Scheme for Igneous Rock Identification CRYSTAL TEXTURE
SIZE
Obsidian
(usually appears black)
Basaltic glass
non-
crystalline
Non-
vesicular
Glassy
Vesicular
Pumice
Vesicular rhyolite
Scoria
Vesicular basalt
(gas
Vesicular
andesite
pockets)
Fine
Basalt
IGNEOUS ROCKS
ess than
mim
EXTRUSIVE
(volcanic)
ENVIRONMENT OF FORMATION
INTRUSIVE
(Plutonic)
Andesite
Rhyolite
Diabase
Granite
Diorite
Peri-
dotite
10 mm
Coarse
Gabbro
Non-
vesicular
Pegmatite
To mm
arger
Very
coarse
LIGHTER
DARKER
CHARACTERISTICS
LOWER
FELSIC
(rich in SI, AD
100%
COLOR
+ DENSITY +
COMPOSITION
HIGHER
MAFIC
(rich in Fe, Mg)
100%
Potassium
feldspar
(pink to white)
75%
Quartz
(clear to
-75%
volume)
white)
Plagioclase feldspar
(white to gray
(relative DOMPOSITION
50%
-50%
MINERAL
Pyroxene
(green)
Biotite
(black)
25%
Olivine
(green)
-25%
Amphibole
(black)
0%
Physical Setting/Earth Scienne
-0%