Metamorphism and Metamorphic Rocks - The Changing Earth: Exploring Geology and Evolution - page 174

Unaltered Low grade

(200˚C)

Intermediate
grade

High grade
(800˚C)

Clay

Chlorite

Muscovite

Biotite

Garnet

Minerals

Staurolite

Kyanite Sillimanite

Feldspar

Quartz

Melting
Migmatite

Rocks Shale Slate Phyllite Schist Gneiss

◗ Figure 7.8 Metamorphic Grade Change in mineral assemblage and rock type with increasing
metamorphism of shale. When a clay-rich rock such as shale is subjected to increasing metamorphism,
new minerals form, as shown by the colored bars. The progressive appearance of certain minerals, known
as index minerals, allows geologists to recognize low-, intermediate-, and high-grade metamorphism.

different metamorphic grades are not sharp, the distinc-
tion is nonetheless useful for communicating in a general
way the degree to which rocks have been metamorphosed.
The presence of index minerals thus helps determine meta-
morphic grade. For example, when a clay-rich rock such
as shale undergoes regional metamorphism, the mineral
chlorite first begins to crystallize under relatively low tem-
peratures of about 200°C. Its presence in these rocks thus
indicates low-grade metamorphism. If temperatures and pres-
sures continue to increase, new minerals crystallize and replace
chlorite because they are more stable under those changing
conditions. Thus, there is a progression in the appearance of
new minerals from chlorite, whose presence indicates low-
grade metamorphism, to biotite and garnet, which are good
index minerals for intermediate grade metamorphism, to sil-
limanite, whose presence indicates high-grade metamorphism
and temperatures exceeding 500°C (Figure 7.8).
Different rock compositions develop different sets of
index minerals. For example, clay-rich rocks such as shale
will develop the index minerals shown in Figure 7.8. On the
other hand, a sandy dolomite will produce a different set of
index minerals as metamorphism progresses because it has a
different mineral composition than shale. Thus, a particular
set of index minerals will form based on the original
composition of the parent rock undergoing metamorphism.

Although such common minerals as mica, quartz, and
feldspar can occur in both igneous and metamorphic rocks,
other minerals such as andalusite, sillimanite, and kyanite
generally occur only in metamorphic rocks derived from
clay-rich rocks such as shale. Whereas these three minerals
all have the same chemical formula (Al 2 SiO 5 ), they differ in
crystal structure and other physical properties because each
forms under a different range of pressures and temperatures,
and is thus refl ective of a different metamorphic grade. For
this reason, they are useful index minerals for metamorphic
rocks that formed from clay-rich rocks.

HOW ARE METAMORPHIC ROCKS
CLASSIFIED?
For purposes of classifi cation, metamorphic rocks are com-
monly divided into two groups: those exhibiting a foliated
texture (from the Latin folium , “leaf ”) and those with a
nonfoliated texture (Table 7.1).

Foliated Metamorphic Rocks
Rocks subjected to heat and differential pressure during meta-
morphism typically have minerals arranged in a parallel fash-
ion, giving them a foliated texture (

◗ Figure 7.9). Low-grade