1. Igneous Rock Classification Lab
What is a rock?
rocks are composed of minerals, mineraloid or organic materials
Granite (composed of minerals)
Opal (composed of mineraloids)
Coal (composed of organics)
Elements Minerals Rocks

2. Igneous Rock Classification Lab
Let’s form an igneous rock
Two major types of igneous rocks
Intrusive – magma solidifies below the Earth’s surface
magma cools very slow
crystals form interlocking “mosaic” textures
very coarse-grained visible minerals
referred to as Plutonic rocks
Extrusive – magma solidifies above the Earth’s surface
magma cools very fast
minerals can not be seen with un-aided eye
very fine-grained texture (no visible minerals
referred to as Volcanic rocks
Extrusive rocks
Intrusive rocks

3. Igneous Rock Classification Lab
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

4. Igneous Rock Classification Lab
pegmatitic texture – crystals are very large (>1cm)
Large orthoclase and plagioclase minerals
Extrusive Igneous Rocks (textural terms)
aphanitic texture – crystals are too small to see
See, you really can not see any mineral crystals

5. Igneous Rock Classification Lab
porphyritic texture – crystals can be separated into two distinct visible sizes. There can be small grains or large grains, but crystals appear in 2-distinct sizes
Phenocrysts
Groundmass (matrix)

6. 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)

7. Intrusive Extrusive Igneous Rock Textures Rock cooled slowly
Rock cooled rapidly
Texture types:
phanartitic
pegmatitic
Texture types:
aphanitic
porphyritic
vesicular
pyroclastic
tuffaceous

8. 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

9. 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

10. Texture Origin Intrusive Extrusive X Igneous Rock Classification
Felsic
Mafic
80%
Origin
Texture
40%
20%
Rock Descriptions
Very coarse crystal grains, usually all
grains are approximately the same size
w/ respect to each other. Grains> one inch
Coarse
Very
Granite
Pegmatite
Mafic
Pegmatite
Intrusive
A mosaic of coarse-grains that are
easily visible to the unaided eye – grains
greater than 1 mm
Phane-
ritic
Grano-
diorite
Diorite
Gabbro
Peridotite
Granite
Aphan-
itic
Fine to very fine grains, usually to small
to be distinguished with the unaided
eye or even with a hand lens
Rhyolite
Dacite X
Andesite
Basalt
Glassy
Glassy – non-crystalline, non-granular
Color varies between black and brown
Obsidian
Extrusive
Por-
ous
Highly vesicular, finely crystalline to glassy
Pumice (light), Scoria (dark)
Pumice
Scoria
Composed of fragments of all sizes
Produced in volcanic environments, Ash
Partially re-fused or cemented
Rhyolitic
Tuff
Basaltic
Tuff
mental
Frag-

11. Igneous Rock Classification Lab
Let’s use Texture and Composition to identify igneous rocks
Steps for identifying igneous rocks
Identify the texture (phaeritic/aphanitic)
If the rock is phaneritic, estimate the % of felsic and mafic minerals
High mafic = dark colored rock
High felsic = light colored rock
50% felsic, 50% mafic = intermediate color
Use your igneous rock schematic and cross reference the texture and composition (light, intermediate, dark) and name the rock. Add modifying terms (porphyritic, vesicular ect…….)