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Imagine
a solid cube as an object floating in space.
Note that a cube has faces or geometric planes on
six sides. If
you draw a straight line though the center of the three
opposing faces, you end up with three lines that meet in
the center of the cube, intersecting at 90-degree angles.
Each line is an axis of the cube.
The three axes are equal length lines inside of the
cube. A cube
is isometric or equal in proportion.
Six
major mineral systems or classes describe crystals in
3-dimensional space.
Each class has unique axis lengths and angles
between axes. Mineral
examples for each system are in parentheses:
1)
Isometric:
three equal length axes, intersecting at 90 degrees or
right angles (Galena, Pyrite);
2)
Hexagonal,
hexagonal division: four axes, consisting of three vertical equal
axes intersecting at 60 degree or 120 degree angles, and
one horizontal unequal length axis at 90 degrees to the
other three axes (Beryl, Quartz).
3)
Hexagonal:
trigonal or rhombohedral division: four axes, consisting of three
vertical equal length axes intersecting at 60 degrees, and
one horizontal unequal length axis at 90 degrees to the
other three axes (Calcite, Tourmaline).
4)
Tetragonal:
three axes consisting of two equal and one unequal length
axes, intersecting at 90 degree or right angles (Rutile,
Chalcopyrite);
5)
Orthorhombic:
three axes of unequal length, intersecting at 90 degrees
or right angles (Sulfur, Topaz).
6)
Monoclinic:
three axes of unequal length, with two axes intersecting
at 90 degrees or right angles, and one axis angle oblique
or greater/less than 90 degrees to the other axes
(Pyroxene, Malachite).
7)
Triclinic: three axes of unequal length, with all axes intersecting at oblique
angles or greater/less than 90 degrees (Rhodonite, Kyanite).
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