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Andrew Alden

Rethinking the QAPF Lozenge

By July 15, 2013

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qapf lozengeDuring the last few days I've been constructing my own versions of the standard rock classification diagrams. Maybe you recognize this one for classifying plutonic rocks, the quartz–alkali feldspar–plagioclase–feldspathoid or QAPF diagram. It consists of the QAP ternary on the top and the FAP ternary, turned upside-down, on the bottom.

It works for conceptual purposes. It would also work if you printed it out, sat down with a table of rock samples analyzed by mineral modes, and plotted each one with a pencil and ruler—but who's going to do that in this day and age? Drawing the diagram as if you were going to actually use it, instead of merely learning from and referring to it, means that it doesn't work as a graphic on a computer screen. I thought, what if you took the lozenge and pushed it into a square instead? As far as I can tell, no one has ever tried. So I did that. See what you think.


July 16, 2013 at 12:54 am
(1) C. Sims says:

I personal like the double terminal cyrstal look the best.
I could work with the square, yet not as easily. Plus I would have trouble with recalling the information if I was only using it.
I am sure there will others that one or the other.
I am sure that why you did both.
Thank You

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