Lecture 11 (10/18/2006) Crystallography Part 4: Crystal Forms Twinning

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Presentation transcript:

Lecture 11 (10/18/2006) Crystallography Part 4: Crystal Forms Twinning

Crystal Forms Habit: the general external shape of a crystal (e.g., prismatic, cubic, bladed...) Form: refers to a group of faces which have the same relation to the elements of symmetry. Crystals with higher degrees of symmetry tend to generate more form faces. open form closed form

Unique Attributes of Crystal Forms Anthophyllite Pyrite Quartz NaCl Faces of particular forms commonly share unique physical or chemical attributes

Form Indexes defined by the Miller index (hkl) of the face in the positive quadrant enclosed in brackets {hkl} a {100}, r {111}, c {001} c a2 a1 Tetragonal 4/m 2/m 2/m

Low Symmetry Forms Pyramids and Dipyramids Prisms

Trapezohedron, Scalehedron, Rhombehedron, Disphenoid

Isometric Forms

Twinning Symmetrical intergrowth of two or more crystals related to a symmetry operation (twin element) that is otherwise absent in a single crystal. Twin elements includes mirrors, rotation axes (usually 2-fold) and roto-inversion that usually do not align with symmetry elements in the crystal. Twin Laws define the twin element and its crystallographic orientation (twin planes are identified by its Miller index (hkl), twin axes are defined by a zone symbol [hkl]).

Twin Types Contact Twins Composition planes Potential Twin Plane (111) Composition Plane Contact Twins Composition planes correspond to twin planes (mirrors) Mirror Reflection Penetration Twins Composition planes are irregular; formed by rotational twin axis operation Twin Axes

Multiple Twins Formed from 3 or more twinned crystals Polysynthetic twins where composition planes are parallel Cyclic twins where composition planes are not parallel

Twin Laws in the Triclinic System Albite combined with Pericline Law [010] twin axis results in “tartan twinning” in microcline (K-feldspar) Albite Law {010} twin plane

Twin Laws in the Monoclinic System Note that twin planes do not align with mirror planes

Twin Laws in the Orthorhombic System contact & cyclic twinning Contact or Penetrative??

Twin Laws in the Tetragonal System

Twin Laws in the Hexagonal System {0112} is most common and may produce single contact or polysynthetic twins exception: twin axis || C

Twin Laws in the Isometric System Penetrative twins with twin axes parallel to rotational axes

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