Introduction
Mineral crystallization occurs under specific conditions. Those processes
include growth from solutions, melts and vapors. Due to the environment and
chemical elements in the parent rock, specific minerals are formed.
Metamorphism is between a total melt of solution that form igneous rocks and the
deposition of sediment under a normal environment that forms sedimentary rocks.
In conditions of metamorphism, some minerals form that are not found in any
other environments on earth. Largely this is a function of phase changes
associated with heat and pressure exerted on the rock. This project will
look at the fundamental properties of metamorphism and the minerals that form as
its result.
Contents
Processes of Metamorphic Rocks
- Under metamorphic conditions, two changes in the minerals occur,
re-crystallization and chemical reactions. If a melt results,
then it is no longer metamorphic rock but an igneous rock. Thus, all
reactions must take place in a solid state. Small amounts of
fluid do interact with the metamorphosed rock, but this is primarily H2O
in the form of water, steam, and super critical fluid.
- Pressure and temperature are the primary factors that cause rocks to
morph, but dictate pressure (stress), shear, or chemically active solutions
can also lead to metamorphic rocks. These factors effect preexisting
sedimentary, igneous or other metamorphic rocks to change the mineral
composition, texture and/or structure.
- When re-crystallization and chemical reactions take place from
metamorphism, the bulk chemical composition of the rock may remain constant
or be changed due to circulating fluids. Isochemical, the most
common form, is when the chemical composition does not change.
Metasomatism, in which the chemical composition changes, is less common.
- Environments where we see metamorphic rocks are usually associated with mountains.
Two common kinds are Contact Metamorphism and Regional Metamorphism.
- Contact Metamorphism occurs in concentric zones, or aureoles, around hot
igneous intrusive bodies (Klein, 2002, p. 109). These zones tend to lack schistosity.
There is generally a large temperature gradient from the edge of the contact
with unchanged country rock. Rock zones appear due to the change in
temperature gradient.
- Regional Metamorphism is large scale from hundreds to thousands of
miles. This form is a function of temperature, pressure, or both. This form is
associated with either mountain building or burial of rocks.
- Some chemical components can be loss due to high heat or pressure in the
sense of dehydration or decarbonization. In dehydration, minerals can become
less hydrous due to the heat driving out H2O. An example of
dehydration is shale, a hydrous silicate clay, heated to schist.
Decarbonization is when CO2 gets baked out of a carbonate rich
rock. An example of decarbonization is when calcite (with a silicate chert) is
converted to wollastonite. Both of these losses are considered
metasomatic (Klein, 2002, p. 109).
Photo by
S.W. Salley, 2001
| The East Spanish Peak in Southern Colorado is a small example of contact
metamorphism. The aureole of the East Peak extends approximately two miles
from the zone of contact. |
Phase Changes
- Just like the change from gas to water to gas, minerals have phases they
go through. A phase is "a homogeneous substance with a well defined set
of physical and chemical properties" (Klein, 2002, p. 115). The word
phase can be substituted with mineral if the mineral is chemically
homogeneous.
- Phase regions are regions where a polymorph of a substance takes on
different physical properties.
- More phases can be present when the conditions allows it to be bordering
the phase line or the critical point.
- Polymorphs are two or more minerals with the same chemical
composition, but with different crystal structures such as graphite and
diamond.
- Since we know that some minerals are exclusive to metamorphic rocks,
polymorphs of minerals are only found if they result from heat and pressure.
Minerals
- Kyanite (Al2SiO5) is an example of a metamorphic
exclusive.
- Found in regional metamorphism of pelitic rocks, usually in schist.
- Occurs only in high pressure zones.
- The phase diagram for kyanite is below.
Figure taken from:
http://www.uoregon.edu/
~jrice/geol_311/stability.html.
| There are three polymorphs of Al2SiO5: andalusite,
kyanite and sillimanite. They are stable in all regions of pressure and
temperature. At normal atmospheric conditions, only kyanite is
thermodynamically stable, but all three persist at room temperature because of
kinetic laws.
|
- Graphite and Diamond are another common metamorphic polymorph.
- Diamond is not usually seen on the earths surface due to its instability
(relative to graphite).
- Diamond still persists due to kinetic factors.
- Graphite is common in metamorphic crystalline limestones, schists, and
gneisses as large crystalline plates.
Conclusion
Metamorphic rocks form from heat and pressure. Some of the minerals that make up those rocks also
change properties as heat and pressure is applied. Polymorphs of minerals at different phases result and thus some minerals are only found in metamorphic rocks.
Copyright 2002 © S.W. Salley. All rights reserved.
