Page 155 - Laboratory Manual in Physical Geology

IGNEOUS ROCK ANALYSIS AND CLASSIFICATION

STEP 1 & 2: MCI and Mineral Composition STEP 3: Texture

Quartz Pegmatitic
hard, transparent, gray, mostly crystals larger than
crystals with no cleavage 1 mm: very slow cooling of
magma
INTRUSIVE ORIGIN
Sheets 1 and 2 for tools to visually estimate MCI. the percent of mafic (green, dark gray, black) minerals in the rock. FELSIC MINERALS often striated lens: slow cooling of magma
Phaneritic
Plagioclase Feldspar
hard, opaque, usually pale gray
crystals about 1–10 mm,
to white crystals with cleavage,
can be identified with a hand
Porphyritic
Potassium Feldspar
large and small crystals:
hard, opaque, usually pastel
slow, then rapid cooling
orange, pink, or white crystals
and/or change in magma
with exsolution lamellae
viscosity or composition
Aphanitic
Muscovite Mica
crystals too small to identify
flat, pale brown, yellow, or
with the naked eye or a hand
colorless, crystals that scratch
Biotite Mica
5.2 and GeoTools easily and split into sheets EXTRUSIVE (VOLCANIC) ORIGIN lens; rapid cooling of lava
Glassy
flat, glossy black crystals that
rapid cooling and/or very
scratch easily and split into
poor nucleation
sheets
Mafic Color Index (MCI): See the top of Figure MAFIC MINERALS hard, dark gray to black, brittle like meringue: rapid cooling
Amphibole
Vesicular
crystals with two cleavages that
of gas-charged lava
intersect at 56 and 124 degrees
Pyroxene (augite)
Vesicular
hard, dark green to green-gray
some bubbles:
crystals with two cleavages that
gas bubbles in lava
intersect at nearly right angles
Olivine (gemstone peridot) Pyroclastic or Fragmental:
hard, transparent to opaque, particles emitted from
pale yellow-green to dark green volcanoes
crystals with no cleavage
STEP 4: Igneous Rock Classification Flowchart
K-spar > Plagioclase quartz present... GRANITE 1,2
Feldspar > mafic minerals
Texture is no quartz........... SYENITE 1,2
pegmatitic K-spar < Plagioclase....................................... DIORITE 1,2
or phaneritic MCI = 45–85.................................................... GABBRO 1,2
Feldspar < mafic minerals
MCI = 85–100 (< 15% felsic minerals) ........... PERIDOTITE
Also refer to
felsic (MCI = 0–15) and/or pink, white, or pale brown............................................... RHYOLITE 2,3 Figure 5.2
Texture is intermediate (MCI = 15–45) and/or green to gray...................................................... ANDESITE 2,3
aphanitic and/or mafic (MCI > 45) and/or dark gray to black.............................................................. BASALT 2,3
vesicular mafic with abundant vesicles (resembles a sponge)................................................ SCORIA
intermediate or felsic with abundant tiny vesicles—like meringue, floats in water.. PUMICE
Glassy texture ................................................................................................................................................. OBSIDIAN

fragments < 2mm.................................................................. TUFF
Pyroclastic (fragmental) texture
fragments > 2mm.................................................................. VOLCANIC BRECCIA
1
Add pegmatite to end of name if crystals are > 1 cm (e.g., granite-pegmatite).
2
Add porphyritic to front of name when present (e.g., porphyritic granite, porphyritic rhyolite).
3
Add vesicular to front of name when present (e.g., vesicular basalt).
FIGURE 5.4 Igneous rock analysis and classification. Step 1—Estimate the rock’s mafic color index (MCI). Step 2—Identify the main
rock-forming minerals if the mineral crystals are large enough to do so, and estimate the relative abundance of each mineral (using a Visual
Estimation of Percent chart from GeoTools Sheet 1 or 2). Step 3—Identify the texture(s) of the rock. Step 4—Use the Igneous Rock Classification
Flowchart to name the rock. Start on the left side of the flowchart, and work toward the right side to the rock name.

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