Presentation on theme: "Crystallography: Forms and Planes"— Presentation transcript:
1Crystallography: Forms and Planes MineralogyCarleton College
2Miller Indices (hkl)The orientation of a surface or a crystal plane may be defined by considering how the plane (or indeed any parallel plane) intersects the main crystallographic axes of the solid. The application of a set of rules leads to the assignment of the Miller Indices, (hkl); a set of numbers which quantify the intercepts and thus may be used to uniquely identify the plane or surface.
3Miller IndicesA set of parallel crystallographic planes is indicated by its Miller Index (hkl). The Miller Index of a plane is derived from the intercepts of the plane with the crystallographic axes.
4Miller Indices: Example 1 The intercepts of the plane are at 0.5a, 0.75b, and 1.0cTake the reciprocals to get (2, 4/3, 1)Reduce common factors to get Miller Index of (643)
5Miller Indices: Example 2 The intercepts of the plane are at 1a, infinity b, and 1.0cTake the reciprocals to get (1, 0, 1)Reduce common factors to get Miller Index of (101)
6Miller Indices: Example 2 The intercepts of the plane are at 1a, 1b, and 1.0cTake the reciprocals to get (1, 1, 1)Reduce common factors to get Miller Index of (111)
9The intercepts of the line are at 1a1, infinity 2a2, -2/3 a3 and infinity with a3 Take the reciprocals to get (1, 1/2, -3/2, , 1/«)Reduce common factors to get Miller Index of ( )
10Hexagonal coordinates Except for (0001) plane, the geometry of this lattice requires both positive and negative terms in the indexA quick check on the correctness of hexagonal indices is that the sum of the first two digits times (-1) should be equal to the third digit.
11Stable Cleavage Planes and Forms The most stable surfaces are those with the lowest Miller Indices (e.g. 100, and 110). Surfaces with high Miller Indices have atoms with very incomplete coordination.
12Stable Cleavage Planes and Forms For a hexagonal lattice, stable cleavage planes will be (-1010) and (0-110) to give cleavage angles of 120 degrees.
13Crystal FormsA form is a set of crystal faces that result by applying the symmetry elements of the crystal to any face.
14Crystal FormsAny group of crystal faces related by the same symmetry is called a form. There are 47 or 48 crystal forms depending on the classification used.
15Crystal Forms, Open or Closed Closed forms are those groups of faces all related by symmetry that completely enclose a volume of space. It is possible for a crystal to have entirely faces of one closed form.
16Crystal Forms, Open or Closed Open forms are those groups of faces all related by symmetry that do not completely enclose a volume of space. A crystal with open form faces requires additional faces as well.
17Crystal Forms, Open or Closed There are 17 or 18 open forms and 30 closed forms.
18Triclinic, Monoclinic and Orthorhombic Forms PedionA single face unrelated to any other by symmetry. Open
19Triclinic, Monoclinic and Orthorhombic Forms PinacoidA pair of parallel faces related by mirror plane or twofold symmetry axis. Open
20Crystal Forms Dihedron A pair of intersecting faces related by mirror plane or twofold symmetry axis. Some crystallographers distinguish between domes (pairs of intersecting faces related by mirror plane) and sphenoids (pairs of intersecting faces related by twofold symmetry axis). All are open forms
21Crystal Forms, 3-, 4- and 6 Prisms Prisms. A collection of faces all parallel to a symmetry axis. All are open.
22Crystal Forms, 3-, 4- and 6 Pyramids Pyramid. A group of faces at symmetry axis. All are open. The base of the pyramid would be a pedion.
23Crystal Forms, 3-, 4- and 6 Dipyramids Dipyramid. Two pyramids joined base to base along a mirror plane. All are closed, as are all all following forms.
24Scalenohedra and Trapezohedra Disphenoid. A solid with four congruent triangle faces, like a distorted tetrahedron. Midpoints of edges are twofold symmetry axes. In the tetragonal disphenoid the faces are isoceles triangles and a fourfold inversion axis joins the midpointsof the bases of the isoceles triangles.
25Scalenohedra and Trapezohedra Scalenohedron. A solid made up of scalene triangle faces (all sides unequal)
26Scalenohedra and Trapezohedra Trapezohedron. A solid made of trapezia (irregular quadrilaterals)
27Scalenohedra and Trapezohedra Rhombohedron. A solid with six congruent parallelogram faces. Can be considered a cube distorted along one of its diagonal three-fold symmetry axes.
28Tetartoidal, Gyroidal and Diploidal Forms The general form for symmetry class congruent irregular pentagonal faces. The name comes from a Greek root for one-fourth because only a quarter of the 48 faces for full isometric symmetry are present.
29Tetartoidal, Gyroidal and Diploidal Forms The general form for symmetry class congruent irregular pentagonal faces.DiploidThe general form for symmetry class 2/m3*. 24 congruent irregular quadrilateral faces. The name comes from a Latin root for half, because half of the 48 faces for full isometric symmetry are present.
30Tetartoidal, Gyroidal and Diploidal Forms PyritohedronSpecial form (hk0) of symmetry class 2/m3*. Faces are each perpendicular to a mirror plane, reducing the number of faces to 12 pentagonal faces. Although this superficially looks like the Platonic solid with 12 regular pentagon faces, these faces are not regular.
32Hextetrahedral Forms Tetrahedron Trapezohedral Tristetrahedron Four equilateral triangle faces (111)Trapezohedral Tristetrahedron12 kite-shaped faces (hll)
33Hextetrahedral Forms Trigonal Tristetrahedron Hextetrahedron 12 isoceles triangle faces (hhl). Like an tetrahedron with a low triangular pyramid built on each face.Hextetrahedron24 triangular faces (hkl) The general form.
34Crystal Forms: Cube Six square faces (100). Octahedron Eight equilateral triangle faces (111)Rhombic Dodecahedron12 rhombic faces (110)Trapezohedral Trisoctahedron24 kite-shaped faces (hhl). Note that the Miller indices for the two trisoctahedra are the opposite of those for the tristetrahedra.
35Crystal Forms: Trigonal Trisoctahedron 24 isoceles triangle faces (hll). Like an octahedron with a low triangular pyramid built on each face.Tetrahexahedron24 isoceles triangle faces (h0l). Like an cube with a low pyramid built on each face.Hexoctahedron48 triangular faces (hkl) The general form
37Crystal Forms: Octahedral Example In Cubic symmetry, the face (111) will generate the faces (111), (-111), (11-1), (-1-1-1), (1-1-1), (-11, -1) and (-1-11). The resulting set of faces is designated (111) and is called an octahedron.