Download Metamorphic Rocks Examples - Uplift Community High School

Metamorphic Rocks Examples
Gneiss
• .
Description
Banded foliation composed of
layers/zones of light colored quartz,
feldspar interlayered with layers/zones of
dark colored biotite and amphibole. The
minerals bands in this specimen are just
beginning to develop, but are distinctly
separate from each other putting it in the
gneissic category
Tectonic Association
Gneisses are typically associated with major mountain building
events when shales or clay rich sandstones (wackes), or felsic igneous rocks
(e.g. granite, granodiorite, etc.) are
metamorphosed through depth of burial, and proximity to
batholiths
Hornfels
Composition
Hornfels can have any of a wide
diversity
of microscopic minerals, all too
small to see
or identify without thinsection and a
specialized microscope.
Description
Hornfels is a nondescript rock which
varies widely in appearance. This
specimen
is typical, a dark gray to black,dense,
hard,
compact, massive rock breaking with
conchoidal
fracture into sharp angular pieces. It is
harder than
glass and has a dull luster.
Be cautions; variations on this,
especially color, are common –
as well as confusion with similar looking
rocks, such as basalt.
Hornfels Continued
• Tectonic Association
•
Hornfels results from the contact
metamorphism of shales or other clay rich
rocks in the viscinity of an igneous
intrusion. As a result they have no specific
tectonic association or meaning. They can
form just about anywhere.
Marble-Limestone
•
•
Description
Recrystallized limestone producing interlocking grains of calcite. This
is a saw cut specimen, wet with water to bring out the grains.
Marble is an extremely variable rock in appearance since it depends on
what limestone was the parent, and the degree of metamorphism it
underwent. It can be virtually any color from white to black to red to green,
etc., and sometimes it will contain metamorphosed fossils or other
components. The most definitive test for the rock would be acid (vigorous
bubbling), and a hardness less than glass.
Marble-Limestone
• Type of Metamorphism
•
None specific. Limestone will convert into marble with virtually
any type of metamorphism. The main exception is hydrothermal
metamorphism which introduces silica rich fluids, in which case the
rock turns into calcium rich silicate skarn minerals.
• Composition
• Calcite: CaCO3
Quartzite
Composition
Fused quartz sand grains.
Typically white or gray, although
other pale colors are common;
rarely a darker color.
Description
This is a typical quartzite.
They are dense, hard rocks,
generally uniform in texture,
composed of fused quartz sand grains.
Often they rock has a granular
appearance
on a weathered surface, but along a
broken
surface the sand grains are usually split.
Also, unlike most of the nondescript rocks,
quartzite has a glassy luster
Quartzite Continued
Tectonic Association
Quartzite results from the almost
any metamorphism of a quartz sandstone,
and as such have no specific tectonic
association
or meaning. They can form just about
anywhere.
Schist
• Composition
•
NORMAL
Quartz
Feldspar
Biotite ACCESSORY
In red if present
Garnet
Staurolite
Schists have intermixed quartz, feldspar, and mica (biotite and/or
muscovite), and often amphibole. This specimen is dominated by biotite,
quartz, and feldspar. Often other minerals are present too, such as garnet
and staurolite, but none are present in this particular specimen.
• Tectonic Association
•
Schists are typically associated with major mountain building events
when shales or clay rich sandstones (wackes) are metamorphosed through
depth of burial, and proximity to batholiths
Schist Continued
•
•
Description
Schistose foliation composed of intermixed quartz, feldspar, and biotite (with some
possible amphibole). The minerals in this specimen are well intermixed, characteristic
of the schistose pattern, although when looking at the enlarged specimen it begins to
look more like gneissic banding. It is a matter of scale though; if you have to get this
close to see the minerals separating into bands you are still in the schist realm.
Schists are hard to photography and capture their details; all the minerals tend to
be mashed together. The details are even hard to see when the specimen is in front
of you. However, go to a very large picture, and you can see the intermixing of the
minerals better.
Slate
Composition
NORMAL
Chlorite ACCESSORY
In red if present
Muscovite
Individual crystals are too small to
be seen by eye. Color typically
green (color of chlorite) but ranges
gray to black (organic matter), to
green,
red, purple, brown (iron oxides).
At higher grades of metamorphism
organic matter and iron oxides are
recrystallized and rock becomes color
of minerals.
Slate Continued
• Description
•
Slatey cleavage foliation composed of the lining up of the basal
cleavages in the new chlorite and/or muscovite minerals. The result is slatey
cleavage, the breaking of the rock into sheets along smooth, flat faces - as
seen in this specimen.
A distinct, weak sheen (light reflectance) is generally visible on cleavage
faces, as seen here. The sheen shows up best when the rock is rotated in
the light.
The rock will often " ring " like a bell when tapped on a hard surface,
another feature not found in the shale parent, or the next rock in the
metamorphic sequence, phyllite
Tectonic Association
Slates are typically associated with major mountain
building events
when shales or clay rich sandstones (wackes)
undergo low grade (Barrovian greenschist facies) metamorphism