Calcite crystallizes in the rhombohedral system, and the most common scalenohedron form has the Miller index [ 211 ].
2.
Both crystallize in the ditrigonal pyramidal ( hemimorphic-hemihedral ) class of the rhombohedral system, possessing the same degree of symmetry as tourmaline.
3.
If the crystal system is trigonal, then the lattice system is hexagonal unless the space group is one of the seven in the rhombohedral lattice system consisting of the 7 trigonal space groups in the table above whose name begins with R . ( The term rhombohedral system is also sometimes used as an alternative name for the whole trigonal system . ) The hexagonal lattice system is larger than the hexagonal crystal system, and consists of the hexagonal crystal system together with the 18 groups of the trigonal crystal system other than the seven whose names begin with R.
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