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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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