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database (NAVDAT) of western North   trimethylpentane in the IUPAC system.   between reducing the number of root
          American igneous rocks (Walker et al.,   When decoded, this provides a complete   names for simplicity and retaining root
          2006), 105 of them unique. The first 18   description of the molecule.  names because they are sufficiently use-
          names make up 90% of the samples; the   Plutonic rocks are a chemical contin-  ful to warrant it. The following examples
          remaining 87 names are rarely applied.    uum rather than a collection of discrete   of our approach therefore are meant to be
          We contend that these surplus names    compounds; therefore, a system that is   illustrative rather than definitive; if the
          (e.g., sannaite, malignite), although locally   analogous to IUPAC organic nomencla-  system is adopted, optimal names will
          useful, are largely noise that obscures the   ture cannot be constructed. However,    arise organically rather than by fiat.
          signals of petrologic processes.   a broadly similar approach can achieve   Finally, there are many reasons that
            It is somewhat bewildering that petrolo-  much the same goals.      root names alone should continue to be
          gists have used modal mineralogy to clas-  We suggest that an optimal system of   used to name map-scale bodies of plu-
          sify plutonic rocks for a century, and yet   plutonic rock names will use a small   tonic rock. Addition of a numerical vector
          these data are rarely recorded in digital   number of root names along with the   to a root name is intended only to be used
          databases. In NAVDAT, only ~5% of the   modal proportions of the minerals used in   at the scale of individual samples from a
          analyses have associated modal data, and   the classification. Each root name corre-  map-scale body. Indeed, by making the
          one-third of those are from just two sources.  sponds to a particular combination of   boundaries around rock names fuzzy,
                                             major minerals and thus indicates the    usage in pluton names and rock names
          A SIMPLER, QUANTITATIVE            relevant classification triangle or tetra-  becomes more consistent.
          APPROACH                           hedron. The absolute minimum number
            We propose the following classification   of root names is four: “granite” for rocks   Modified IUGS Method for
          methods for three major groups of rocks:   classified on the basis of quartz, alkali   Classification of Plutonic Rocks
          quartz/feldspathoid- + feldspar-bearing,   feldspar, and plagioclase; “foid syenite”   The following procedure involves the
          ultramafic, and gabbroic. We do not pro-  or some other moniker for those domi-  same observations needed to classify a
          pose equivalent schemes for other igneous   nated by feldspathoids, alkali feldspar,   rock with the IUGS method, and thus the
          rock groups (e.g., lamprophyres, carbon-  and plagioclase; “peridotite” for olivine,   full IUGS name can always be applied.
          atites, agpaites), but a similar approach   orthopyroxene, and clinopyroxene; and   All percentages are modal (volume %).
          could be applied to all plutonic rocks, and   “gabbro” for plagioclase, olivine,
          the same reasoning could be applied to   orthopyroxene, and clinopyroxene. In this   Rocks with Quartz + Feldspars >10%
          chemical classifications (e.g., total alkalis-   extreme rendering of our approach, the 27   (Upper Half of IUGS Diamond)
          silica) for volcanic rocks.        fields in the Streckeisen double triangle   1. Estimate the proportions of quartz (Q),
            Our approach borrows conceptually   are replaced by two root names, granite   alkali feldspar (A), plagioclase (P), and
          from the International Union for Pure and   for the upper triangle and foid syenite for   mafic minerals (M), and the identities
          Applied Chemistry (IUPAC) system for   the lower triangle.              of mafic minerals and accessories
          naming organic compounds. In the 1950s,   The simplicity of the extreme approach   (example: 20,20,50,10; biotite, horn-
          nomenclature for organic compounds was   is appealing, but we see reasons for a less   blende, titanite).
          bloated and tangled. Names and properties   radical trimming of the list of recognized   2. Assign a root name based on where the
          of thousands of known organic compounds   rock names. First, some combinations of   QAP estimate, normalized to 100, falls
          were compiled originally by Beilstein   names trace a process; e.g., lherzolite,    on the upper triangle in Figure 6
          (1881), much as names and properties of   harzburgite, and dunite trace the composi-  (example: granodiorite).
          igneous rocks were compiled by Johannsen   tion of the mantle residue of basalt extrac-  3. Prefix the rock name with unnormal-
          (1938, and accompanying volumes). The   tion. Second, a small number of root   ized QAP; e.g., 20,20,50. The propor-
          Beilstein compilation includes the name of   names can provide a convenient shorthand   tion of mafic minerals (color index)
          the compound and assigns it a unique   for differences between related rocks that   is implicit in these numbers as 100
          number. However, the common name and   are awkward to express by differences in   minus their sum (example: 20,20,50
          Beilstein number provide limited informa-  the mineral proportions. Finally, allowing   granodiorite; color index is 10).
          tion about the nature of a compound.  a given set of mineral components to map   4. Prefix the resulting name with relevant
            IUPAC therefore set out to create a   into more than one rock name permits the   mafic minerals, using defined abbrevia-
          nomenclatural scheme that would be more   boundaries between the rock names to be   tions if desired. The prefix should list
          systematic and informative. The IUPAC   fuzzy, reflecting the real variability of the   these in increasing abundance so that
          system consists of a set of root names that   rocks noted by Bowen (1928). This inher-  the most abundant is closest to the root
          correspond to chemically and structurally   ent flexibility can allow the rock names   name (Shelley, 1993, p. 7) (example:
          simple organic molecules (e.g., the   that are retained to emerge from their   hbl-bio 20,20,50 granodiorite).
          alkanes methane, ethane, propane, etc.),   actual usage rather than being imposed    5. Important accessory minerals can be
          preceded by modifiers that identify and   by a committee.               denoted by, for example, titanite-
          describe the spatial arrangement of atoms   We therefore propose an approach to   bearing or ttn-bearing (example:
          added to the root molecule to form a par-  naming rocks that consists of a root name   ttn-bearing hbl-bio 20,20,50
          ticular compound. For example, the com-  preceded by a vector of mineral abun-  granodiorite).
          pound on which the octane rating system   dances. As this is a new system, we do   Note that these steps are identical to
          of gasoline is based, isooctane, is 2,2,4   not claim to know the optimal balance   the IUGS method except that there are

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