RHOMBOHEDRAL  CLASS (Parallel-faced hemihedral)." The only elements of symmetry are the triad See also:

• AXIS (Lat. for " axle ")

axis and a centre of symmetry . The See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general See also:

• FORM (Lat. forma)

form {hkl} is a rhombohedron, and is a hemihedral form, with parallel faces, of the scalenohedron . The form {hkl}, where h–2k+l=o, is also a rhombohedron, being the hemihedral form of the hexagonal bipyramid . The dihexagonal See also:

• PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism {hkl} of the See also:

• HOLOSYMMETRIC

holosymmetric class becomes here a hexagonal prism . The rhombohedra (hkk), hexagonal prisms 12'1] and {IOT and the basal pinacoid 1111} are geometrically the same in this class as in the holosymmetric class . Fig . 76 represents a crystal of See also:

• DIOPTASE

dioptase with the fundamental rhombohedron r{See also:

• IOO

Ioo) and the hexagonal prism of the second See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order m See also:

• TOLL (etymologically, that which is numbered or counted; from a common Teutonic form, cf. " tale," " tell ")
• TOLL, JOHAN KRISTOFFER, COUNT (1743-1817)

toll combined with the rhombohedron s 1o3Y} . Examples of minerals which crystallize in this class are See also:

• PHENACITE

phenacite,581 dioptase, See also:

• WILLEMITE

willemite, See also:

• DOLOMITE

dolomite, See also:

• ILMENITE

ilmenite and pyrophanite: amongst artificial substances is ammonium periodate ((NH4)4I209.3H20) .