Download Rock Identification by Type

The Scheme for
Igneous Rock Identification
A Brief Tour
Crystal
size
Grain Size
Description
Igneous rocks have
“Intergrown Crystals”
Intergrown
Intergrown
Intergrown
Not Intergrown
Outside
theEarth:
volcano:
Extrusive
Inside the
Intrusive
Where it was formed
IGNEOUS ROCKS
Slow cooling deep
Rapid cooling near or
beneath the Earth’s at the Earth’s
surface, produces
surface allows
many small crystals
crystals to grow to
that are not readily
large size (1/8” or
seen by the
more). This group
unaided eye. This
of igneous rocks as group of igneous
INTRUSIVE.
rocks is called
EXTRUSIVE.
Granite - intrusive
quartz
biotite
mica
(*)
feldspar
(*)
GRANITE is a coarse to
medium-grained rock
that forms from the
cooling of magma deep
within the Earth
(intrusive). It is made up
mainly of varying
amounts of the minerals:
quartz, orthoclase,
muscovite, biotite and
hornblende. The name
is from the Latin granum,
for “grains”.
Granite - intrusive
Graphic Granite
mica
Porphyritic
feldspar
Pegmatite
Granite - intrusive Pegmatite a Special Case
tourmaline
feldspar
PEGMATITES are
classified as intrusive igneous
rocks, but there is a
difference. They are VERY
coarse grained and strictly
speaking are not crystallizing
out of a magma.
The coarse grained nature is
the result of crystal growth in
aqueous solutions rather than
in the molten liquid state.
mica
quartz
The resulting freedom of ion
motion allows the crystal to
grow much larger in a shorter
length of time. (*)
Diorite - intrusive
DIORITE is very similar
to granite, but is
distinguished in the
hand specimen by the
absence of visible
quartz.
feldspar
Generally it has a salt
and pepper
biotite (*) appearance (about ½
black and ½ white).
Gabbro - intrusive
Black minerals are primarily amphibole (like
hornblende) and plagioclase feldspar.
(*)
GABBRO is a coarsegrained rock that is high
is iron & magnesiumbearing minerals
(pyroxenes, amphiboles,
plagioclase feldspar,
olivene). The rocks will
be dark in color,
somewhat heavier than
granitic rocks and devoid
of quartz.
Peridotite - intrusive
PERIDOTITE or DUNITE
is composed of 90100% olivine. As a
result it is
characteristically olivegreens in color.
This material is thought
to have originated in the
upper mantle of the
Rhyolite – extrusive
This is another sample of rhyolite.
This has a crystal of garnet that
grew after the rhyolite was
This is an example of
deposited. (*)
banded rhyolite. (*)
If you look closely, you might see tiny clear phenocrysts of
feldspar. (*)
Andesite - extrusive
ANDESITE is the finegrained equivalent
of DIORITE. It
tends to be a darker
gray than rhyolite
hornblende
and is often
phenocrysts (*)
porphyritic, with
visible hornblende.
Basalt - extrusive
Vesicular (Scoria)
Volcanic Bomb (*)
Gases released near the surface
of a lava flow create bubbles or
vesicles that are “frozen” in
stone. (*)
Amygdaloidal
(*)
Bubbles?
Yes= Vesicular
Bubbles?
No=Non-vesicular
Very Light
Color
Neither Light nor Dark
NotVery Light Not Very Dark Very Dark
Light
Dark
Darkest
Lightest
Very Light
Density
Medium Density
Light
Dense Very Dense
Finding The Minerals
Identify the rock.
Unless you have other information,
work in the middle of the rock’s box.
This is the amount of Potassium Feldspar in the rock.
This is the amount of Quartz in the rock.
This is the amount of Plagioclase Feldspar in the rock.
Use tick marks on a scrap paper to measure the percentage.
Potassium Feldspar 25%
Quartz 40%
Practice
Name a light-colored, fine-grained
rock with no bubbles.
Name a coarse-grained, dense rock.
Name a very light-colored, glassy,
extrusive rock with bubbles.
Sedimentary Rocks




Made from sediments or rock
material that has been broken
down in some way.
Sedimentary rocks are usually
formed in a watery environment.
Often layered
Are the only rocks that contain
fossils
Types of Sedimentary Rocks


Clastic (fragmental) –made by
compaction and cementation of
sediments.
Clastic rocks are identified by
the size of the fragments.
Mixed
Conglomerate has
rounded fragments
Breccia has angular
fragments
Chemically formed rocks:
evaporites


Formed when water carrying
minerals evaporates and leaves
the minerals behind.
Are identified by the minerals
present (ie halite hardness of
2.5)

Bioclasticbio= life
 clastic= fragments


Made from accumulated shells
(limestone) or plants (coal)
Metamorphic Rocks
 Metamorphic
rocks have
been changed from a
pre-existing rock.
 Caused by extreme heat
and/or pressure.
Regional Metamorphism
 Caused
by extreme
pressure and heat.
 Happens over wide
“regions”.
Contact Metamorphism
 Caused
by contact with
extreme heat.
 Meta.
rocks will often
have foliation or a
distorted structure.
Distorted Structure

Foliation is a “grain” to the
rock.
Types of Foliation:

Gneissic Foliation- minerals
are squished into bands.
“Zebra Stripes”
Banding vs. Layers
Bands
Layers

Schistose foliation
minerals have been squashed
into flakes of mica.

Slaty foliation
Rock splits into flat, thin
layers.
Slate
+
+
=
Phyllite
Phyllite
+
+
=
Schist
Schist +
+
=
Gneiss