HEXAGONAL SECTION OF HEXAGONAL SYSTEM. 45 



of cleavage) is 90° ; and in oligoclase and the other triclinic 

 feldspars it is 3° to 5° from 90°, being in oligoclase 93° 50', and 

 in anorthite 94° 10'. This difference in angle involves oblique 

 intersections between the axes b and c, and c and a, which are 

 rectangular in monoclinic forms. There is a similarly close re- 

 lation between the triclinic form of rhodonite and that of pyrox- 

 ene, and a resemblance also in composition. 



The diametral prism in this system is similar to fig. 7 on 

 p3ge 41, under the monoclinic system, but differs in having the 

 planes all rhomboidal instead of part rectangular. The form 

 correspond uig to the oblique rhombic prism of the monoclinic 

 system (fig. 8, p. 41) also has rhomboidal instead of rhombic 

 planes ; moreover, the two prismatic planes have unequal in- 

 clinations to the vertical diametral section, and are therefore 

 dissimilar planes. The prism, consequently, is made of two 

 hemiprisms, and the basal plane is another, making in all three 

 hemiprisms. 



2. Cleavage. — Cleavage takes place independently in differ- 

 ent diametral or diagonal directions. In the triclinic feldspars 

 it conforms to the directions in orthoclase, with only the excep- 

 tion arising from the obliquity above explained. 



VL HEXAGONAL SYSTEM. 



This system is distinguished from the others by the charac- 

 ter of its symmetry — the number of planes of a kind around 

 the vertical axis being a multiple of 3. The number of lateral 

 axes is hence 3. It is related to the dimetric system in having 

 the lateral axes at right angles to the vertical and equal, and is 

 hence like it also in the optical characters of its crystals. Its 

 hexagonal prismatic form approaches trimetric crystals in the 

 obtuse angle (120°) of the prism, some trimetric crystals having 

 in angle of nearly 120°. 



Under this system there are two sections : 



1. The Hexagonal section, in which the number of planes 

 of a kind around each vertical axis above or below the basal 

 section is 6 or 12. 



2. The Rhombohedral section, in which the number of 

 planes of a kind around each half of the vertical axis, above or 

 below the basal section, is 3 or 6 ; and, in addition, the planes 

 above are alternate in position with those below. The forms 

 are mathematically hemihedral to the hexagonal, Vut not so 

 in their real nature. 



