This document describes several crystal classes and forms. It discusses: 1) The pyritahedral class which includes forms like the pyritahedron and diploid bounded by 12 or 24 faces respectively. 2) The tetrahedral class including forms like the tetrahedron, deltoid dodecahedron, trigonal tristetrahedron, and hextetrahedron bounded by 4, 12, 12, and 24 faces. 3) Prism forms in the tetragonal, hexagonal, and orthorhombic crystal systems defined by their faces being parallel to the c-axis including first, second, and third order prisms.

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: