Page 219 - Laboratory Manual in Physical Geology

■ Schistosity — a scaly glittery layering of visible ■ Crystalline texture (nonfoliated) —a medium- to
(medium- to coarse-grained) platy minerals (mainly coarse-grained aggregate of intergrown, usually
micas and chlorite) and/or linear alignment of long equal-sized (equigranular), visible crystals. Marble is a
prismatic crystals (tourmaline, hornblende, kyanite). nonfoliated metamorphic rock that typically exhibits
Rocks with schistosity break along scaly, glittery folia- an equigranular crystalline texture ( FIGURE 7.11 ).

tions and are called schist ( FIGURE 7.9 ). Schists are a prod-

uct of intermediate-to-high grades of metamorphism. ■ Microcrystalline texture —a fine-grained aggregate of
intergrown microscopic crystals (as in a sugar cube).
■ Gneissic banding — alternating layers or lenses of light Hornfels ( FIGURE 7.12 ) is a nonfoliated metamorphic

and dark medium- to coarse-grained minerals . Rock with rock that has a microcrystalline texture.
gneissic banding is called gneiss ( FIGURES 7.4 and 7.10 ). ■ Sandy texture —a medium- to coarse-grained
Ferromagnesian minerals usually form the dark bands. aggregate of fused, sand-sized grains that resembles
Quartz or feldspars usually form the light bands. Most sandstone. Quartzite is a nonfoliated metamorphic
gneisses form by high-grade metamorphism (including rock with a sandy texture ( FIGURE 7.13 ) remaining

recrystallization) of clay- or mica-rich rocks such as from its sandstone parent rock, but the sand grains

shale (see FIGURE 7.1 ), but they can also form by meta- cannot be rubbed free of the rock because they are

morphism of igneous rocks such as granite and diorite.
fused together.
■ Glassy texture —a homogeneous texture with no
Nonfoliated Metamorphic Rocks visible grains or other structures and breaks along
Nonfoliated metamorphic rocks have no obvious layering glossy surfaces; said of materials that resemble glass,

(i.e., no foliations), although they may exhibit stretched fossils such as anthracite coal ( FIGURE 7.14 ).
or long, prismatic crystals (tourmaline, amphibole) that have
grown parallel to the pressure field. Nonfoliated metamorphic Besides the main features that distinguish foliated and
rocks are mainly characterized by the following textures: nonfoliated metamorphic rocks, there are some features
that can occur in any metamorphic rock. They include the
following:
Visible well-formed
Visible well-formedisible well-formed
muscovite crystals
muscovite
muscovite crystals ■ Stretched or sheared grains —deformed pebbles,
crystals
fossils, or mineral crystals that have been stretched
out, shortened, or sheared.

Biotite

Quartz

weak
of
Edges of weak slaty
slaty
Edges
Edges of weak slaty
surfaces
cleavage surfaces
cleavage
cleavage surfaces
surfaces,
1
op
Top view of foliation surfaces, 1x
Top view of foliation surfaces, 1x
view
foliation
of
FIGURE 7.9 Schist. Schist is a medium- to coarse-grained, scaly
(like fish scales), foliated metamorphic rock formed by intermediate- 1 Edges of foliation surfaces
grade metamorphism of mudstone, shale, slate, phyllite, or other
rocks rich in clay, chlorite, or mica. Schist forms when clay, chlorite, FIGURE 7.10 Gneiss. Gneiss is a medium- to coarse-grained
and mica mineral crystals are foliated as they recrystallize to larger, metamorphic rock with gneissic banding (alternating layers or
more visible crystals of chlorite, muscovite, or biotite. This gives lenses of light and dark minerals). Generally, light-colored layers
schist its scaly foliated appearance called schistosity . Slaty cleavage or are rich in quartz or feldspars and alternate with dark in biotite
crenulations (sets of tiny folds) may be present, but schist breaks along mica, hornblende, or tourmaline. Most gneisses form by high-grade
its scaly, glittery schistosity. It often contains porphyroblasts of garnet, metamorphism (including recrystallization) of clay or mica-rich
kyanite, sillimanite, or tourmaline mineral crystals. The schist grade rocks such as shale ( FIGURE 7.1 ), mudstone, slate, phyllite, or schist.

of metamorphism is intermediate between the lower grade that However, they can also form by metamorphism of igneous rocks
produces phyllite ( FIGURE 7.8 ) and the higher grade that produces such as granite and diorite. The compositional name of the rock in

gneiss ( FIGURES 7.10 ). Also see chlorite schist in FIGURE 7.15 . this picture is biotite quartz gneiss.

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