Page 156 - Laboratory Manual in Physical Geology

IGNEOUS ROCKS CLASSIFICATION
1. Mafic Color
Index (MCI) Felsic Intermediate Mafic Ultramafic
Estimate the rock’s
percent of mafic
(green, dark gray, and
black) mineral crystals.
You can also use visual 0 15 45 85 100%
estimators in GeoTools Felsic Intermediate Mafic Ultramafic
1 and 2. (0 to 15% mafic crystals) (16 to 45% mafic crystals) (46 to 85% mafic crystals) (> 85% mafic crystals)
100%
Muscovite
MINERALS

Quartz MINERALS
80
Plagioclase black)
Feldspar brown/orange)
2. Minerals
Identify minerals gray,
in the rock, if
possible, and 60 dark
the percent pale
of each one. You
can use visual Potassium gray, green,
estimators in Feldspar
GeoTools 1 and 2. (K-Spar) Olivine
Skip this step if the light
FELSIC (Light-Colored: white, Pyroxene
rock is glassy or 40
aphanitic. (Ferromagnesian:

20 MAFIC (augite)

Biotite Amphibole
(hornblende)
0
3. Texture(s)
Identify the rock’s 4. Rock Name: Select name below, based on data from steps 1–3.
texture(s).
Pegmatitic: PEGMATITIC PEGMATITIC PEGMATITIC PEGMATITIC
very
INTRUSIVE ORIGIN coarse-grained (SYENITE, if no quartz) DIORITE GABBRO PERIDOTITE
DIORITE
GABBRO
GRANITE
PERIDOTITE
coarse-grained
Phaneritic:
GRANITE
Phenocrysts
in a phaneritic
GRANITE
GABBRO
PERIDOTITE
DIORITE
groundmass 1 PORPHYRITIC PORPHYRITIC PORPHYRITIC PORPHYRITIC
Phenocrysts 1 PORPHYRITIC PORPHYRITIC PORPHYRITIC
in an aphanitic RHYOLITE ANDESITE BASALT
groundmass
Aphanitic:
fine-grained RHYOLITE ANDESITE BASALT KOMATIITE
EXTRUSIVE ORIGIN Vesicular very lightweight; white or gray; floats in water) (resembles a sponge) criss-crossing needles
OBSIDIAN
Glassy
(resembles basalt but
has 1–10 cm long
SCORIA
PUMICE
of olivine or pyroxene)
(abundant tiny vesicles–like meringue;
VESICULAR BASALT
Pyroclastic or VOLCANIC TUFF (fragments < 2 mm) (has few scattered vesicles)
Fragmental
VOLCANIC BRECCIA (fragments > 2 mm)
1
Phenocrysts are crystals conspicuously larger than the finer grained groundmass (main mass, matrix) of the rock.

FIGURE 5.5 Igneous Rock Classification Chart. Obtain data about the rock in Steps 1–3, then use that data to select the name of the

rock (Step 4). Also refer to FIGURE 5.4 and the examples of classified igneous rocks in FIGURES 5.8 – 5.14 .

Igneous Rocks and Processes ■ 137