GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 2
Classification of Igneous Rock
or
How many ways can you skin a rock?
Jan. 20, 2016
GOALS OF A ROCK CLASSIFICATION SCHEME
Practical – especially for field use
 Descriptive – minimal genetic terms
 Systematic – allows for easy information retrieval
 Hierarchical – allows for greater levels of detail
 Natural – defines boundaries that agree with nature

Most common schemes based on mode (vol. %) of
“essential” primary minerals ...
(When observation of primary minerals is not possible (e.g.,
too fine, altered) chemistry is commonly used)
...with modification by texture and structural features
and occurrence of “accessory” and minor minerals
“ESSENTIAL” MINERALS
Basic
Spinel
LIGHT MINERALS
•Leucocratic
•Felsic Minerals
Synonymous
Terms
Synonymous –
Terms
DARK MINERALS
•Melanocratic
•Mafic Minerals
Color Index-CI
=% Dark Minerals
Acid
A SIMPLE (BUT LIMITING)
CLASSIFICATION SCHEME
CLASSIFICATION OF INTERMEDIATE
AND FELSIC ROCKS
Foid Minerals
Nepheline
Leucite
Kalsilite
Analcime
Sodalite
International Union of Geological Sciences
CLASSIFICATION OF PHANERITIC MAFIC IGNEOUS
ROCKS RECOMMENDED BY THE IUGS*
*International Union of Geological Sciences
CLASSIFICATION OF ULTRAMAFIC IGNEOUS ROCKS
RECOMMENDED BY IUGS
Olivine
Dunite
90
Peridotites
Lherzolite
Olivine Orthopyroxenite
40
Olivine Clinopyroxenite
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
Websterite
10
Orthopyroxene
Clinopyroxenite
Clinopyroxene
NORMALIZING AND PLOTTING MINERAL MODES
ON TERNARY DIAGRAMS
Rock A
Mineral X – 59.5%
Mineral Y – 17%
Mineral Z – 8.5%
Mineral A – 12%
Mineral B – 3%
Normalization to XYZ only
X+Y+Z = 85%
Xn = 59.5/85 = 70%
Yn = 17/85 = 20%
Zn = 8.5/85 =10%
MODIFIERS – FOR A COMPLETE ROCK
DESCRIPTION
Alteration
Foliation/Layering
Absolute Grain Size
Bulk Rock Texture –based on pyroxene habit
Accessory/Minor Minerals
For example:
Moderately serpentinized, modally layered,
moderately foliated, medium-grained, subophitic,
biotitic, oxide-bearing TROCTOLITE
PRIMARY PLANAR FEATURES
Foliation
Alignment of elongate or tabular mineral phases
Scale - % aligned within 10 of a common plane





non-foliated (<25%) (decussate)
poorly foliated (25-50%)
moderately foliated (50-75%)
well foliated (75-90%)
very well foliated (>90%)
Layering
Type: modal, isomodal, graded modal, grain size, textural, phase,
combination
Contrast (or demarcation): strong, moderate, weak, subtle
Frequency: single layer, rhythmic, intermittent, irregular
Scale: centimeter, decimeter, meter, inconsistent, variable
Lateral continuity: continuous, discontinuous, lenticular, wispy
Other descriptors: wavy, cross-bedded, schlerien, colloform, trough,
corrugated, deformed, slumped, convoluted
PCFO
Igneous
Layering
PO
Cm-scale Rhythmic Layering
PCFO
Phase Layering
Intermittent Graded Layering
Wavy Layering
ABSOLUTE AND RELATIVE GRAIN SIZE TERMINOLOGY
Generalized Scale:
Fine (<1 mm) Medium (1-5 mm) Coarse (5-12 mm)
Very Coarse/Pegmatitic (>12 mm)
Detailed Scale:
Very Fine (<0.2 mm) Fine (0.2-0.8 mm)
Medium Fine (0.8-1.5 mm)
Medium (1.5-3 mm)
Medium Coarse (3-7 mm)
Coarse (7-12 mm) Very Coarse (12-30 mm)
Pegmatitic (>30 mm)
Terms describing Distribution of Grain Size
equigranular – generally equal sizes for all granular phases
seriate – gradational range in grain size of all granular phases
hiatial – bimodal range in grain size of all granular phases
porphyritic – bimodal range in grain size of one (or rarely two) primary phase types
modifiers based on contrast in grain size– weakly, moderately, strongly
alternate terminology – "phase"-phyric (e.g., plagioclase-phyric)
BULK ROCK TEXTURES OF MAFIC IGNEOUS ROCKS
Bulk rock texture for mafic igneous rocks
commonly based on pyroxene habit

ophitic – multiple lath-shaped crystals of
plagioclase totally enclosed in crystals of
pyroxene

subophitic – multiple lath-shaped crystals
of plagioclase partially enclosed in crystals of
pyroxene

