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Laboratory #2: Igneous Rocks
Materials Needed (to be provided by the instructor):
1. Igneous Rocks Set
2. Scratch Glass, Porcelain Plate, Copper Penny, Iron Nail, Magnets, HCl
3. Hand lens
Introduction to Igneous Rocks
Igneous rocks form by solidification of molten rock (magma). There are two basic types of
igneous rocks: Intrusive and extrusive. Magma that solidifies within the Earth’s surface forms
intrusive igneous rocks. Extrusive igneous rocks solidify on the Earth’s surface. Molten rock
that is erupted from a magma chamber and flows onto the ground is called lava. Sometimes
magma is ejected from a volcano and into the air, where it cools before it falls to the ground.
These extrusive igneous rocks are called pyroclasts.
Mineral Composition
Igneous rocks are comprised primarily of silicate minerals, and can be divided into two groups:
light colored (or felsic) minerals and dark colored (or mafic) minerals.
Common Felsic Minerals
Common Mafic Minerals
Plagioclase feldspar
Alkali feldspar
Quartz
Muscovite
Olivine
Pyroxene
Hornblende
Biotite
If only mafic minerals are present, then the rock is said to be ultramafic. Rocks that contain
mostly mafic minerals are called mafic, and if only felsic minerals are present, then the rock is
said to be felsic. Rocks that contain roughly equal amounts of both felsic and mafic minerals are
intermediate.
Igneous Rock Textures
Intrusive igneous rock textures: The cooling of magma inside of the Earth produces crystals
that interlock with one another. The resulting rock is usually quite hard.
 Coarse-grained intrusive rocks have visible crystals that are all roughly the same size.
This texture is typical of plutonic rocks such as granite, diorite, gabbro and peridotite.
 Pegmatite texture is typical of intrusive rocks that form in dikes. These rocks are very
coarse-grained and crystal sizes can be several centimeters in size.
 Fine-grained intrusive rocks have crystals that are so small you cannot see them with
the naked eye. Not many intrusive rocks have fine-grained textures, although it can
develop in shallow intrusive bodies such as dikes or sills.
Extrusive igneous rock textures: Extrusive igneous rocks such as lavas cool as one massive
unit. This produces an interlocking crystal pattern, similar to that seen in intrusive igneous rocks.
Pyroclastic igneous rocks, such as ashes and tuffs, were molten prior to being ejected into the air.
These pyroclastic fragments cool and solidify before they reach the ground. This can produce a
rock that may have individual fragments and layering.
Page 1

Aphanitic (fine-grained) textures develop when the molten rock is cooled very rapidly.
Individual crystals are not visible to the naked eye. This texture is typical of extrusive
igneous rocks such as basalt, rhyolite, and andesite.

Porphyritic texture contains large crystals (called phenocrysts) in an aphanitic matrix.
The two different grain sizes must be distinct, and not gradual. This type of texture
develops in two stages. First, cooling takes place slowly in the Earth, producing the
phenocrysts. The magma, with the phenocrysts, is then ejected from the volcano where
it cools quickly, forming the aphanitic groundmass. A porphyry is a rock that contains
25% or more phenocrysts; a porphyritic rock contains less than 25% phenocrysts.

Glassy textures form from highly viscous lavas that are often felsic in composition. They
can also form when the magma cools so rapidly that minerals do not have a chance to
crystallize. Obsidian and pumice are two examples of extrusive igneous rocks with
glassy textures.

Vesicular textures form as gasses are released from the magma as it cools, and are
typical of extrusive igneous rocks. Each small cavity is called a vesicle. Rocks that are
very vesicular are called scoria; when there are more vesicles than matrix, the rock is
called pumice. Sometimes minerals will be deposited in the vesicles after the rock has
cooled. These are called amygdules and the rock is said to have an amygdaloidal
texture.

