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Crystalllographic Systems ~
Minerals usually form distinct crystals and their shape plays an important part in
their identification. The study of crystals is called crystallography and includes
the study of natural crystal but crystals formed by metal alloys, chemicals, and
other synthetic materials. Specific tools, such as an x-ray spectrometer, are used
to find and distinguish new minerals and verify or correct the identification of
specimens. How does this help an ordinary rockhound to identify minerals? The
arrangement of component atoms and/or ions is responsible for the
outward shape of the crystal. Rarely does a single mineral form crystals that are
completely unique to itself. Generally minerals form crystals that are consistent
with the symmetry class that the mineral falls into, based on its own structure.
This symmetry also affects a other properties including cleavage, luster,
hardness, and sometimes color. Understanding what symmetry class a mineral belongs
to is very helpful in identifying its crystals.
Symmetry Operations
The different symmetry operations help define the crystal's outward symmetry.
They represent the way a crystal can repeat the facets or faces on their crystal's
surface.
- Mirror Plane A mirror plane
reflects a face from one side of the crystal to the other. This means the
reflected face must be identical but reversed in orientation. In other words,
if the original face has any right handed characteristics, then the reflected
face must have the same characteristics but with a left handed slant to them.
- Axis of Rotation A rotational axis is
imaginary line drawn through the crystal that acts as an axis just like the axle
for a tire. A face can be repeated on a crystal when the crystal is rotated
around this axis and a new face is left at various intervals during the
rotation. The new faces must be identical to the original face in
orientation. If the face has a right handed slant and is rotated, the
rotated faces must keep the same right handed slant.
- Fold or Interval of Rotation The
interval for dropping a face is determined by a division of the full turn into
equal segments. For example, to drop four faces on a crystal the rotation
requires a stop at every 90 degrees and this type of rotation is called a four
fold rotational axis. Rotational axes can have rotations of 1, 2, 3, 4 and 6
fold. A 1 fold axis rotates the crystal in 360 degree intervals, the 2 fold
interval is 180 degrees, the 3 fold interval is 120 degrees, the 4 fold
interval is 90 degrees and the 6 fold interval is 60 degrees.
- Rotoinversion Axis After rotating once
and before dropping a face, it inverts the face through the crystal's center
to the other side. The resulting face is completely flipped, i.e., up is down
and right is left. The rotoinversion continues until it returns to the
original starting face. Rotoinversion is constrained by the same rules for the
simple rotational axes with the same folds or turns and degrees.
- Center Symmetry A center is an operation
that takes a face on one side of a crystal and inverts it through the center
of the crystal. This has the same effect as the inversion in a rotoinversion
operation in that the face is completely flipped up to down and right to left.
Every point in a crystal is inverted to the other side of the crystal.
Usually, a center is one operation that is all but ignored in most crystals
because it is often caused by the juxtaposition of other symmetry operations.
However in the triclinic system it is the only possible symmetry operation
except for a one fold rotational axis, which is actually just returning a
crystal face to its original position.
This information compiled from
sources including mineralogy
database ,
www.mineral.galleries.com,
and
thierry.chauvier.free.fr/systemes.html |
