Unveiling the Missing Puzzle Pieces: The Absence of Pyroxenites and Peridotites in Streckeisen’s QAPF Diagram

Unveiling the Missing Puzzle Pieces: Why You Won’t Find Pyroxenites and Peridotites on Streckeisen’s QAPF Diagram

Ever stared at a QAPF diagram and wondered, “Where do those rocks fit in?” You know, the really dark, heavy ones like pyroxenites and peridotites? Devised by the brilliant Albert Streckeisen, this diagram is a classic for classifying those coarse-grained plutonic rocks we find deep within the Earth. It neatly sorts rocks based on how much Quartz (Q), Alkali Feldspar (A), Plagioclase Feldspar (P), and Feldspathoids (F) they contain. It’s elegant, it’s efficient… but it’s also missing a couple of key players.

So, what’s the deal? Why are pyroxenite and peridotite absent from this seemingly comprehensive chart? Well, the QAPF diagram was really designed with felsic and intermediate plutonic rocks in mind. Think granites and their kin. These rocks flaunt a good amount of Q, A, P, and F minerals, making the diagram perfect for pinning them down. The very way the QAPF diagram is set up, relying on the proportions of those four mineral groups, just naturally leaves out rocks that don’t play by those rules. It’s like trying to fit a square peg in a round hole.

Now, pyroxenites and peridotites? They’re ultramafic – a completely different beast! These guys are all about the mafic minerals, mainly olivine and pyroxene. Feldspars and quartz? Usually nowhere to be found, or maybe just a tiny sprinkle. Peridotites are swimming in olivine (we’re talking over 40%), while pyroxenites are practically pure pyroxene (more than 90%!). Since the QAPF diagram is obsessed with quartz and feldspars, trying to classify these ultramafic rocks with it would be… well, pointless. You’d get results that are misleading at best.

Think of it this way: it’s like using a ruler to measure volume. Sure, you can try, but you’re not going to get very far!

Streckeisen himself knew this, of course. He understood that different rock groups need different tools. The QAPF diagram is fantastic for granites, granodiorites, and the like, but it’s hopeless for rocks with a totally different mineral makeup. Ultramafic rocks like pyroxenites and peridotites need diagrams that highlight their key ingredients. That’s where diagrams like the olivine-pyroxene-hornblende ternary diagram come in. These specialized diagrams give you a much more accurate picture of what’s going on with these unique rocks.

So, the fact that you won’t find pyroxenites and peridotites on the QAPF diagram isn’t a mistake. It’s just a matter of using the right tool for the job. The QAPF diagram is still incredibly useful for classifying a whole bunch of plutonic rocks. Just remember that it has its limits, and sometimes you need to pull out a different chart to really understand what you’re looking at. Geology, after all, is all about having the right tools in your kit!