PETROLOGIC TERMINOLOGY 

P ETROGRAPHIC DESCRIPTIONS are essential 

parts of many geologic reports, but such descriptions are not easy reading, and they demand care to 

keep them as free as possible from abstruse phraseology. The geologist who includes petrographic 

descriptions in a geologic report should make them 

an essential part of the story. Unless a paper is aimed 

primarily at other petrologists, the author should 

avoid overly detailed petrographic descriptions. Instead, include only matter that bears directly on the 

overall purpose of the report. Thus, a report on a 

mining district is hardly the place to introduce a new 

rock name or to exhaustively describe all the rocks 

and thin sections that have been examined. On the 

other hand, even minute features of host rocks warrant full description if they bear on the origin of the 

ore deposits or on the search for more ore. 

General definitions of standard petrologic terminology, including rock textures, fabrics, color, and 

state of aggregation, are given by Bates and Jackson 

(1987) and Tomkeieff (1983). Recent additional information on the appearance of certain textures and 

rock types in thin section may be found in MacKenzie 

and Guilford (1981), MacKenzie and others (1982), 

and Adams and others (1984). 

The Subcommittee on the Systematics of Igneous 

Rocks of the International Union of Geological Sciences (IUGS) has proposed standards for the use 

of plutonic and volcanic rock names (Streckeisen, 

1967, 1976, 1979). This system is now widely used 

internationally for the modal and normative classification of igneous rocks. Another useful classification is 

based solely on chemistry, without regard for modal 

expression or recalculated norms (De La Roche and 

others, 1980); authors working with intermediate to 

mafic igneous rocks or silica-deficient rocks may find 

this system of classification more useful than that of 

Streckeisen. STA recommends that authors use one 

or the other of these systems for naming igneous 

rocks. In any event, to avoid any misunderstanding, 

authors should clearly identify whatever system they 

have used. 

SEDIMENTARY AND METAMORPHIC 

ROCK NAMES 

Nomenclatures of sedimentary and metamorphic 

rocks have not been standardized. Authors should 

consult widely used texts in sedimentary and metamorphic petrology for naming these rocks. Also, many 

review volumes, symposia proceedings, and individual 

periodical issues devoted to individual rock types provide guidelines on their nomenclature. 

Petrologists should be chary about proposing new 

rock names. Henry S. Washington, who himself proposed 31 rock names, spoke many years ago of the 

host of new names as "impossible to remember and 

not worth remembering." He once remarked also that 

he was spending his later years in sackcloth and ashes 

over his early rock-naming sins, and then related a 

story about himself. On receiving a paper from 

England that contained a rock name meaningless to 

him, he asked Norman L. Bowen if he knew what the 

Englishman was talking about, and Bowen replied, 

"You should know, you proposed that name yourself." 

Applying modifiers, such as mineral prefixes, to 

existing rock names is generally more satisfactory 

than introducing new names into the literature. In 

using such modifiers, Survey petrologists should 

follow a uniform scheme for the use of hyphens, 

based on the principle that like names are connected 

by hyphens and unlike names are not. Modifiers used 

for such terms generally fall into four classes: (1) rock 

names, (2) mineral names, (3) textural terms, such as 

porphyritic, gneissic, or vitrophyric, and (4) names 

expressing kinds of clastic aggregation, such as 

agglomerate, breccia, or tuff. Names within any 

one class are hyphenated; others are not: 

biotite-pyroxene andesite 

albite-epidote-chlorite schist 

porphyritic nepheline syenite 

trachyte tuff 

98 General petrologic terms 
An unhyphenated compound term should remain 

unhyphenated when it becomes a unit modifier: 

"quartz monzonite dike," for example. Expressions 

such as "granite-syenite contact" or "sandstonesiltstone transition" should be avoided; they seem to 

refer to single rocks. Instead, say "contact of the 

granite and the syenite" or "transition from sandstone to siltstone." 

Polysyllabic adjectives should be avoided in favor of 

shorter and simpler terms. Words such as "aphanitic," "melanocratic," and "arenaceous" are not as 

good as "fine-grained," "dark-colored," and "sandy." 

The terms "acid," "basic," and "alkaline" for 

describing rocks and minerals have been frowned 

upon since the time of F.W. Clarke. Rocks characterized by quartz are "silicic" rather than "acidic." 

Rocks are "alkalic," "calcic," "sodic," or "potassic" 

but not "alkaline." Similarly, the terms "mafic" and 

"ferromagnesian" are preferred over "basic." 

Authors are urged to follow the mineral symbology 

of Kretz (1983) when using mineral-phase abbreviations as symbols, subscripts, or superscripts in a 

manuscript. These symbols (table 3, p. 108) conform 

to the following guidelines: 

1. The mineral symbol should consist of two or three 

letters, the first capitalized and the other@) 

lowercase. 

2. The first letter of the symbol should be the same 

as the first letter of the mineral name; the other 

letter(@ should be selected from the mineral 

name, preferably from the consonants. 

3. The symbol should not be identical to any symbol 

of the elements in the periodic table. 

4. The symbol should not spell out a common word 

of any language used in scientific writing. 

These guidelines should be followed by authors who 

intend to compose new symbols for minerals not listed 

in table 3. 

Mineral-phase symbols and mineral-component symbols must be clearly distinguished from one another 

(Kretz, 1983). Mineral-phase symbols should always 

begin with uppercase letters; mineral-component 

symbols should consist solely of lowercase letters; for 

example, Di = the mineral phase diopside, di = the diopside component in a pyroxene or a melt. 

ROCK ANALYSES AND 

SAMPLE DESCRIPTIONS 

A representative table showing one way to report 

rock analyses is given by table 10, page 220. To help 

the reader interpret the analyses, a description of the 

samples can be provided either by footnotes or by a 

separate table. 

Rock analyses and sample descriptions 99