symmetry elements ii 3-d symmetry we now have 8 unique 3d symmetry operations: 1 2 3 4 6 m 3 4 1 2 3...

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Symmetry Elements II Symmetry Elements II

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Symmetry Elements IISymmetry Elements II

3-D Symmetry3-D Symmetry

We now have We now have 88 unique unique 3D3D symmetry operations: symmetry operations:

1 2 3 4 6 m1 2 3 4 6 m 3 4 3 4

CombinationsCombinations of these elements are also possible of these elements are also possible

A A completecomplete analysis of analysis of symmetry about a point in spacesymmetry about a point in space requires that we try all possible combinations of these requires that we try all possible combinations of these symmetry elementssymmetry elements

Point GroupPoint Group

The set of symmetry operations that leave The set of symmetry operations that leave the appearance of the crystal structure the appearance of the crystal structure unchanged. unchanged.

There are 32 possible point groupsThere are 32 possible point groups(i.e., unique combinations of symmetry (i.e., unique combinations of symmetry operations). operations).

Try combining a 2-fold rotation axis with a mirrorTry combining a 2-fold rotation axis with a mirror

The result is The result is Point Group 2mmPoint Group 2mm

““2mm” indicates 2mm” indicates 22 mirrors mirrors

The mirrors are differentThe mirrors are different

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Step 1: reflectStep 1: reflect

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Step 1: reflectStep 1: reflect

Step 2: rotate Step 2: rotate 11

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Step 1: reflectStep 1: reflect

Step 2: rotateStep 2: rotate 2 2

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Step 1: reflectStep 1: reflect

Step 2: rotate Step 2: rotate 33

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Any other elements?

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Yes, two more mirrors

Any other elements?

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

Point group name??

Yes, two more mirrors

Any other elements?

2-D Symmetry2-D Symmetry

Now try combining a 4-fold rotation axis with a mirrorNow try combining a 4-fold rotation axis with a mirror

4mm

Point group name??

Yes, two more mirrors

Any other elements?

2-D Symmetry2-D Symmetry

Why not 4mmmm?

3-fold rotation axis with a mirror creates point group 3-fold rotation axis with a mirror creates point group 3m3m

Why not 3mmm?Why not 3mmm?

2-D Symmetry2-D Symmetry

6-fold rotation axis with a mirror creates point group 6-fold rotation axis with a mirror creates point group 6mm6mm

2-D Symmetry2-D Symmetry

The original 6 elements plus the 4 combinations The original 6 elements plus the 4 combinations creates creates 1010 possible possible 2-D Point Groups2-D Point Groups::

1 2 3 4 6 m 2mm 3m 4mm 6mm1 2 3 4 6 m 2mm 3m 4mm 6mm

AnyAny 2-D pattern of objects surrounding a point 2-D pattern of objects surrounding a point must conform to one of these groupsmust conform to one of these groups

2-D Symmetry2-D Symmetry

3-D Symmetry3-D Symmetry

As in 2-D, the number of possible combinations is As in 2-D, the number of possible combinations is limited only by limited only by incompatibilityincompatibility and and redundancyredundancy

There are only There are only 2222 possible unique 3-D possible unique 3-D combinations, when combined with the combinations, when combined with the 1010 original 3-D elements yields the original 3-D elements yields the 32 3-D Point 32 3-D Point GroupsGroups

3-D Symmetry3-D SymmetryThe 32 3-D Point GroupsThe 32 3-D Point Groups

Every 3-D pattern must conform to Every 3-D pattern must conform to oneone of them. of them.

This includes every crystal, and every point within a This includes every crystal, and every point within a crystalcrystal

Rotation axis only 1 2 3 4 6

Rotoinversion axis only 1 (= i ) 2 (= m) 3 4 6 (= 3/m)

Combination of rotation axes 222 32 422 622

One rotation axis mirror 2/m 3/m (= 6) 4/m 6/m

One rotation axis || mirror 2mm 3m 4mm 6mm

Rotoinversion with rotation and mirror 3 2/m 4 2/m 6 2/m

Three rotation axes and mirrors 2/m 2/m 2/m 4/m 2/m 2/m 6/m 2/m 2/m

Additional Isometric patterns 23 432 4/m 3 2/m

2/m 3 43m

Increasing Rotational Symmetry

Table 5.1 of Klein (2002) Manual of Mineral Science, John Wiley and Sons

Crystal SystemsCrystal Systems

A grouping point groups that require a A grouping point groups that require a similar arrangement of axes to describe similar arrangement of axes to describe the crystal lattice. |the crystal lattice. |

There are seven unique crystal systems. There are seven unique crystal systems.

