Download common rock-forming minerals

GEOLOGY 12
CHAPTER 2 NOTES
MINERALS AND ROCKS
Name __________________
COMMON ROCK-FORMING MINERALS
quartz**
K-feldspars**
Ca/Na feldspars**
pyroxenes**
olivines**
epidotes**
calcite
dolomite
clay minerals**
gypsum
anhydrite
halite
biotite**
muscovite**
amphiboles**
chlorites
garnets**
magnetite
hematite
limonite
pyrite
pyrrhotlte
chromite
Of the more than 3,000 known minerals, less than 30 make up the bulk of the Earth's
crust and mantle (>90%). These, along with an additional 20 less common minerals,
serve as the basis for naming most of the rocks exposed on the Earth's surface. Nearly
all common rocks can be named by determining the presence of one, two, or a few of
the minerals listed above.
Each of the common rock-forming minerals can be identified on the basis of its chemical
composition and its crystal structure (i.e. the arrangement of its atoms and ions).
However, fairly expensive equipment and sophisticated procedures are required for
such determinations. Therefore, it is fortunate that macroscopic examination, along
with one or more tests, is usually sufficient to identify these minerals as they occur in
most rocks.
MINERAL CLASSIFICATION BY COMPOSITION
Because the chemical elements Si (silicon) and O (oxygen) make up 95% by volume
(75% by weight) of the earth's crust, the most common minerals found there are
silicates.** When one atom of Si and four atoms of O combine in nature, the resulting
configuration forms a tetrahedron. Within the mineral, these silicate tetrahedra link in
various degrees to form a variety of silicate structure types. Most of the rock-forming
minerals are classified as silicates.
Silicate Structure Types (label diagram on next page)
Arrangement of Tetrahedrons
1A
1B
1C
1D
1E
1F
1G
1H
individual oxygen-silicon tetrahedron
SiO4 schematic
double tetrahedron
ring tetrahedron
single chain
double chain
sheet silicate
framework silicate
Example(s)
1A
1B
1C
1D
1E
1F
1G
1H
olivine, garnet, topaz
oivine
epidote
tourmaline
pyroxene / augite
amphibole / hornblende
biotite, muscovite, talc, clay
feldspars, quartz
THE FELDSPAR GROUP
The feldspar group is a fairly large group with nearly 20 members, but only nine are well
known and common. Those few, however, make up the greatest percentage of
minerals found in the Earth's crust. Often, feldspars are simply referred to as
plagioclase and orthoclase (a K-feldspar) because identification to greater precision is
difficult with ordinary methods. The following are some of the more common feldspar
minerals:
The Plagioclase Feldspars:
The K-Feldspars or Alkali Feldspars:
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• Microcline (potassium aluminum silicate)
• Sanidine (potassium sodium aluminum silicate)
• Orthoclase (potassium aluminum silicate)
Albite (sodium aluminum silicate)
Oligoclase (sodium calcium aluminum silicate)
Andesine (sodium calcium aluminum silicate)
Labradorite (calcium sodium aluminum silicate)
Bytownite (calcium sodium aluminum silicate)
Anorthite (calcium aluminum silicate)
OTHER COMPOSITIONAL GROUPS
NATIVE ELEMENTS → gold, silver, sulfur, copper, carbon (diamond/graphite)
SULFIDES (__S) → pyrite, galena, sphalerite, chalcopyrite
OXIDES (__O) → hematite, magnetite
HALIDES (__Cl/F) → halite, fluorite
CARBONATES (__CO3) → calcite, dolomite
PHOSPHATES (__PO4) → apatite
SULFATES (__SO4) → barite, gypsum
Two important rock-forming materials that are not minerals are major components of a
few rocks. These are glass and macerals. Glass forms when magma (molten rock
material) is cooled so rapidly that the constituent atoms do not have time to arrange
themselves into crystals. Natural glass is the major constituent of a few volcanic rockse.g., obsidian. Macerals are macerated bits of organic matter, primarily plant materials.
One or more of the macerals are the chief original constituents of all the diverse coals
and several other organic-rich rocks such as oil shales.