intergranular – generally equigranular
euhedral to anhedral primary minerals (need
not be just augite and plagioclase), none
enclosing the others
Textural terms for other primary non-granular phases
 poikilitic – one phase completely envelops many other more granular phases
(e.g., plagioclase-poikilitic)
 subpoikilitic – one phase partially envelops other more granular phases
(e.g., olivine-subpoikilitic)
PYROXENE-BASED
BULK ROCK TEXTURES
in
MAFIC INTRUSIVE
ROCKS
Subophitic
Ophitic
Oikocrysts
Intergranular
Prismatic
OPHITIC
INTERGRANULAR
MODAL ROCK CLASSIFICATION
FOR
GABBROIC ROCKS
LeMaitre (2002)
Streckeisen (1976)
-Cotectic Troctolite
72%Pl 28%Ol
Duluth Complex
Schemes
Phinney (1972)
Davidson (1969)
Severson and Hauck (1990)
RECOMMENDED MODAL CLASSIFICATION
From Miller, Green,
and Severson, 2002
►
it uses all five major essential mineral phases (Pl, Ol, Cpx, Opx, and Feox)
► it defines modal boundaries that bracket natural modal populations and cotectic
proportions determined from experimental data
► it uses simple mafic mineral ratios (3:1 or 1:1) which are easy to estimate in the field.
MODAL GROUPS
From Miller, Green,
and Severson, 2002
STILLWATER COMPLEX
CUMULATE CLASSIFICATION SCHEME
PROPOSED “CUMULATE” CLASSIFICATION FOR DC ROCKS
(MILLER, GREEN, AND SEVERSON, 2002)
Main Attributes:
 It applies to rocks that show some igneous foliation or modal layering and
therefore solidified under conditions in which mineral phases segregated from
their parent magma
 It lists abbreviations of all minerals composing greater than about 2 modal
percent in decreasing order of abundance regardless of their cumulus status
 It denotes granular (cumulus) mineral phases with upper case letter
abbreviations and interstitial (intercumulus) mineral phases with lower case
abbreviations
Cumulus/Intercumulus Mineral Codes
PP*/P/p - plagioclase
F/f
- Fe-Ti oxide
O/o
- olivine
A/a
- apatite
C/c
- clinopyroxene (augite)
-/b
- biotite
I/i
- inverted pigeonite -/a
- amphibole
H/h
- hypersthene, bronzite
-/g
- granophyre
* used with anorthositic group rocks
EXAMPLES OF CUMULATE ROCK CODES
Cumulate Code Translation of Some Common Rock Types in
the Duluth Complex
 Ophitic augite troctolite
POcf
 Subophitic augite-bearing oxide troctolite POFc
 Olivine gabbroic anorthosite with poikilitic olivine
PPoc
 Ophitic olivine gabbro
PcOf
 Biotitic feldspathic dunite with poikilitic plagioclase
Opb
 Intergranular apatitic oxide olivine gabbro PCFOA
 Ophitic biotitic augite leucotroctolite
POcb

Possible modifications of the code:
- preface with grain size designator (e.g. mPOcf, mcPPcfab)
-designate well developed foliation of plagioclase with a bar over the P or PP
-designate poikilitic texture with a ^ over the phase abbreviation (e.g. cPPĥ)
-designate subpoikilitic texture with a ´ over the phase abbreviation (e.g mfPCó)
O
PO
PCFO
PcOf
CLASSIFICATION OF INTERMEDIATE
AND FELSIC ROCKS
Intermediate
(“ferro-”)
Felsic
Root
Rock
Name
From Streckeisen
(1976)
Attach mafic mineral
prefix
to root rock name for
intermediate rocks...
or “ferro” if uncertain of
mafic mineral phase
THE MAFIC TO INTERMEDIATE TRANSITION



Typically based on average An content (> or < An50)
Hyndman (1972) suggests also considering:
 rock associations (diorite with more granitoid rocks, gabbro with
more mafic rocks)
 mafic mineral assemblage (diorite = hornblende or biotite 
pyroxene; gabbro = pyroxene  olivine  hornblende)
 plagioclase color (diorite whitish or nearly so; gabbro - greenish
gray to gray)
Blatt and Tracy (1995) suggest using diorite for rock that contains
hornblende over pyroxene and that contain less than 35 modal
percent high-temperature mafic minerals (olivine, pyroxene).
Problems
Can’t “see” AN content in hand samples
Even DC felsic rocks are dominated by pyx over hb and bio
Suggested Field Criteria for DC
Intermediate
Dark Mineral Habit
Felsic Mesostasis
prismatic, subprismatic
>5 %
Mafic
granular to poikilitic
<5 %
Q
CLASSIFICATION OF
VOLCANIC ROCKS
RECOMMENDED BY
THE IUGS
60
60
Rhyolite
Dacite
20
20
Basalt/Andesite Distinctions
Trachyte
SiO2
Basalt
< 52%
Andesite
> 52%
CI
> 35%
< 35%
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
10
Tephrite
60
60
(Foid)ites
F
P
CHEMISTRY-BASED TAS CLASSIFICATION OF
APHANITIC OR METAMORPHOSED IGNEOUS ROCKS
CIPW
NORMATIVE
CALCULATIONS
PSEUDOMINERALOGY
Calculated from
whole rock
geochemical
analyses;
distributes major
elements among
rock-forming
minerals
CLASSIFICATION OF PYROCLASTIC ROCKS