3-D Symmetry3-D SymmetryThe 32 3-D Point GroupsThe 32 3-D Point Groups

Regrouped by Regrouped by Crystal SystemCrystal System

Crystal System No Center Center

Triclinic 1 1

Monoclinic 2, 2 (= m) 2/m

Orthorhombic 222, 2mm 2/m 2/m 2/m

Tetragonal 4, 4, 422, 4mm, 42m 4/m, 4/m 2/m 2/m

Hexagonal 3, 32, 3m 3, 3 2/m

6, 6, 622, 6mm, 62m 6/m, 6/m 2/m 2/m

Isometric 23, 432, 43m 2/m 3, 4/m 3 2/m

Table 5.3 of Klein (2002) Manual of Mineral Science, John Wiley and Sons

TriclinicTriclinic

Three axes of Three axes of unequal length unequal length

Angles between axes Angles between axes are not equalare not equal

Point group: 1Point group: 1

MonoclinicMonoclinic

Three axes of Three axes of unequal length unequal length

Angle between two Angle between two axes is 90axes is 90°°

Point groups: Point groups: 2, m, 2/m2, m, 2/m

OrthorhombicOrthorhombic

Three axes of Three axes of unequal length unequal length

Angle between all Angle between all axes is 90axes is 90°°

Point groups: 222Point groups: 2222/m2/m/2/m, 2mm2/m2/m/2/m, 2mm

TetragonalTetragonal

Two axes of equal Two axes of equal length length

Angle between all Angle between all axes is 90axes is 90°°

Point groups: 4, 4, Point groups: 4, 4, 4/m, 4mm, 422, 42m, 4/m, 4mm, 422, 42m, 4/m2/m2/m4/m2/m2/m

HexagonalHexagonal Four axes, three equal Four axes, three equal

axes within one plane axes within one plane

Angle between the 3 Angle between the 3 co-planar axes is 60co-planar axes is 60°°

Angle with remaining axis Angle with remaining axis is 90is 90°°

Point groups: 6, 6, 6/m, Point groups: 6, 6, 6/m, 6mm, 622, 62m, 6mm, 622, 62m, 6/m2/m2/m6/m2/m2/m

Trigonal Trigonal (Subset of Hexagonal)(Subset of Hexagonal)

Four axes, three equal Four axes, three equal axes within one plane axes within one plane

Angle between the 3 Angle between the 3 co-planar axes is 60co-planar axes is 60°°

Angle with remaining axis Angle with remaining axis is 90is 90°°

Point groups: 3, 3, 3/m, Point groups: 3, 3, 3/m, 32, 32/m32, 32/m

Cubic / IsometricCubic / Isometric

All axes of equal All axes of equal length length

Angle between all Angle between all axes is 90axes is 90°°

Point groups: 23, 423, Point groups: 23, 423, 2/m3, 43m, 4/m32/m2/m3, 43m, 4/m32/m

Crystal System CharacteristicsCrystal System Characteristics

Isometric/CubicIsometric/Cubic

HexagonalHexagonal

TetragonalTetragonal

OrthorhombicOrthorhombic

MonoclinicMonoclinic

TriclinicTriclinic

ALL AXES EQUAL

AXES UNEQUAL

BirefringenceBirefringence

Isometric/CubicIsometric/Cubic

HexagonalHexagonal

TetragonalTetragonal

OrthorhombicOrthorhombic

MonoclinicMonoclinic

TriclinicTriclinic

ISOTROPIC

ANISOTROPIC

Crystal System CharacteristicsCrystal System Characteristics

Isometric/CubicIsometric/Cubic

HexagonalHexagonal

TetragonalTetragonal

OrthorhombicOrthorhombic

MonoclinicMonoclinic

TriclinicTriclinic

ALL AXES EQUAL

ALL AXES UNEQUAL

TWO AXES EQUAL

Interference FigureInterference Figure

Isometric/CubicIsometric/Cubic

HexagonalHexagonal

TetragonalTetragonal

OrthorhombicOrthorhombic

MonoclinicMonoclinic

TriclinicTriclinic

UNIAXIAL

BIAXIAL

Crystal System CharacteristicsCrystal System Characteristics

Isometric/CubicIsometric/Cubic

HexagonalHexagonal

TetragonalTetragonal

OrthorhombicOrthorhombic

MonoclinicMonoclinic

TriclinicTriclinic

ALL AXES EQUAL

AXES NON-ORTHOGONAL

AXES ORTHOGONAL

ExtinctionExtinction

Isometric/CubicIsometric/Cubic

HexagonalHexagonal

TetragonalTetragonal

OrthorhombicOrthorhombic

MonoclinicMonoclinic

TriclinicTriclinic

PARALLEL

INCLINED