Basal Pinacoid
Feature relating to crystal symmetry. A columnar or prismatic crystal with flat ends may be referred to as having a basal pinacoid.
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Base or basal pinacoid: This has 2 faces in it and is denoted by Miller indices as (0001) and (000-1).
First order prism
Second order prism ...
5.5b Prism and Basal Pinacoid
5.5c
With Figure 5.5, we have 3 sets of prisms expressed by the letter designations of m, l, and n, and a pinacoid face letter-designated as a.
It crystallizes in the orthorhombic group and its crystals are mostly prismatic terminated by pyramidal and other faces, the basal pinacoid often being present.
Habit: Usually well-formed prisms, with a basal pinacoidal termination on one end and a small pinacoidal face surrounded by pyramid and horizontal prism faces at the other.
Hardness: 8
Cleavage: 1; perfect, perpendicular to c axis.
Beryl is especially abundant in the Kunar area, but the gem varieties, aquamarine and morganite, are found mainly in the Laghman district, where they occur with the basal pinacoid characteristically well developed.
crystal is generally vertically striated or grooved and pleochroism is distinct in deeply colored crystals. Occasionally, some specimens of aquamarine display chatoyancy. Aquamarine has imperfect cleavage which runs parallel to the basal pinacoid.
 See also: Crystal, Orient, Mineral, Faces, Minerals
  
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