Lecture 8 (10/09/2006) Crystallography Part 1: Symmetry Operations
Crystallography The study of crystalline solids and the principles that govern their growth, external shape, and internal structure Crystal – a solid bounded by geometrically-arranged smooth faces Perfection of crystal face development: Euhedral – perfect crystal faces Subhedral – imperfectly developed crystal faces Anhedral – no crystal faces
Recognition of Mineral Symmetry External Crystal Form (Morphology) - macroscopic observations/measurements of crystal faces - 32 crystal classes (point groups) possible symmetry elements or combinations of elements - 1st Half of Chapter 5 (p. 170-213) Internal Atomic Arrangement - determined from X-ray diffraction - takes into account 3D translation - 14 lattice types (Bravais lattices) - Combination of 32 point groups and 14 lattice types yields 230 space groups - 2nd Half of Chapter 5 (p. 213-239)
External Symmetry Elements Crystal Morphology Rotation Reflection Center of Symmetry Rotation with Inversion
External Symmetry Elements Motif Patterns Rotation Reflection Center of Symmetry Rotation with Inversion
Rotational Axes As an external symmetry element, rotation increments (n) range from 1 to ∞ When considering the limits of translation, rotation increments are limited to 1, 2, 3, 4, and 6-fold
Rotation with Inversion (Rotoinversion) Equivalent to other symmetry operations Equivalent to Center of Symmetry
Combination of Symmetry Elements – Multiple Rotational Axes Axes at 90º (except 3-fold axes in cubic symmetry at 54º44’) Axes intersect at point Possible symmetry combinations: 422, 622, 222, 32, 23, 432 (View 422 Symmetry.ai)
Combination of Symmetry Elements – Multiple Rotational Axes with 32 Symmetry motif projections do not require a second set of 2-fold axes
Combination of Symmetry Elements – Multiple Rotational Axes in the Cubic System 432 Point Group
Next Lecture External Symmetry (Continued) Combination of Rotation Axes and Mirror Planes 32 Crystal Classes/Point Groups 6 Crystal Systems Crystal Morphology Read: Chapter 5, p. 181-192