Name __________________________
Date ___________
PS/ES Per ___
Mr. Landsman
Classifying Common Metamorphic Rocks
In the classification of rocks, a geologist usually uses what is called a field
classification. Field classification represents a broad framework into which the majority
of rocks can be placed (the name of the thing covers a wide range of actual things). The
framework is primarily based on the texture and mineral composition of the rock.
Metamorphic rocks are “changed rocks” (metamorphosed). They were once
another type of rock but have had their textures and composition changed by heat, pressure,
and/or chemical reactions. They may still possess some of the characteristics of the rocks
from which they were made. As a result, it is easy to make a mistake in the rock’s
classification.
Texture:
The differences in the orientation, or alignment, of the crystals and the size of the
crystals determine the texture of a metamorphic rock. There are two general texture
groups, Foliated, and Non-Foliated.
Foliated textures are those in which platy or leaf-like minerals such as mica are
nearly aligned parallel to one another. As a result, a rock may split along the nearly
parallel cleavages of the aligned minerals (Ex: mica schist).
Non-foliated textures are composed of either randomly arranged (not aligned) platy
minerals, or simply altered grains of the same minerals as the original rocks. Non-foliated
rocks are often referred to as massive (Ex: marble, quartzite).
There are four basic textures found in metamorphic rocks.
Description
This is a coarsely foliated texture in which the minerals have
been segregated into discontinuous bands, each of which is
dominated by one or two minerals. These bands range in
thickness from 1 mm to several centimeters. The individual
mineral grains are macroscopic and impart a striped appearance
to a hand specimen. Light-colored bands commonly contain
quartz and feldspar, and the dark bands are commonly
composed of hornblende and biotite. A rock with a gneissic
texture is called a gneiss.
This is a foliated texture resulting from the sub-parallel to
parallel orientation of platy minerals such as micas. Other
common minerals present are quartz and amphiboles. A
schistose texture lies between the parallel platy appearance of
phyllite and the distinct banding of gneissic texture. The average
grain size of the minerals is generally smaller than in a gneiss. A
rock with schistose texture is called a schist
This texture is caused by the parallel orientation of microscopic
grains. The name for the rock with this texture is slate , and the
rock is characterized by a tendency to separate along parallel
planes. This feature is a property known as slaty cleavage. (Slaty
cleavage or rock cleavage is not to be confused with cleavage in
a mineral, which is related to the internal atomic structure of the
mineral.)
Non-foliated
Texture
Gneissic
Example Rock Names
Gneiss
Schistose
Schist
Slaty
Slate
Massive
Marble, Quartzite, Hornfels
Using the text above and your ESRT, complete the questions below.
1. The classification of rocks is primarily based on _______________________ and
_____________________________ ____________________________.
2.
Name the four basic textures of Metamorphic Rocks.
a. ____________________________________
b. ____________________________________
c. ____________________________________
d. ____________________________________
3. Name the eight common classifications of Metamorphic Rocks.
a. _____________________________________
b. _____________________________________
c. _____________________________________
d. _____________________________________
e. _____________________________________
f. _____________________________________
g. _____________________________________
h. _____________________________________
4. Name the rocks that may result from metamorphism of the following “original”
rocks.
a. Limestone _________________________
b. Sandstone _________________________
c. Granite ___________________________
d. Shale (see notes)
_____________________________________________________________
_____________________________________________________________
5. For each description, indicate the type of metamorphic rock.
a. Fine to coarse-grained, positive acid reaction
________________________________________________________
b. Very fine-grained well-foliated, looks like compressed shale
________________________________________________________
c. Coarse-grained with alternating bands of light and dark minerals
________________________________________________________
d. Very hard, sugary-textured, massive, non-foliated, no acid reaction
________________________________________________________
BONUS: What is the correct vocabulary for: a coarsely foliated rock that shows banding
and the minerals within the rock break along planes of weak atomic bonds.
__________________________________________________