BASIC GEOLOGY 1. CRYSTALLOGRAPHY Crystal is a convex polyhedron.
Crystal is a solid material in which atoms and/or ions are arranged and chemically bounded in a regular and repeating long-range pattern. MINERAL is a (1) naturally occuring (2) crystalline solid with a
(3) definite chemical composition. - Naturally occuring, i.e. it is formed without human action or intervention, but it may be biotic (biomineralization).
- Crystalline solids, but some minerals are amorphous (mineraloids). - Definite chemical composition, but it is not necessarily fixed.
Olivine (Fe,Mg)2SiO4 a) Spot repeated by translating parallel to a vector row of dots b) Repeated translations parallel to a and b plane lattice c) Repeated plane lattices one above the other space lattice
Unit cell: the smallest group of particles in the material that constitutes the repeating pattern. The unit cell completely defines the symmetry and structure of the entire crystal lattice, which is built up by repetitive translation of the unit cell along its principal axes. Triclinic
Tetragonal Hexagonal Monoclinic Trigonal Orthorhombic
Isometric SYMMETRY ELEMENTS Rotational symmetry involves repeating a motif by a set of uniform rotation about an axis passing through the crystal. The entire crystal can
be produced 1-, 2-, 3-, 4- or 6-fold rotations. If a crystal has inversion symmetry, any line drawn through the origin will find identical features equidistant from the origin on opposite side of the crystal. Inversions move faces to a reversal of position; the old and new positions lie upon a line, at the middle of which is the centre of inversion. Improper rotations are rotations followed by inversions (called
rotoinversions). Reflections exchange the parts of the crystal on the two sides of a plane of symmetry (mirror plane) so the pattern on one side is a mirror image of the pattern on the other. A rotation (2, 3, 4, 6)
B rotoinversion or improper rotation (3,4) C inversion D reflection The amount of light able to be passed through a mineral determines its transparency.
- Opaque minerals do not let any light through. - Translucent minerals partially let light pass through. - Light is able to pass through transparent minerals. Index of refraction (n) n=Vv/Vm
Vv velocity in vacuum Vm velocity in the material Double refraction or birefrigence
ISOTROPIC ANISOTROPIC Properties are direction independent. A single refractive index.
No birefringence. Properties are direction dependent. More than one refractive indices. Birefringence.
Cubic crystals and amorphous materials. All crystals except cubic ones. POLARIZING MICROSCOPE
a) Unpolarized light vibrates in all directions. b) Polarized light vibrates in a single plane Plane light view
Crossed polar view
