Laboratory Manual for Introductory Geology 4e

7.2.2 Changes in Mineralogy
The original protolith minerals may not be able to survive the new temperature and pressure
conditions and may therefore change in the solid state to new metamorphic minerals
that are more stable under those new conditions. Such changes may occur in two ways:
1. Phase changes: A phase change is a change in a mineral from one polymorph to
another that is more stable under the new conditions. Polymorphs are minerals
that have the same chemical composition but different internal atomic structures,
so a phase change causes a change in atomic structure, but not in chemical
composition. A common example is the conversion of andalusite (Al 2
SiO 5
)
to sillimanite (Al 2
SiO 5
) with increasing heat and pressure.
2. Chemical reactions between minerals: Ions from two or more minerals may be
recombined to form new metamorphic minerals that are more stable than
the original assemblage. An example is the conversion of muscovite and quartz
to sillimanite, potassium feldspar, and water:
KAl 2
(AlSi 3
O 10
)(OH) 2
1 SiO 2 → Al 2
SiO 5
1 KAlSi 3
O 8
1 H 2
O
(muscovite) (quartz) (sillimanite) (potassic (water)
feldspar)
7.2.3 Changes in Composition
Less commonly, hot fluids circulating through rocks during metamorphism may
change the chemical composition of the protolith by adding or removing large
amounts of ions. In extreme cases, this composition change is so great that metamorphic
minerals crystallize that could not have formed in the protolith. For example,
if a large number of calcium ions are added to a shale, calcareous metamorphic
minerals can form that could not have formed in the original clay-based protolith.
Sometimes a rock changes only slightly during metamorphism, so that most of its
original sedimentary, igneous, or prior metamorphic characteristics are still recognizable.
In many cases, however, the changes are so drastic that a metamorphic rock can
look as different from its protolith as a butterfly does from a caterpillar (FIG. 7.1). If a
rock changes only a little during metamorphism, we say it has undergone low-grade metamorphism;
if it has changed drastically, it has experienced high-grade metamorphism. We
will see how to determine the metamorphic grade of a hand specimen in section 7.5.3.
7.3 Agents of Metamorphism
Geologists refer to the heat, pressure, stress, and hydrothermal fluids that cause metamorphic
changes as agents of metamorphism. Each of these agents acting alone
can cause metamorphism, but in most geologic settings two or more act simultaneously.
In this section, we will examine how individual agents of metamorphism cause
change and their effects on protoliths.
7.3.1 The Effect of Heat
When rocks are heated, either by an intrusive magma or during burial by sedimentation,
mountain building, or subduction, their ions vibrate more rapidly. The chemical
bonds holding those ions in mineral structures stretch, and some begin to break.
The freed ions migrate slowly through the solid rock by a process called diffusion,
much as atoms of food coloring spread when dripped into a glass of water. Eventually,
the wandering ions bond with other ions to produce metamorphic minerals that are
stable under the higher temperatures. Heat is thus the major cause of phase changes
and chemical reactions that replace protolith minerals with metamorphic minerals.
7.3 AGENTS OF METAMORPHISM
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