Metamorphism and Metamorphic Rocks - The Changing Earth: Exploring Geology and Evolution - page 175

TABLE 7.1 Classifi cation of Common Metamophic Rocks

Metamorphic
Rock

Metamorphic
Grade

Characteristics
of Rocks Texture Typical Mineral Parent Rock

Foliated Slate Clays, micas, chlorite Low Fine-grained, splits

easily into fl at pieces

Mudrocks, volcanic
ash

Phyllite Fine-grained quartz,

micas, chlorite

Low to medium Fine-grained, glossy or

lustrous sheen

Mudrocks

Schist Micas, chlorite, quartz,

talc, hornblende,
garnet, staurolite,
graphite

Low to high Distinct foliation,

minerals visible

Mudrocks,
carbonates, mafi c
igneous rocks

Gneiss Quartz, feldspars,

hornblende, micas

High Segregated light and

dark bands visible

Mudrocks,
sandstones, felsic
igneous rocks

Amphibolite Hornblende,

plagioclase

Medium to
high

Dark, weakly foliated Mafi c igneous rocks

Migmatite Quartz, feldspars,

hornblende, micas

High Streaks or lenses of

granite intermixed
with gneiss

Felsic igneous
rocks mixed with
sedimentary rocks

Nonfoliated Marble Calcite, dolomite Low to high Interlocking grains of

calcite or dolomite,
reacts with HCl

Limestone or
dolostone

Quartzite Quartz Medium to

high

Interlocking quartz
grains, hard, dense

Quartz sandstone

Greenstone Chlorite, epidote,

hornblende

Low to high Fine-grained, green Mafi c igneous rocks

Hornfels Micas, garnets,

andalusite, cordierite,
quartz

Low to medium Fine-grained,

equidimensional
grains, hard, dense

Mudrocks

Anthracite Carbon High Black, lustrous,

subconcoidal fracture

Coal

metamorphic rocks, such as slate, have a fi nely foliated tex-
ture in which the mineral grains are so small that they cannot
be distinguished without the aid of magnifi cation. High-grade
foliated rocks, such as gneiss, are coarse-grained, such that the
individual grains can easily be seen with the unaided eye. Foli-
ated metamorphic rocks can be arranged in order of increas-
ingly coarse grain size and perfection of foliation.
Slate is the lowest-grade foliated metamorphic rock and
commonly exhibits slaty cleavage (

rock ( ◗ Figure 7.12). Metamorphism of many rock types can

yield schist, but most schist appears to have formed from
clay-rich sedimentary rocks (Table 7.1).
All schists contain more than 50% platy and elongated
minerals, all of which are large enough to be clearly visible.
Their mineral composition imparts a schistosity or schistose
foliation to the rock that usually produces a wavy type of part-
ing when split. Schistosity is common in low- to high-grade
metamorphic environments, and each type of schist is known
by its most conspicuous mineral or minerals, such as mica
schist, chlorite schist, or garnet-mica schist (Figure 7.12).
Gneiss is a high-grade metamorphic rock that is streaked
or has segregated bands of light and dark minerals. Gneisses
consist of mostly granular minerals such as quartz, feldspar,
or both, with lesser percentages of platy or elongated miner-
als such as micas or amphiboles (

Figure 7.10). It results
from regional metamorphism of shale or, more rarely, vol-
canic ash. Because it can easily be split along cleavage planes
into fl at pieces, slate is an excellent rock for roofi ng and fl oor
tiles, billiard and pool table tops, and blackboards. The differ-
ent colors of slate are caused by minute amounts of graphite
(black), iron oxide (red and purple), and chlorite (green).
Phyllite is similar in composition to slate but is coarser
grained. The minerals, however, are still too small to be iden-
tified without magnification. Phyllite can be distinguished
from slate by its glossy or lustrous sheen (

◗

Figure 7.13). Quartz and
feldspar characteristically make up the light-colored bands of
minerals, whereas biotite and hornblende compose the dark
mineral bands. Gneiss typically breaks in an irregular man-
ner, much like coarsely crystalline nonfoliated rocks.
Most gneiss probably results from recrystallization of
clay-rich sedimentary rocks during regional metamorphism
(Table 7.1). Gneiss also can form from igneous rocks such as
granite or older metamorphic rocks.

◗

Figure 7.11), and
represents an intermediate grain size between slate and schist.
Further coarsening in grain size produces s chist —a meta-
morphic rock that is most commonly produced by regional
metamorphism. The type of schist formed depends on the
intensity of metamorphism and the character of the original

◗