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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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