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EARTH SCIENCE LAB
Igneous Rock Identification: Virtual LAB
Answer Sheet
Igneous Rocks
Igneous rocks are rocks which solidify from molten material (magma). Cooling of
the magma can occur beneath the surface (plutonic) or on the surface (volcanic).
Igneous rocks can be identified by the determination of the composition and
texture of the rock. Once these two characteristics have been identified, the
Igneous Rock Identification chart is used to identify the rock name.
Igneous Rock Identification Chart
COMPOSITION
TEXTURE
Felsic
Intermediate
Mafic
Pegmatitic
Granite Pegmatite Diorite Pegmatite Gabbro Pegmatite
Phaneritic
Granite
Diorite
Gabbro
Aphanitic
Rhyolite
Andesite
Basalt
Porphyritic
Rhyolite
Andesite
Basalt
Glassy
Vesicular
Pyroclastic
Obsidian
Pumice
Ultramafic
Dunite
Basaltic Glass
Scoria
Volcanic Tuff
Composition
Composition of igneous rocks is properly identified by determination of the rock's
chemical composition. This, however, requires chemical equipment and
apparatus that is unavailable in this lab. Fortunately determination of the exact
chemical composition is not necessary. Color is often an indicator of the
composition of a rock or mineral and can be effectively used to identify the
composition of most igneous rocks. Light colors, including white, light gray, tan
and pink, indicate a felsic composition. Felsic compositions are rich in silica
(SiO2). Dark colors, such as black and dark brown, indicate a mafic or ultramafic
composition. Mafic compositions are poor in silica, but rich in iron (Fe) and
magnesium (Mg). Intermediate compositions have an intermediate color, often
gray or consisting of equal parts of dark and light mineral . Beware that even
though an igneous rock may have a felsic composition (light color), the rock can
contain dark colored minerals. Mafic rocks may contain light colored minerals as
well. As mentioned above, the composition of most igneous rocks can be
identified using this system, formally known as the Color Index. However, there
are exceptions. The two most notable are obsidian and dunite. Obsidian is
volcanic glass which erupts as a lava flow. Most obsidian is felsic in
composition, yet typically it will have a very dark color (dark brown to black).
Dunite has an ultramafic composition yet is apple green to yellowish green in
color. Dunite is composed almost entirely of the mineral olivine which usually
contains both iron and magnesium.
Texture
The texture of an igneous rock does not refer to the roughness or smoothness
of the surface. Textures are based primarily on crystal size. Pegmatitic texture
is composed of very large crystals (larger than 2-3 cm). Phaneritic texture is
composed of crystals which are large enough to see but smaller than
pegmatitic texture, and the entire rock is composed of crystals. Aphanitic
texture is a fine grained texture but the crystals are too small to see.
Porphyritic texture is composed of crystals of two different sizes. Typically, the
large crystals (phenocrysts) are visible while the smaller crystal are not
(referred to as groundmass). Glassy texture is the most readily recognized.
The rock is composed entirely of glass. Few, if any, crystals will be visible.
Vesicular texture is formed when lava solidifies before gases are able to
escape. The result is a "bubbly" appearance. Lastly, pyroclastic texture is
composed of volcanic fragments. These fragments or clasts can be very fine
(ash) or coarse (lapilli) or very coarse (bombs and blocks).
Sample Identification
On each of the following pages you will find an image of a rock and a
Igneous Rock Identification Chart. Identify the composition by identifying
the color and determine the texture by examining the crystal sizes that are
present. Once this is done the rock name is easily determined using the
Classification of Igneous Rock chart. Click each answer, then check to see if
you have correctly identified the rock sample.
Igneous Rock Identification Chart
Sample
Number
1
2
3
4
5
6
7
8
9
10
11
12
Texture
Chemical
Composition
Rock Name
EARTH SCIENCE LAB
Sedimentary Rock Identification: Virtual LAB
Answer Sheet
Sedimentary Rocks
Sedimentary rocks are rocks composed of sediment. Sediment is deposited in a
number of environments of deposition, by both moving air and moving water.
Sedimentary rock identification is primarily based on composition. Texture will still
be used but in a different sense than for igneous rocks.
Texture
Texture of sedimentary rocks in this lab will be taken to indicate origin or type of
sediment found in the rock. Three types of "texture" will be used - clastic,
chemical, and biologic.
Clastic Rocks
Clastic sedimentary rocks contain clasts. These are fragments or pieces of rock
or minerals. The composition of clastic sedimentary rocks is divided into three
types - clay/silt, sand and gravel. Clay and silt are less than 1/16 mm. These
are not visible to the unaided eye. Sand is clasts between 1/16 and 2 mm in
size, and gravel is greater than 2 mm.
Chemical Rocks
Chemical sedimentary rocks are identified by identifying the mineral from
which they are composed. In this lab there are four minerals that need to be
identified - quartz, halite, gypsum and calcite. Quartz has a hardness of 7
and is very difficult to scratch, even with a good quality knife blade. Gypsum
is relatively soft (Hardness =2) and can be scratched easily with a fingernail.
