitely confirms some information and diagrams I got are wrong or too vague. $\endgroup$ Aug 19, 2022 at 17:35
  • $\begingroup$ Also, just to clarify something in your original post - albite is the sodium feldspar endmember, not the calcium endmember. Anorthite is the calcium endmember. $\endgroup$
    – desander
    Aug 20, 2022 at 13:49
  • $\begingroup$ Yes my bad, you're right. $\endgroup$ Aug 20, 2022 at 20:05

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