Roger Weller, geology instructor
|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
Metamorphic Rock Photos
visual examples of each of these
metamorphic rock types.
Foliated (Layered) Metamorphic Rocks
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.
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.
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.
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
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.
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.
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.
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.