8 Common Metamorphic Rocks
Cochise College
Virtual Geology
Museum
Roger Weller, geology instructor
wellerr@cochise.edu
copyright
2007-R.Weller
| 1. slate | 2. phyllite | 3. schist | 4. gneiss |
| 5. marble | 6. quartzite | 7. serpentine | 8. hornfels |
Metamorphic rocks fall into two major categories:
foliated
metamorphic
sedimentary rocks non-foliated metamorphic rocks
Refer to
Metamorphic Rock Photos
for
more
visual examples of each of these
metamorphic rock types.
Foliated (Layered) Metamorphic Rocks
1.
slate
Slate is
metamorphosed shale. Slate is tougher than shale and it breaks
into thin,
flat layers. Slate is usually
dark
gray, but it can also be
red
colored. Slate has been
used in some countries
as roofing and more recently, it has been utilized as
floor tiles.
2.
phyllite
Phyllite is metamorphosed
slate.It is still foliated (layered), but unlike slate, the layers
are not
completely flat but have a slightly
undulating pattern.
Phyllite also has a slightly
silky appearance due to the growth of tiny mica plates oriented parallel to the foliation.
3.
schist
Schist is metamorphosed phyllite. The mica crystals in schists are larger than those
in phyllites
and so schists tend to
distinctly sparkle. Mica schists often also
contain
garnet crystals or
staurolite crystals, producing a bumpy surface.
Schists might also be
made of
talc,
kyanite,
pyrophyllite,
chlorite, or
sillimanite.
4.
gneiss
Gneiss is metamorphosed schist. It is a highly
metamorphosed that is almost a granite.
It differs from schist due to the
lenses
of feldspar between the
mica layers.
The minerals
in
gneiss may occur either as layers (foliation)
or elongated in one direction (lineation).
Intensely
crumpled layers are another means of identifying gneiss.
Non-Foliated Metamorphic Rocks
5. marble
Marble is metamorphosed limestone. In the process of being
metamorphosed, the
limestone is
recrystallized, creating a change in color and texture and the destruction
of included fossils.
There are hundreds of recognized commercial marble with a wide
range of
colors and patterns.
However, since marble is calcite, it still bubbles vigorously
when strong
hydrochloric acid is applied
to it.
The
gray
streaks in typical marble are
made up of graphite.
Marble can
range from
white
to
pink to
brown, and even
black.
Limestones that have broken by tectonic forces as they are being
metamorphosed
produce
brecciated marbles. Cut and polished marble is used for statues and
flooring.
6. quartzite
Quartzite is
metamorphosed sandstone. It is often difficult to distinguish it from
a
sandstone
that has been cemented by quartz. The sand grains in
quartzite are so
tightly cemented together than
when a rock of quartzite is broken in half. the break
actually
cuts the individual sand grains.
7. serpentine
The name serpentine is used for both a mineral and a
metamorphic rock.
It is formed by the
metamorphic transformation of olivine and pyroxene to the
serpentine mineral
group.
Serpentine
varies from a
light green to a
dark green
color with
veins and fractures. It strongly resembles
some varieties of jade.
It is used for flooring and
tabletops.
8.
hornfels
Hornfels is a non-foliated, baked rock that
is formed by contact metamorphism.
The color, grain size,
and mineral composition shows wide variation.
Colors can range from a
light gray to a dark black. The
darkest colored varieties
of hornfels
may have have originally been dark shales, siltstones, or
even basalt.