Classification of Igneous Rocks


TEXTURES
    DESCRIPTION INTERPRETATION
   
PHANERITIC
1/16 mm
to
3 cm
Average size of crystal-grains is more or less uniform and the average grain size is in the range, 1/16 mm to 3 cm.  For instance, in a granite typically 85% or more of the crystals might be close to 1 cm in size.  There may be some smaller grains and some larger grains, but not many of each.  The average crystal size would be about 1 cm. Plutonic.  Slow rate of cooling.
>3 cm
PEGMATITIC texture, if most of the volume of the rock is made up of mineral grains larger than 3 cm. Again, there may be some smaller grains, but they don't account for much of the volume. Plutonic.  Pegmatitic rocks are generally believed to crystallize from H2O-dominated fluid, sort of like a brine.  The fluid is not a conventional silicate magma, but is generally associated with the evolution of conventional granitic magmas. 
APHANITIC
< 1/16 mm
Essentially all of the grains are less than 1/16 mm in size.  There may be rare larger crystal, but if the larger crystals don't account for more than about 5% or 10 % of the volume of the rock, the texture may still be considered aphanitic. Volcanic.  Rapid cooling.
PORPHYRITIC Bimodal distribution of crystal-grain sizes.  This is a very common texture in volcanic rocks.  "Bimodal" grain-size distribution means two distinct grain-size populations.  In a porphyritic andesite, for instance, 75% of the volume of the rock may be aphanitic (<1/16 mm) and the remaining 25% of the volume may be represented by a population of larger (~8 mm) crystals (e.g., hornblende, plagioclase). Volcanic or plutonic.  In volcanic rocks, the small grain-size part of the rock (matrix) is aphanitic, and large grain-size crystals are generally phaneritic.  In plutonic rocks, the small grain-size part of the rock (matrix) is phaneritic, and the large grain-size crystals just larger. The texture usually implies a change in the rate of cooling from slow to rapid.
GLASSY Natural glass (not crystalline).  Smooth, vitreous, usually black, but translucent.  Breaks along conchoidal (smooth, curved) surfaces. Volcanic, very rapid cooling.
VESICULAR A high proportion of the volume of the rock (say 40% or more) consists of spherical or elliptical empty spaces.  The texture may be likened to an ordinary kitchen sponge, soap suds, or foam rubber.  Because a significant portion of the rock is air-space, the rocks may be very light. Volcanic.  Gas bubbles, formed as lava cools on the Earth's surface.
   


MINERALOGY
Compositions of igneous rocks are expressed by a limited number of combinations of the minerals identified at the top of the CLASSIFICATION CHART below.  The combinations involving OLIVINE, AUGITE (PYROXENE), and Ca-rich PLAGIOCLASE crystallize from magmas that originated in the ASTHENOSPHERE.  The resulting plutonic rocks are GABBRO, PERIDOTITE, DUNITE, and the resulting volcanic rock is BASALT.  Except for the olivine in a dunite, these combinations of minerals produce very dark-colored rocks.  The combinations K-FELDSPAR+QUARTZ, and Na-rich PLAGIOCLASE+HORNBLENDE crystallize from magmas that originated by melting near the base of continental crust or oceanic island arc crust adjacent to convergent plate boundaries.  The resulting plutonic rocks are GRANITE and DIORITE, and the corresponding volcanic rocks are RHYOLITE and ANDESITE.  In GRANITE the dominant minerals (quartz, K-feldspar) are both light in color, so the rock as a whole is light in color.  In DIORITE, one of the dominant minerals (Na-rich plagioclase) is light in color (usually white) while the other dominant mineral (hornblende) is nearly black, so the rock has a coarse salt-and-pepper look.  RHYOLITE is light in color or red. ANDESITE is intermediate in color.


CLASSIFICATION CHART
 Igneous Rocks  --      
 MINERAL COMPOSITION
K-FELDSPAR
QUARTZ
+/-
plagioclase
biotite
muscovite
Na-rich
PLAGIOCLASE
HORNBLENDE
+/-
biotite
quartz
augite
Ca-rich
PLAGIOCLASE
AUGITE
+/-
olivine
hornblende
PYROXENE(S)
OLIVINE
OLIVINE
  ROCK NAME
----
T
E
X
T
U
R
E
-----
Phaneritic
(Plutonic)
grains 
< 3 cm
GRANITE
DIORITE
GABBRO
PERIDOTITE
---DUNITE---
grains 
> 3cm
PEGMATITIC
GRANITE
rare or
unknown
rare or
unknown
rare or
unknown
rare or
unknown
Aphanitic (volcanic)
RHYOLITE
ANDESITE
BASALT
rare or
unknown
rare or
unknown
Porphyritic (volcanic)
PORPHYRITIC
RHYOLITE
PORPHYRITIC
ANDESITE
PORPHYRITIC
BASALT
rare or
unknown
rare or
unknown
Glassy (volcanic)
OBSIDIAN
rare or
unknown
rare or
unknown
rare or
unknown
rare or
unknown
Vesicular (volcanic)
PUMICE
(small vesicles)
rare or
unknown
VESICULAR
BASALT (Scoria)
rare or
unknown
rare or
unknown
      
COLOR
Light Shades or red, 
except obsidian
which is usually black
Intermediate
Shades
Dark-shades of green, to nearly black
Light to 
Intermediate green



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