Halite is common table salt and is most easily identified by taste. However,
this is not a sensible practice in a large lab with many different people
handling the samples. Halite has a hardness of 2.5 and cannot be scratched
by a fingernail (unpolished fingernail). Calcite readily reacts with a small
drop of HCl.
Biologic Rocks
Biologic sedimentary rocks are which form as the result of the
accumulation of organic material or biologic activity. Coal is usually
obvious to most students even though few people seem to have ever
actually examined it up close. The dark brown to black color is the most
obvious characteristic. Coquina and limestone are both composed of
calcite . Coquina is composed almost entirely of shell or fossil fragments.
Limestone may or may not contain fossils fragments. Both will react to
HCl. Limestone containing fossils is referred to as fossiliferous limestone.
Sedimentary Rock Identification Chart
TEXTURE
Clastic
Chemical
Biologic
GRAIN
SIZE
COMPOSITION
ROCK NAME
>2 mm
rounded quartz, feldspar and rock fragments
Conglomerate
>2 mm
angular quartz, feldspar and rock fragments
Breccia
1/16 - 2
mm
quartz, feldspar
Sandstone
>1/16 mm
feldspar, quartz
Arkose
<1/16 mm
quartz, clay minerals
Siltstone
(Mudstone, Shale)
<1/256 mm
quartz, clay minerals
Claystone
silica (quartz)
Chert
dolomite
Dolostone
calcite
Limestone
halite
Rock Salt
gypsum
Rock Gypsum
silica (quartz)
Chert
loosely compacted organic material and plant
fragments
Peat
densely compacted organic material and plant
fragments
Bituminous Coal
calcite
Limestone
calcite, micro-skeletal fragments
Chalk
calcite, almost entirely shell and skeletal
fragments
Coquina
calcite with some shell and skeletal fragments
Fossiliferous
Limestone
dolomite with some shell and skeletal fragments
Fossiliferous
Dolostone
Sedimentary Rock Identification Chart
Sample
Number
1
2
3
4
5
6
7
8
9
10
11
12
Texture
Grain Size
Composition
Rock Name
EARTH SCIENCE LAB
Metamorphic Rock Identification: Virtual
LAB Answer Sheet
Metamorphic Rocks
Metamorphic rocks are rocks that have undergone a change from their original
form due to changes in temperature, pressure or chemical alteration. The
classification of metamorphic rocks is based on the minerals that are present and
the temperature and pressure at which these minerals form. Determination of this
information is not easily accomplished in this lab. Therefore, a simplified system is
used based on texture and composition.
Texture
Texture is divided into two groups. Foliated textures show a distinct planar
character. This means that the minerals in the rock are all aligned with each
other. This planar character can be flat like a piece of slate or folded. Nonfoliated textures have minerals that are not aligned. Essentially, the minerals are
randomly oriented.
Foliation
Foliated textures show four types of foliation. Slaty cleavage is composed of
platy minerals that are too small to see. Typically, these rocks split along
parallel, planar surfaces. Phyllitic foliation is composed of platy minerals that
are slightly larger than those found in slaty cleavage, but generally are still too
small to see with the unaided eye. The larger size gives the foliation a slightly
shiny appearance. Schistose foliation is composed of larger minerals which are
visible to the unaided eye. Platy minerals tend to dominate. Gneissic banding is
the easiest of the foliations to recognize. It is composed of alternating bands
of dark and light minerals.
Non-Foliation
Non-foliated textures are identified by their lack of planar character. Further
identification of non-foliated rocks is dependent on the composition of the
minerals or components in the rock. Anthracite coal is similar to bituminous
coal. Both are black in color , and is composed of carbon. Anthracite coal is
generally shiny in appearance and breaks with a conchoidal fracture (broken
glass also shows this type of fracture). Metaconglomerate is composed of
pebbles and gravel that have been flattened due to directed pressure.
Quartzite is composed of quartz sand grains. Quartz has a hardness of 7,
which makes it difficult to scratch. Marble is composed of calcite and will
readily react to a small drop of HCl.
Metamorphic Rock Identification Chart
COMPOSITION
TYPE
PARENT
ROCK
ROCK NAME
slaty
mica
Regional
Mudstone
Slate
phyllitic
quartz, mica,
chlorite
Regional
Mudstone
Phyllite
schistose
mica, quartz
Regional
Slate
Schist
schistose
amphibole,
plagioclase
Regional
Basalt or
Gabbro
Amphibolite
gneissic
banding
feldspar, mica,
quartz
Regional
Schist
Gneiss
carbon
Contact or
Regional
Bituminous
Coal
Anthracite Coal
quartz, rock
fragments
Contact or
Regional
calcite
Contact or
Regional
Limestone
Marble
quartz
Contact or
Regional
Sandstone
Quartzite
TEXTURE FOLIATION
Foliated
NonFoliated
Conglomerate Metaconglomerate
Metamorphic Rock Identification Chart
Sample
Number
1
2
3
4
5
6
7
8
Texture
Foliation
Composition
Parent Rock
Rock Name