Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . \JkJ in Fig. 138, there is shown also the unit pyramid p, and a thirdorder pyramid v, {^a : ^ a : a: 3c). HEXAGONAL SYSTEM. 39 SCALENOHEDRAL CLASS. 31. No. 13. Rhombohedral Heraihedry, LieUsch. No. 19. Rhombohedral Group, Dana. This is by far the most important group in the hexagonal sys-tem, and-includes the crystals of such minerals as calcite, corun-dum, hematite and chabazite. Symmetry of the Class. Fig. 139 shows the three planes of symmetry at sixty degreeswith the three axes diagonal to these

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Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . \JkJ in Fig. 138, there is shown also the unit pyramid p, and a thirdorder pyramid v, {^a : ^ a : a: 3c). HEXAGONAL SYSTEM. 39 SCALENOHEDRAL CLASS. 31. No. 13. Rhombohedral Heraihedry, LieUsch. No. 19. Rhombohedral Group, Dana. This is by far the most important group in the hexagonal sys-tem, and-includes the crystals of such minerals as calcite, corun-dum, hematite and chabazite. Symmetry of the Class. Fig. 139 shows the three planes of symmetry at sixty degreeswith the three axes diagonal to these and one vertical axis. Theaxes of symmetry are taken as the crystal axes. The General Form or Scalenohedron. Weiss, na: - a?mc; Dan3i,m — n: Miller, ]M/i]. Composed of twelve faces, Fig. 140, each cutting all theaxes. In the ideal form the faces are scalene triangles. The ad-jacent polar edges are necessarily unequal. Fig. 139. Fig.

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