FORMS OF CRYSTAL SYSTEM
Closed and open forms of Non-Isometric system
Open forms of non-isometric system
1. Pedions:
Single faced form. It is a form consisting of a single face so situated on a
crystal that is not repeated by any of the elements of symmetry.
2. Pinacoids: 2 parallel faces. It is a form composed of a pair of parallel faces on the opposite side of the centre.
3. Prisms: A form whose faces are parallel to vertical axis only. Prisms may be composed of 3,4,6,8, or 12 faces.
4. Dome:
2 non parallel faces related by a mirror reflection. It resembles a gabble-roof
of a house. The faces are parallel to a or b crystallographic axis.
5. Sphenoids:
2 nonparallel faces related by a
2-fold rotation. It is a wedge like form which is
obtained by repetition of a face around a daid axis.
6. Pyramid:
3 or more faces meeting in a vertex. The faces of this form meet three axis, a ,
b, and c. Pyramids may be composed of 3,4,6,8, or 12 faces.
Closed forms of Non-isometric system
1. Dipyramid: A
closed form consisting of 6, 8, 12, 16 or 24 faces. A dipyramid is formed of
two pyramids by reflection of one of them across a horizontal plane.
2. Disphenoid: A closed form having two upper faces that alternate with two lower faces, offset by 900.
3. Trapezohedron:
A closed form that has trapezium shaped 6, 8, 12, faces.
4. Rhombohedron: A closed form of 6 rhomb-shaped faces.
Note:
ALL FORMS BELONGING TO ISOMETRIC SYSTEM ARE CLOSED FORMS
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