Classification of Igneous Rocks

by Russell B. Travis
Web pages adapted from Quarterly of the Colorado School of Mines, vol. 50, no. 1

Essential
Minerals
Potash Feldspar > 2/3 Total Feldspar
Quartz > 10% Quartz < 10%
Feldspathoid < 10%
Feldspathoid > 10%
Characterizing
Accessory
Minerals
Chiefly: Hornblende, Biotite, Pyroxene, Muscovite
Also: Sodic Amphiboles, Aegirine, Cancrinite, Sodalite, Tourmaline
Color Index 10 15 20
Average
Chemical
Composition
(DALY)
SiO2 71.5 60.4 56.0
Al2O3 14.0 17.0 19.2
Fe2O3 1.5 2.7 2.9
FeO 1.4 2.9 1.6
MgO 0.6 1.8 0.6
CaO 1.6 3.7 2.0
Na2O 3.4 4.2 8.5
K2O 4.3 5.1 5.3
Phaneritic Equigranular
Batholiths, lopoliths, stocks, large laccoliths, thick dikes, and sills

Granite
ALASKITE:
few dark minerals
GRAPHIC GRANITE:
Graphic texture
ALKALI GRANITE:
abundant albite and sodic
amphibole or pyroxene
CHARNOCKITE:
with orthopyroxene
LUXULLIANITE:
tourmalinized

Syenite
QUARTZ SYENITE:
a little quartz
ALKALI SYENITE:
no plagioclase except albite
PULASKITE:
a little nepheline
NORDMARKITE:
a little quartz
LARVIKITE:
with "blue" feldspar
SHONKINITE:
abundant FeMg minerals
Nepheline Syenite
LEUCITE SYENITE:
pseudoleucite only feldspathoid
SODALITE SYENITE:
sodalite only feldspathoid
FOYAITE:
abundant feldspar
MALIGNITE:
abundant FeMg minerals
DITROITE:
with nepheline and sodalite
Porphyritic Phaneritic Groundmass
Laccoliths, dikes, sills, plugs, small stocks, margins of larger masses
Granite Porphyry Syenite Porphyry Nepheline Syenite Porphyry
Aphanitic Groundmass
Dikes, sills, laccoliths, surface flows, margins of larger masses, welded tuffs
Rhyolite Porphyry Trachyte Porphyry Phonolite Porphyry
Aphanitic Microcrystalline
Dikes, sills, surface flows, margins of larger masses, welded tuffs
Rhyolite Trachyte Phonolite
LEUCITE PHONOLITE:
(Leucite trachyte) leucite only feldspathoid
TINGUAITE:
abundant aegirine
WYOMINGITE:
leucite and phlogopite

Glassy
Surface flows, margins of dikes and sills, welded tuffs

Obsidian: black
Pitchstone: resinous
Vitrophyre: porphyritic
Perlite: concentric fractures
Pumice: finely cellular, light colored
Scoria: coarsely cellular, dark colored
Normally it is not possible to determine the composition of these rocks. They are customarily designated by the names at the left of this column. Basic glass is rare so rocks named, except scoria, will normally be silicic. If the approximate composition (by close association) or silica content (by refractive index or analysis), can be determined, the name may be prefixed by the name of the appropriate aphanitic rock, for example, "trachyte obsidian," or "latite vitrophyre." In general, scoria is basic; basic obsidian is called "tachylite"; and spherulitic tachylite is "variolite."

Essential
Minerals
Potash Feldspar 1/3 - 2/3 Total Feldspar
Quartz > 10% Quartz < 10%
Feldspathoid < 10%
Feldspathoid
> 10%
Characterizing
Accessory
Minerals
Chiefly: Hornblende, Biotite, Pyroxene
Also: Sodic Amphiboles, Aegirine
Color Index 20 25 30
Average
Chemical
Composition
(DALY)
SiO2 66.8 57.0 54.1
Al2O3 15.8 17.1 21.0
Fe2O3 2.3 3.4 1.8
FeO 1.3 3.6 3.3
MgO 1.0 2.3 1.1
CaO 2.8 5.4 3.2
Na2O 3.7 4.7 6.2
K2O 4.2 3.7 5.9
Phaneritic Equigranular
Batholiths, lopoliths, stocks, large laccoliths, thick dikes, and sills
Quartz Monzonite
(ADAMELLITE)
Monzonite Nepheline Monzonite
Porphyritic Phaneritic Groundmass
Laccoliths, dikes, sills, plugs, small stocks, margins of larger masses
Quartz Monzonite Porphyry Monzonite Porphyry Nepheline Monzonite Porphyry
Aphanitic Groundmass
Dikes, sills, laccoliths, surface flows, margins of larger masses, welded tuffs
Quartz Latite Porphyry Latite Porphyry Nepheline Latite Porphyry
Aphanitic Microcrystalline
Dikes, sills, surface flows, margins of larger masses, welded tuffs
Quartz Latite
(DELLENITE)
Latite
(TRACHY-ANDESITE)
Nepheline Latite

Glassy
Surface flows, margins of dikes and sills, welded tuffs

Obsidian: black
Pitchstone: resinous
Vitrophyre: porphyritic
Perlite: concentric fractures
Pumice: finely cellular, light colored
Scoria: coarsely cellular, dark colored
Normally it is not possible to determine the composition of these rocks. They are customarily designated by the names at the left of this column. Basic glass is rare so rocks named, except scoria, will normally be silicic. If the approximate composition (by close association) or silica content (by refractive index or analysis), can be determined, the name may be prefixed by the name of the appropriate aphanitic rock, for example, "trachyte obsidian," or "latite vitrophyre." In general, scoria is basic; basic obsidian is called "tachylite"; and spherulitic tachylite is "variolite."

Essential
Minerals
Plagioclase Feldspar > 2/3 Total Feldspar
Potash Feldspar > 10% Total Feldspar Potash Feldspar < 10% Total Feldspar
Sodic Plagioclase Calcic Plagioclase
Quartz > 10% Quartz > 10% Quartz < 10%
Feldspathoid < 10%
Quartz < 10%
Feldspathoid < 10%
Feldspathoid > 10%
Pyroxene > 10%
Characterizing
Accessory
Minerals
Chiefly: Hornblende, Biotite, Pyroxene (in Andesite)
Also: Pyroxene, Feldspathoid, Sodic Amphiboles
Chiefly: Pyroxene, Uralite, Olivine
Also: Hornblende, Biotite, Quartz, Analcite, Aegirine, Sodic Amphiboles
Color Index 20 20 25 50 60
Average
Chemical
Composition
(DALY)
SiO2 65.3 61.6 58.2 48.6 47.4
Al2O3 16.1 16.2 17.0 16.8 15.4
Fe2O3 2.1 2.5 3.2 4.8 4.9
FeO 2.3 3.8 3.7 6.0 5.4
MgO 1.7 2.8 3.5 5.1 5.0
CaO 3.9 5.4 6.3 8.9 9.7
Na2O 3.8 3.4 3.5 3.7 3.8
K2O 2.7 2.1 2.1 1.9 3.5
Phaneritic Equigranular
Batholiths, lopoliths, stocks, large laccoliths, thick dikes, and sills
Granodiorite Quartz Diorite
(TONALITE)
Diorite

Gabbro
GABBRO:
with clinopyroxene
NORITE:
with orthopyroxene
OLIVINE GABBRO: with olivine
TROCTOLITE: olivine and plagioclase only
ANORTHOSITE: plagioclase only
QUARTZ GABBRO: with quartz

Diabase
(Dolerite of British)
Phaneritic diabasic
texture,
normally
medium
or fine
grained
Theralite
(ESSEXITE, NEPHELINE GABBRO)
TESCHENITE: analcite only feldspathoid
OLIVINE THERALITE:
with olivine
Porphyritic Phaneritic Groundmass
Laccoliths, dikes, sills, plugs, small stocks, margins of larger masses
Granodiorite Porphyry Quartz Diorite Porphyry Diorite Porphyry Gabbro Porphyry Theralite Porphyry
Aphanitic Groundmass
Dikes, sills, laccoliths, surface flows, margins of larger masses, welded tuffs
Dacite Porphyry

Andesite
Porphyry

Basalt Porphyry   Tephrite Porphyry
Aphanitic Microcrystalline
Dikes, sills, surface flows, margins of larger masses, welded tuffs
Dacite Andesite Basalt
OLIVINE BASALT:
with olivine
ANALCITE BASALT:
with analcite
QUARTZ BASALT:
with quartz
OCEANITE:
with abundant olivine
Tephrite
LEUCITE TEPHRITE:
leucite only feldspathoid
BASANITE:
with olivine
LEUCITE BASANITE:
with olivine and leucite

Glassy
Surface flows, margins of dikes and sills, welded tuffs

Obsidian: black
Pitchstone: resinous
Vitrophyre: porphyritic
Perlite: concentric fractures
Pumice: finely cellular, light colored
Scoria: coarsely cellular, dark colored
Normally it is not possible to determine the composition of these rocks. They are customarily designated by the names at the left of this column. Basic glass is rare so rocks named, except scoria, will normally be silicic. If the approximate composition (by close association) or silica content (by refractive index or analysis), can be determined, the name may be prefixed by the name of the appropriate aphanitic rock, for example, "trachyte obsidian," or "latite vitrophyre." In general, scoria is basic; basic obsidian is called "tachylite"; and spherulitic tachylite is "variolite."

Essential
Minerals
Little or No Feldspar Special Types
Chiefly Pyroxene and/or Olivine Chiefly Ferro-Magnesian Minerals and Feldspathoids
Characterizing
Accessory
Minerals
Chiefly: Serpentine, Iron Ore
Also: Hornblende, Biotite
Hornblende, Biotite, Iron Ore
Color Index 95 55  
Average
Chemical
Composition
(DALY)
SiO2 41.1 42.0
Al2O3 4.8 17.9
Fe2O3 4.0 5.7
FeO 7.1 5.7
MgO 32.2 3.4
CaO 4.4 10.3
Na2O 0.5 8.0
K2O 1.0 2.4
Phaneritic Equigranular
Batholiths, lopoliths, stocks, large laccoliths, thick dikes, and sills

Peridotite
PERIDOTITE:
clinopyroxene and olivine
HARZBURGITE:
orthopyroxene and olivine
PICRITE:
pyroxene and olivine with some plagioclase
DUNITE:
olivine only
PYROXENITE:
pyroxene only
SERPENTINE (SERPENTINITE):
chiefly serpentine

MISSOURITE:
pyroxene, olivine, and pseudoleucite
IJOLITE:
pyroxene and nepheline
FERGUSITE:
pyroxene and pseudoleucite
UNCOMPAHGRITE
(MELILITE PYROXENITE):
pyroxene and melilite

Pegmatite:
phanerocrystalline, normally silicic, dike rock (or small irregular mass) having a conspicuously coarser texture than parent

Aplite:
phanerocrystalline rock having
sugary (fine-grained
allotriomorphic-granular) texture

Lamprophyre:
dark dike rock with exclusive FeMg phenocrysts and/or euhedral FeMg minerals in ground mass

Porphyritic Phaneritic Groundmass
Laccoliths, dikes, sills, plugs, small stocks, margins of larger masses
Peridotite porphyry
KIMBERLITE:
peridotite porphyry or breccia
 
Aphanitic Groundmass
Dikes, sills, laccoliths, surface flows, margins of larger masses, welded tuffs
Limburgite porphyry  

Trap:
dark-colored aphanitic rock

Felsite:
light-colored aphanitic rock

Aphanitic Microcrystalline
Dikes, sills, surface flows, margins of larger masses, welded tuffs
Limburgite NEPHELINITE:
pyroxene and nepheline
LEUCITITE:
pyroxene and leucite
MELILITITE:
pyroxene and melilite
OLIVINE NEPHELINITE
(NEPHELINE BASALT):
pyroxene, nepheline, and olivine
ETC.

Glassy
Surface flows, margins of dikes and sills, welded tuffs

Obsidian: black
Pitchstone: resinous
Vitrophyre: porphyritic
Perlite: concentric fractures
Pumice: finely cellular, light colored
Scoria: coarsely cellular, dark colored
Normally it is not possible to determine the composition of these rocks. They are customarily designated by the names at the left of this column. Basic glass is rare so rocks named, except scoria, will normally be silicic. If the approximate composition (by close association) or silica content (by refractive index or analysis), can be determined, the name may be prefixed by the name of the appropriate aphanitic rock, for example, "trachyte obsidian," or "latite vitrophyre." In general, scoria is basic; basic obsidian is called "tachylite"; and spherulitic tachylite is "variolite."