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Olivine • For quartz/feldspathoid rocks, the color
index is implicit in the name; for gabbros,
dunite this gives the proportion of minerals
other than plagioclase, pyroxenes, and
olivine; for ultramafic rocks, this exercise
gives the proportion of minerals other
than olivine and pyroxene, such as garnet
or spinel. Such information is neglected
e w by the IUGS naming scheme.
t
i e • There are many fewer rock names and
g peridotite h
r r field boundaries to commit to memory,
u l
b i freeing the mind to think about pro-
t
z e cesses rather than classification.
r • Freight trains of syllables in hybrid
a pyroxenite
h names such as quartz monzodiorite are
avoided. We contend that 10,20,60 dio-
ortho- clino- rite is both less cumbersome and more
pyroxenite websterite pyroxenite informative than quartz monzodiorite.
• Quantitative modal and mineralogical
Orthopyroxene Clinopyroxene data become part of the rock name,
allowing for more complete and easily
Figure 7. Proposed fuzzy classification of ultramafic rocks in the olivine-orthopyroxene-
clinopyroxene (OOC) triangle. accessible information in databases,
which in turn allows for new scientific
opportunities in data mining (e.g.,
Hazen et al., 2011).
Plagioclase We hope that this modification of the
venerable IUGS scheme will be adopted
anorthosite
in order to simplify and take the stress out
of naming plutonic rocks, with the added
benefit of adding significant quantitative
information that can be easily assimilated
in digital form.

ACKNOWLEDGMENTS
We thank editor Mihai Ducea for encouraging
lite
troctolite
this work, Jade Star Lackey, Doug Walker, Ryan
dunite Mills, Andy Barth, and Alan Boudreau for
comments on the manuscript, and Bob Hazen
Olivine and an anonymous reviewer for helpful reviews.
Orthopyroxene This work is supported by National Science
Foundation grant EAR-1551990 to Glazner.
orthopyroxenite REFERENCES CITED
Aitchison, L., Corradi, N., and Latham, P.E.,
2016, Zipf’s law arises naturally when there
clinopyroxenite Clinopyroxene are underlying, unobserved variables: PLoS
Computational Biology, v. 12, e1005110,
https://doi.org/10.1371/journal.pcbi.1005110.
Figure 8. Proposed fuzzy classification of gabbroic rocks in the plagioclase-olivine-orthopyroxene- Bateman, P.C., 1992, Plutonism in the central
clinopyroxene (POOC) tetrahedron. All boundaries are meant to be fuzzy. The base of this tetra- part of the Sierra Nevada batholith, California:
hedron is the OOC triangle of Figure 7, and plagioclase-poor rocks can be named using either one U.S. Geological Survey Professional Paper
(see examples). Names in the base of the tetrahedron are omitted for clarity.
1483, 186 p.
Bateman, P.C., Dodge, F.C.W., and Bruggman,
P.E., 1984, Major oxide analyses, CIPW
norms, modes, and bulk specific gravities of
TABLE 1. MODAL MINERALOGY OF ROCKS FROM THE STILLWATER COMPLEX plutonic rocks from the Mariposa 1 degrees
Sample Plagioclase Olivine Orthopyroxene Clinopyroxene Other Name by 2 degrees sheet, central Sierra Nevada,
MA203 90.7 0 2.0 7.0 91,0,2,7 anorthosite California: U.S. Geological Survey Open-File
MA208 82.8 14.4 2.4 0.4 83,14,2,0 norite Report 84-0162, 59 p.
MA161 68.6 0 24.7 6.6 69,0,25,7 norite
MA190 64.9 1.5 9.7 23.9 65,2,10,24 gabbro Bateman, P.C., Chappell, B.W., Kistler, R.W.,
5104EX 2.4 66.5 20.0 0.6 phlogopite plag-phlog 66,20,1 harzburgite Peck, D.L., and Busacca, A.J., 1988,
10.1 or phlog 2,66,20,1 harzburgite Tuolumne Meadows Quadrangle, California;
analytic data: U.S. Geological Survey
Bulletin 1819, 43 p.

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