1. Classification of Igneous Rock

2. 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

3. Textural classification of igneous rocks
Phaneritic: crystals visible with naked eye
Plutonic or intrusive rocks
Aphanitic: crystal too small for naked eye
Volcanic or extrusive rocks
Porphyritic: two different, dominant grain sizes
Large xtals = phenocrysts; small xtals = groundmass
Fragmental: composed of disagregated igneous material
Pyroclastic rocks

4. Textural classification of igneous rocks
Pegmatitic: very large xtals (cm to 10s of cm); i.e., slowly cooled
Forms veins or layers within plutonic body
Glassy: non-crystalline; cools very fast (e.g., obsidian)
Volcanic rocks
Vesicular: vesicles (holes, pores, cavities) form as gases expand

5. Compositional terms for igneous rocks
Felsic: feldspar + silica
~55-70% silica, K-feldspar > 1/3 of feldspars present
light-colored silicate minerals — Continental crust
Intermediate: between felsic and mafic
~55-65% silica, plag > 2/3 of feldspars present
Na-rich plag predominates over Ca-rich plag
Mafic: magnesium + ferric iron
~45-50% silica; Ca-rich plag dominant feldspar
dark silicate minerals — Oceanic crust
Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars
Mantle-derived

6. Classification of common igneous rocks
Composition
Phaneritic
Aphanitic
Color index
(% dark minerals)
Felsic
Granite
Syenite
Monzonite
Rhyolite
Trachyte
Latite 10 15 20
Intermediate
Granodiorite
Diorite
Dacite
Andesite 25
Mafic
Gabbro
Basalt 50
Ultramafic
Peridotite 95

7. Composition of Igneous Rocks

9. Determination of Modal Parameters

10. Classification of Igneous Rocks
Figure 2-1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

11. Classification of Igneous Rocks
Figure 2.1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.

12. Classification of Phaneritic Igneous RocksQ
The rock must contain a total of
at least 10% of the minerals below.
Quartzolite
Renormalize to 100% 90 90
Quartz-rich
Granitoid 60 60
Granite
Grano-
Tonalite
Alkali Feldspar Granite
diorite
Alkali Fs. 20 20
Qtz. Diorite/
Quartz Syenite
Plutonic rocks
Quartz
Quartz
Quartz
Qtz. Gabbro
Alkali Fs.
Syenite
Monzonite
Monzodiorite
Syenite 5 5
Diorite/Gabbro/
Syenite
Monzonite
Monzodiorite A 10 35 65 90
Anorthosite
(Foid)-bearing
(Foid)-bearing
(Foid)-bearing P
Syenite
Monzonite
Monzodiorite
(Foid)-bearing 10 10
Diorite/Gabbro
(Foid)-bearing
Alkali Fs.
Syenite
(Foid) Syenite
(Foid)
(Foid)
Monzosyenite
Monzodiorite
(Foid) Gabbro
Figure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. 60 60
(Foid)olites F

13. Classification of Igneous Rocks
Figure 2.2a. A classification of the phaneritic igneous rocks: Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS.

15. Classification of Aphanitic Igneous Rocks
Volcanic rocks
Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS.

18. Classification of Igneous Rocks
Gabbroic
rocks
Figure 2-2. A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS.
Olivine
Clinopyroxene
Orthopyroxene
Lherzolite
Harzburgite
Wehrlite
Websterite
Orthopyroxenite
Clinopyroxenite
Olivine Websterite
Peridotites
Pyroxenites 90 40 10
Dunite
Ultramafic
rocks
(c)

19. Classification of Igneous Rocks
Figure 2.2b. A classification of the phaneritic igneous rocks: Gabbroic rocks. After IUGS.

20. pl
opx
cpx pl pl ol
classification of ultramafic rocks (Streckeinsen, 1973) and LeMaitre (1989)

21. Classification of Igneous Rocks
Figure 2.2c. A classification of the phaneritic igneous rocks: Ultramafic rocks. After IUGS.

22. Textures of Igneous Rocks (1):
Allotriomorphic-granular (gabbroic) texture is a feature of phaneritic rocks in which all of the minerals have anhedral shapes; it is characteristic of some mafic and ultramafic rocks, such as dunite and pyroxenite.
Hypidiomorphic-granular (granitic) texture: igneous texture in which most of the mineral grains are subhedral. Typical of granite, granodiorite, quartz monzonite, etc.
Porphyritic texture: rocks in which larger grains are contained in a finer grained matrix. May occur in either intrusive or extrusive rocks, but it is most common in extrusive rocks such as basalt, andesite, dacite, and rhyolite.
Intergranular texture: Plagioclase laths with interstitial pyroxene grains that are smaller than the plagioclase; commonly found in basalts.

23. Textures of Igneous Rocks (2):
Intersertal texture: Small feldspars with glass or altered glass interstitial to the feldspars; common texture of basalts.
Ophitic texture: Pyroxene grains partially or completely surround plagioclase laths; common texture of gabbros and basalts.
Trachytic texture: Subparallel feldspars formed during flow in volcanic rocks.
Spinifex texture: Interlacing olivine or pyroxene, a texture formed by quenching in komatiites.

24. Igneous Textures
Figure 3-5. a. Compositionally zoned hornblende phenocryst with pronounced color variation visible in plane-polarized light. Field width 1 mm. b. Zoned plagioclase twinned on the carlsbad law. Andesite, Crater Lake, OR. Field width 0.3 mm. © John Winter and Prentice Hall.

25. Figure 3-18. a. Carlsbad twin in orthoclase
Figure a. Carlsbad twin in orthoclase. Wispy perthitic exsolution is also evident. Granite, St. Cloud MN. Field widths ~1 mm. © John Winter and Prentice Hall.
Figure b. Very straight multiple albite twins in plagioclase, set in felsitic groundmass. Rhyolite, Chaffee, CO. Field widths ~1 mm. © John Winter and Prentice Hall.

26. Figure 3-18. (c-d) Tartan twins in microcline. Field widths ~1 mm
Figure (c-d) Tartan twins in microcline. Field widths ~1 mm. © John Winter and Prentice Hall.

27. Figure 3-19. Polysynthetic deformation twins in plagioclase
Figure Polysynthetic deformation twins in plagioclase. Note how they concentrate in areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. © John Winter and Prentice Hall.

28. Figure Myrmekite formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L. Collins.

29. Michel-Levy method for determining feldspar composition
In XPL, find
uniform
extinction in
N-S direction 1
Using albite
twins 3
Angle between
CW and CCW
measurement
should be within
a few degrees;
measure 5-10
grains and take
highest angle. 2
Rotate counterclockwise…
Rotate clockwise…

30. Descriptive Terminology
Pyroxene-based (augite) bulk textural terminology
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 augite and plagioclase), none enclosing the others
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)

31. Mineral Textures Poikilitic (Ophitic) Subpoikilitic Subhedral Granular
Ameboidal Anhedral Granular
Euhedral Granular

32. 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 ( mm)
Medium Fine ( mm)
Medium (1.5-3 mm) Medium Coarse (3-7 mm)
Coarse (7-12 mm) Very Coarse (12-30 mm)
Pegmatitic (>30 mm)
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)

33. Planar Features
Alignment of elongate or tabular mineral phases
(igneous lamination, fluxion structure, foliation)
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 (not typically observed at the scale of a thin section)
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