Forms of crystals.
- 1. PYRITOHEDRAL CLASS- 3Aii, 4Aiii, 3P, C TYPICAL FORMS OF THIS CLASS ARE PYRITOHEDRON (PENTAGONAL DODECAHDERON) AND THE DIPLOID (Dyakisdodecahedron) 1) PYRITOHEDRON: • Type mineral pyrite. • Bounded by 12faces. • General symbol is (hk0) and is related to tetrahexahedron. • Positive- if the long axis is vertical; Negative- if the long axis is horizontal. 2 ) DIPLOID: • Bounded by 24faces, the faces are quadrilaterals or trapeziums. • General symbol is (321) and is derived from hexoctahedron. TETRAHEDRAL CLASS- 3A ii, 4A iii, 6P. 1) TETRAHEDRON: • Bounded by four faces (equilateral triangle) • General symbol is (111) • The principal axes join the centres of the opposite edges. • Developed from alternate octants of octahedron. • Hemihedral form of octahedron • Positive and negative forms are present . 2) DELTOID- Dodecahedron: Bounded by 12 faces, each a quadrilateral, having miller indices as (221) Also known as tetragonal tristetrahedron ad is a hemihedral form of Trisoctahedron. 3) TRIGONAL TRIS TETRAHEDRON. • It has the general symbol (hll) and is bounded by 12 like triangular faces. Developed from trapezohedron. 4) HEXTETRAHEDRON: • Bounded by 24 scalene triangular faces. • General symbol is (hkl) • It is developed from Hexoctahedron. PRISM: Prisms, in systems other than the isometric, have been defined to be forms whose faces are parallel to the vertical axis of the crystal while they meet the horizontal axes. TETRAGONAL SYSTEM: - three types of prism depends on the number of faces and the orientation. • I-Order Prism- four faced form parallel to C- axis, general symbol is (110) • II-Order Prism- four faced form parallel to C- axis, general symbol is (100) • Ditetragonal prism- 8 faced form parallel to C- Axis, general symbol is (210) • Third order prism- tripyramidal class,
- 2. Hexagonal system: • I-Order Prism- includes six faces , each one of which is parallel to C- axis, meets two adjacent horizontal axes at equal distance, while it is parallel to the third horizontal axis.general symbol is (10-1 0) • II-Order Prism- includes six faces , each one of which is parallel to C- axis, meets the three horizontal axes, last named axis at unit distances and the others at double this distances..general symbol is (11-20) • Dihexagonal prism- 12 faced form parallel to C- Axis, general symbol is (hk_i 0) • Third order prism- tripyramidal class, ORTHORHOMBIC SYSTEM: