RHOMBOHEDRAL SECTION OF HEXAGONAL SYSTEM. 53 
tllose of a six-sided prism. This last will be seen from the relations 
of the planes & to J on fig. 8, and from the approximation to a prismatic 
form in the planes 16 of fig. 15. For an explanation of the letteringe 
of other planes on rhombohedral crystals, reference must be made to 
the ‘‘ Text-Book of Mineralogy.” 
3. Hemihedrism. Tetartohedrism.— Hemihedrism occurs 
among rhombohedral forms, similar to that in fig. 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-§ are 
ik in number at éath extremity, and are so 
situated that they give a spiral aspect to the 
crystal. If these planes were only three in 
number at each extremity, the alternate 
three 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 
plagthedral, from the Greek for oblique and 
face. Insome 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 scalenohedron, 
or of a rhombohedron, is wanting. For example, in crystals of 
tourmaline rhombohedral planes, and sometimes scalenohedral, 
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 prisms, 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 unfre- 
quently the rhombohedral cleavage is wanting, 
and there is highly perfect cleavage parallel to 
the basal plane, as in graphite, brucite. 
5. Irregularities of Crystals —Distortions oc: 
cur of the same nature with those under the 
other systems. Some examples are given under 
quartz. Some rhombohedral species, as dolomite, 
have the opposite faces convex or roncave, as in fig. 25. 


