|
FORM OF MINERALS.
|
41
|
||
|
|
|||
|
system. Thus dihexagonal pyramids are produced with the general sign mPn, the chief peculiarity being that, whereas in the tetragonal system η might have any rational value from 1 to
changes in the value of η ; and when m=0, the basal pina-coid.
Few hexagonal mineral species form perfect holohedric combinations. Though quartz and apatite appear as such,
|
|||
|
|
|||
|
|
|||
|
|
|||
|
yet
properly the former is a tetartohedral, the latter a hem-ihedral
species. In holohedric species the predominant faces are usually those
of the two hexagonal prisms ooP and
and
2P2 are the most common subordinate forms. Figure 38 represents the
prism, bounded on the extremities by two pyramids ; one, P, forming the
point, the other 2P2, the rhombic faces on the angles, or
crystals the lateral edges of the prism are replaced by the
|
|||
|
|
|||