Pyroclastic textures may have a “powdery” texture or may appear to be composed of
individual fragments stuck together. Volcanic ash is comprised of loose sand to silt size
fragments; Tuffs are rocks comprised of silt to pebble sized ash and/or pumice; Welded
tuffs are tuffs that were deposited while still partially molten, and may appear streaky or
glassy; and Volcanic breccia are rocks with large, broken volcanic rock fragments.
The Lab
The object of the lab is to be able to identify the various types of igneous rocks based upon their
color, mineral composition, and texture. First, determine which minerals are present. Then,
determine the grain size. Determine what color the rock is (light, dark, gray, pink, etc.). Does the
rock have any textures, such as vesicles, phenocrysts, etc.? Use the chart below to help you
determine the name of the rock, and whether the rock is extrusive or intrusive.
Page 2
Identification of Igneous Rocks
Particle size
Physical properties
Dark gray to black in color; minerals include plagioclase,
pyroxene, and olivine.
GABBRO
Dark gray to black in color; minerals include plagioclase,
pyroxene, and olivine. Some minerals may be significantly
larger than others.
PORPHYRITIC GABBRO
Groundmass is dark gray to black in color; commonly contains
olivine phenocrysts
BASALT PORPHYRY OR
PORPHYRITIC BASALT
Dark gray to black in color; individual minerals are too small to
see. May be vesicular
BASALT OR SCORIA
Crystals several centimeters across. Minerals may vary.
Pegmatites typically form in dikes
PEGMATITE
Comprised of dark green to black pyroxene and white to gray
plagioclase feldspar in roughly equal proportions (salt and
pepper appearance). Presence of pyroxene and absence of
quartz are diagnostic features.
DIORITE
porphyritic with coarse
grained groundmass
As that of diorite, but some minerals are significantly larger than
others.
PORPHYRITIC DIORITE
porphyritic with aphanitic
groundmass
As that of andesite, but with phenocrysts of plagioclase,
pyroxene and/or hornblende.
ANDESITE PORPHYRY OR
PORPHYRITIC ANDESITE
Gray, dark tan, purple or brown in color. Minerals in the
groundmass are too small to see. Andesite is the most common
lava-flow rock of stratovolcanoes or composite cones.
ANDESITE
Volcanic glass with conchoidal fracture. Usually black in color
and transparent along thin edges.
OBSIDIAN
Highly vesicular ("frothy") texture, typically light gray in color.
PUMICE
Crystals several centimeters across. Minerals may vary.
Pegmatites typically form in dikes
PEGMATITE
Two types of feldspars are visible - plagioclase (white) and
microcline (pink), in addition to quartz, biotite and hornblende.
GRANITE
Similar to that of granite, but with less alkali feldspars (such as
microcline). Color is more gray than pink.
GRANODIORITE
porphyritic with coarse
grained groundmass
As that of granite, but some minerals are significantly larger
than others.
PORPHYRITIC GRANITE
porphyritic with aphanitic
groundmass
As that of rhyolite, but contains quartz phenocrysts (and
sometimes feldspar). The presence of quartz distinguishes it
from andesite.
May be light tan, pink, beige, yellowish or light gray in color.
Yellowstone National Park is named for the yellow rhyolite found
there.
Volcanic glass with conchoidal fracture. Usually black in color
and transparent along thin edges.
RHYOLITE PORPHYRY OR
PORPHYRITIC RHYOLITE
Highly vesicular ("frothy") texture, typically light gray in color.
PUMICE
PLUTONIC
PERIDOTITE
coarse grained
porphyritic with coarse
grained groundmass
porphyritic with aphanitic
groundmass
fine grained / aphanitic
fine grained / aphanitic
PLUTONIC
VOLCANIC
coarse grained
(MEDIUM COLORED)
INTERMEDIATE
very coarse grained
fine grained / aphanitic
VOLCANIC
(LIGHT COLORED)
coarse grained
PLUTONIC
very coarse grained
FELSIC
Rock name
Dark green in color; minerals include olivine, pyroxene, and
minor amounts of plagioclase. Can be distinguished from
gabbro by having a lower proportion of cleavage surfaces that
catch the light.
VOLCANIC
(DARK COLORED)
ULTRAMAFIC / MAFIC
Color
Page 3
RHYOLITE
OBSIDIAN
Page 4
Page 5
12
11
10
9
8
Pink, buff
Coarse
Aphanitic
porphyritic
Plagioclase,
microcline, quartz,
biotite
None visible
White with
some black
Aphanitic / Fine
Visible: olivine
Dark gray
Black and
white
Coarse
Quartz, plagioclase,
microcline,
hornblende & biotite
Lithic fragments
May have olivine
Like granite, only one
feldspar
Frothy texture
Gray
Aphanitic / Fine
glassy
None visible
Quartz, microcline,
muscovite
7
Two types of feldspar
VERY coarse grained
Glassy
None
(Amorphous silica)
6
Pink, white
and black
Very small clear crystals
Pink, white,
silvery
Coarse
Quartz, plagioclase,
microcline,
hornblende & biotite
5
Buff, rose
Very coarse
Aphanitic / Fine
None visible
4
Black and white = diorite!
Feldspar phenocrysts
OTHER (fragments.
Vesicular, etc.)
Looks like glass
Coarse
Plagioclase &
hornblende
3
Black, dark
gray, white
Dark gray,
white
Black and
white
COLOR
Lithic Tuff
Adamellite
Basalt
(vesicular)
Granodiorite
Pumice
Pegmatite
Obsidian
Granite
Rhyolite
Diorite
Andesite
Porphyry
Norite
NAME
Name ______________________________
Black
Aphanitic
porphyritic
Visible: feldspar
2
Coarse
GRAIN SIZE
(fine or coarse)
Plagioclase &
pyroxene
VISIBLE
MINERALS
1
Sample #
Igneous Rock Identification Table