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Flashcards in Form and stuff Deck (11)
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Symmetry Operations

A transposition of an object

Includes 3 types:

  1. rotations

  2. inversions

  3. translations

There are 10 permissible rotations

  1. proper - 1, 2, 3, 4, 6
  2. improper - i, -2 = m, -3, -4, -6


Point Groups

There are 32 possible combinations of rotational symmetry operations that each correspond to one of the 7 crystal systems



The morphology of a perfect crystal, in general, reflects the maximum symmetry that a crystal can have.


Crystal Form

A crystal fce plus its symmetric equivalents


Special Form

A crystal form that is repeated by symmetry operations onto itself so that there are fewer faces as the order of the group.


General Form

A form that is not repeated onto itself by the symmetry operations so that it has the same number of faces as the order of the group


Closed Form

Encloses a volume

ex: cube, tetrahedron, octahedron


Open Form

Does not enclose a volume

ex: prism, pinnacoid


Miller Indices

A plane of a crystal cuts crystallographic axes at intercepts

May be a cleavage plane, a crystal face, or any diffracting X-ray plane

Always in relation to the crystallographic axes, not any orthogonal system of convenience

General form is (h k l)


32 Crystal Classes

There are 32 point groups - combinations of proper and improper rotations

Each corresponds to one of the 32 crystal classes

Each is assigned to one of the seven crystal systems



The order of the group is the number of operations of the group