They can also be hilarious. Observe:
Note the upper left. One of my geo friends on Twitter posted this ages ago (I wish I remembered who), and it still makes me giggle.
Thanks Dana for sharing and using this.
3-1-12 I also came across this QAPF on igneous rocks @ Sandatlas and have found this very helpful too.
Here it is, the classification diagram used for plutonic rocks:
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QAPF diagram for plutonic rocks. The diagram was created by the Subcommission of the Systematics of Igneous Rocks in the latter part of the 20th century.
old
QAPF
diagram for plutonic rocks. The diagram was created by the
Subcommission of the Systematics of Igneous Rocks in the latter part of
the 20th century.
Example: a plutonic rock that contains no alkali feldspar and no feldspathoids but contains lots of pyroxenes (neglected in this diagram), plagioclase feldspar and few quartz grains is probably gabbro (located at the right edge of the diagram, little bit up from P towards Q). The diagram doesn’t say whether it is gabbro, diorite, or anorthosite. There are another criteria used to decide that. Note that this diagram is not used for all plutonic rocks. Ultramafic rocks are the most important plutonics that have separate classification diagrams.
Here's the QAPF_diagram link to it at Wikipedia
http://en.wikipedia.org/wiki/QAPF_diagram
I did have this at a different location since this is more appropriate for it I've moved it here.
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