Rock Identification Rock Lab
Identification of all three rock groups
is based on
TEXTURE and COMPOSITION
the appearance of the
rock
what the rock is made
out of – type of minerals
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Igneous Rock Classification
Let’s form an igneous rock
Intrusive – magma solidifies below the Earth’s surface
Extrusive – magma solidifies above the Earth’s surface
Igneous Rock Classification
Identification of igneous rocks is based on two main characteristics
Texture – the appearance of the rock due to the rate of magma cooling
Composition – the type of minerals found in the rock (mineral composition)
Textures of Igneous Rocks
Intrusive rocks (Textural terms)
phaneritic texture – crystals are visible and form a mosaic of interlocking mineral aggregates (less than 1 cm)
Interlocking crystal grains
phaneritic texture
Extrusive Igneous Rocks (textural terms)
aphanitic texture – crystals are too small to see
See, you really can not see any mineral crystals
(quenched)
(amorphous solid)- lacks a crystalline
structure
glassy texture
Igneous Rock Classification Lab
Vesicular texture- sponge like appearance, texture contains numerous cavities or holes
Vesicles – gas bubbles cavities
Pyroclastic texture – textures created by rapidly cooling lava that is
“hurled” through the air picking up fragments
(tuffaceous texture)
rock fragments – pieces of rock “incorporated” into the
rock (tuffaceous)
Igneous Rock Classification Lab
Igneous Rock Composition
mineral composition = mineral assemblages= chemistry
The mineral is either ferromagnesian (dark colored) or felsic (light colored
ferromagnesian (mafic)
minerals rich in Fe, Mg – creates a dark colored rocks
Pyroxene (Augite)
Amphibole (hornblende)
Mica - Biotite
Igneous Rock Classification Lab
Igneous Rock Composition
Mineral composition = Mineral assemblages = Chemistry
The mineral is either ferromagnesian (dark colored) or felsic (light colored)
Felsic – mineral composition is light colored minerals
Ca-plagioclase
K-feldspar (orthoclase)
Mica- muscovite
Quartz
Origin
Texture
Intrusive
Phane-
ritic
Very coarse crystal grains, usually all
grains are approximately the same size
w/ respect to each other. Grains> one inch
A mosaic of coarse-grains that are
easily visible to the unaided eye – grains
greater than 1 mm
Granite
Grano-
diorite
Diorite
Gabbro
Peridotite
Granite
Pegmatite
Mafic
Pegmatite
Rhyolite
Dacite
Andesite
Basalt
X
Extrusive
Fine to very fine grains, usually to small
to be distinguished with the unaided
eye or even with a hand lens
Aphan-
itic
Glassy
Por-
ous
Frag-
mental
Obsidian
Pumice
Scoria
Rhyolitic
Tuff
Basaltic
Tuff
80%
40%
20%
Glassy – non-crystalline, non-granular
Color varies between black and brown
Highly vesicular, finely crystalline to glassy
Pumice (light), Scoria (dark)
Composed of fragments of all sizes
Produced in volcanic environments, Ash
Partially re-fused or cemented
Very
Coarse
Rock Descriptions
Igneous Rock Classification
Felsic
Mafic
Sedimentary Rock Identification Lab
Sandstone Arches
How is a sedimentary Rock formed???
Pre-existing
Rock
Erosion
Water, Wind,
Ice
Breaks down
Pre-existing rock
Sediment
Transported
Lakes
Rivers
Deposited
Compacted
Cemented
Lithified
Sedimentary
Rock
Why is the earth’s surface 75% sedimentary rocks?????????
Three major categories of Sedimentary Rocks
Detrital Sedimentary rocks
Chemical Sedimentary rocks
Organic Sedimentary rocks
Detrital (inorganic-detrital)
rocks are composed of particles or grains. Rock
name is dependent on grain size.
Decreasing grain size
2+ mm
2 mm
1/16 mm
1/256 mm
Chemical Rocks – inorganic, non-clastic rocks that are commonly
the result of evaporation and precipitation
Chemical rocks contain no fossils, or biological material but may
react with HCl
The Great Salt Lake, Utah
halite (salt) crystals
Calcium Carbonate (CaCO3) chemical rocks
(evaporation / precipitation processes)
Chemically precipitated SiO2
Organic (bio-clastic, biogenic) rocks –
result from animal and plant secretions. The term clastic
indicates that these rocks contain fossils or parts of plants
shells, compacted plat material
Animal parts
Cemented shell fragments
Lithified plant material
Classification Procedure
Step 1: Determine if the rock is clastic or non-clastic
Step 2: If the rock is clastic, determine the grain size and
diagnostic properties. Make the ID.
Step 3: If the rock is not clastic, determine if the rock is
either chemical or bioclastic
chemical – will not contain particles such as grains and
organic material. Test rock to observe
fizzing (carbonates) Match diagnostic
properties on chart for ID
bio-clastic- rock will be composed of particles that are
organic- animal parts, plant secretions.
Test rock to observe fizzing (carbonate)
Match diagnostic properties on chart for ID
Metamorphic Rocks
Meta (to change) morphic (form) = to change form
Metamorphism– to change a rocks compositional and textural
characteristics by subjecting the rock to
various pressure/temperature relationships
Rocks undergo a solid-state-transformation
The parent rock: represents the pre-existing rock
metamorphism
Example: Shale (parent) Schist
Limestone (parent) Marble
Identification of Metamorphic Rocks
Identified by using the rocks Texture and Composition
Compositional characteristics – types of mineral present
common minerals: quartz, feldspar, muscovite
special minerals: talc, garnet, chlorite
related to P/T relationships
Textural characteristics – minerals are oriented in various
degrees of “parallelism”
platy sub-parallel parallel
Grain size distribution – fine-grain to coarse-grain
reflects P/T relationships
fine-grain coarse-grain
Low P/T High P/T
FOLIATED
Gneissic
Schistose
Slaty
Coarse-grained
High P/T
Fine-grained
Low P/T
Alternating light
and dark mineral
bands
Parallel to sub-parallel
Mineral alignment
“compressed”
layering - platy
Textural characteristics
Non-foliated texture – minerals are not visible and there
orientation does not exist
Rock is composed of single type
chemistry.
NON-FOLIATED
Example: rocks composed of CaCO3
rocks composed of SiO2
Shows NO foliation
Composed of ONE COMPOSITION
Quartz
SiO2
Calcite
CaCO3
Plant Material
C -carbon
How can one
Tell the
Difference ?
“Fussed glass”
Scratches glass
Hard H = 7
“Sometimes xln”
Fizzes in HCl
Soft H = 3
Classification Procedure
Step 1:
no visible grains?
Step 2:
Step 3: