Laboratory Manual for Introductory Geology 4e

TABLE 7.1 Metamorphic rock classification
Texture
*Dominant
minerals
Grain
size Rock name Grade Comments
Very fine Slate Low Breaks along smooth, flat surfaces (slaty
cleavage)
Aligned minerals
Micas, chlorite
Fine Phyllite Low Breaks along wavy, shimmering surfaces;
micas barely visible
Foliated
Layered and
aligned
†
Alternating
light and dark
layers
Coarse *Schist High Strong alignment of visible mica flakes;
often with garnet and other minerals
Medium
to coarse
Gneiss High Light and dark minerals separated into
layers; typically one of these is foliated
and the other nonfoliated
Aligned
deformed grains
Quartz, rock
fragments
Quartz, rock
fragments
Coarse Metaconglomerate Low to
high
Fine to
very fine
Foliation defined by elongate, flattened
and stretched clasts
Mylonite High Grains “smeared” in fault zones by
dynamic metamorphism
*Calcite,
dolomite
Medium
to coarse
*Marble
Medium
to high
Recrystallized grains interlock and get
larger with increasing metamorphic grade
Nonfoliated
Interlocking
equant
grains with
no preferred
orientation
Quartz
Quartz,
feldspars
*Quartz,
feldspars,
micas
Fine to
medium
Fine to
medium
Fine to
medium
Quartzite
Metasandstone
Hornfels
Medium
to high
Medium
to high
Medium
to high
Recrystallized grains interlock and get
larger with increasing metamorphic grade
Matrix clay minerals typically produce
interstitial micas
Forms at contacts with igneous rocks;
may contain coarse index minerals
Graphite
Nearly
glassy
Anthracite coal
Medium
to high
Shiny black surface; resembles obsidian
*Additional metamorphic index minerals may also be present, such as garnet, staurolite, sillimanite, kyanite, andalusite, and actinolite.
†
Mineralogy of gneiss depends on its chemical composition.
Slate: A low-grade metamorphic rock formed from shale or mudstone, composed
mostly of clay minerals. Individual grains are too small to be seen with a hand lens.
Foliation is revealed by the tendency of slate to break along smooth, closely spaced
planes (a pattern called slaty cleavage, which makes the rock useful for roofing and
paving walkways). Slates are commonly gray, black, green, or red.
Phyllite: A rock that forms from shale or mudstone under higher-grade metamorphic
conditions than slate as clay minerals react to form fine-grained flakes of mica and
the platy mineral chlorite. Individual mica or chlorite flakes may be barely visible
with a hand lens, but their foliation typically produces a silky sheen regardless of
their size (see Fig. 7.12a).
Schist: A medium- to coarse-grained rock that forms from phyllite with increasing
metamorphic grade, characterized by well-developed foliation and mica flakes
that are easily seen with the naked eye (see Fig. 7.1c.i). The most common mineral
components of schist are muscovite and biotite; temperatures at this grade
are typically too high for chlorite to survive. Additional minerals such as garnet,
kyanite, or sillimanite may form, depending on protolith composition and temperature
(see the discussion of metamorphic index minerals in section 7.5.3 and
Exercise 7.5).
Gneiss: A coarse-grained high-grade metamorphic rock in which light- and dark-colored
minerals have been segregated by diffusion into parallel bands. Either light or dark
bands are typically foliated. Coarse-grained metamorphic index minerals—most
7.4 STUDYING METAMORPHIC ROCKS
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