BHOMEOHEDKAL SECTION OF HEXAGONAL SYSTEM. 53 



those of a six-sided prism. This last will be seen from the relations 

 of the planes R to / on fig\ 8, and from the approximation to a prism ;tic 

 form in the planes 1(5 of fig-. 15. For an explanation of the lettering 

 of other planes on rhombohedral crystals, reference must be made to 

 the tl Text-Book of 3Iineralogy." 



3. Hemihedrism. Tetartohedrism. 



among; rhombohedral forms, similar to that in fin;. 



Hemihedrism oc-iir» 

 13, page 48> 

 except that the suppressed planes of one pyramid are alternate 

 with those of the other. One of these is 

 represented in fig. 24. The planes 6-4 are 

 six in number at each extremity, and are so 

 situated that they give a spiral aspect to the 

 ciystal. If these planes were only three in 

 number at each extremity, the alternate 

 /hree of the six, the form would be tetarto- 

 hedral to the double six-sided pyramid ; 

 that is, there would be one-fourth the num- 

 ber of planes that exist in the double twelve- 

 sided pyramid, or 6 planes instead of 24. 

 Such cases of hemihedrism and tetartohe- 

 drism are common in crystals of quartz, and 

 when existing, the crystals are said to be 

 plagihedral, from the Greek for oblique and 

 face. In some crystals the spiral turns to the 

 right and in others to the left, and the two kinds are distin- 

 guished as right-handed and left-handed. There are also tetar- 

 tohedral forms in which one whole pyramid of a scaleuohedron, 

 or of a rhombohedron, is wanting. For example, in crystals of 

 tourmaline rhombohedral planes, and sometimes scalenuhedral, 

 may occur at one extremity of the prism and be absent from 

 the other. This dissimilarity in the two extremities of a crys- 

 tal of tourmaline is connected with pyro-electric polarity in the 

 mineral. Three-sided jmsrus, hemihedral to the hexagonal prism, 

 are common in some rhombohedral species, as tourmaline. 



4. Cleavage. — Cleavage usually takes place parallel to the 

 faces of a rhombohedron, as in calcite, corundum. Not unii o 



quently the rhombohedral cleavage is wanti ig ; 



and there is highly perfect cleavage parallel tc 



the basal plane, as in graphite, brucite. 

 3 5. Irregularities of Crystals. — Distortions oc- 

 ££v IbF cur °f tne same nature with those under ihe 

 ~'~ < ^ < jr other systems. Some examples are given under 



quartz. Some rhombohedral species, as dolomite, 

 have the opposite faces convex or concave, as in fig. 25. 



2o. 



