8 Common Metamorphic Rocks
Cochise College    
Virtual Geology Museum          

Geology Home Page

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 sparkleMica 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
actuall
y 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 fracturesIt 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.