HOLOSYMMETRIC CLASS (Holohedral; Bipyramidal.) Here there are three dissimilar dyad axes of symmetry, each coinciding with a crystallographic axis; perpendicular to them are three dissimilar planes of symmetry; there is also a centre of symmetry. There are seven kinds of simple forms : Bipyramid (figs. 54 and 55).—This is the general form and is bounded by eight scalene triangles ; the indices are { III } , 121 I } .
12211, { 1121 , { 3 21 } , { 2i31, 11231, etc., or in general { hkl } , The crystallographic axes join opposite corners of these pyramids in the fundamental bipyramid {III } parametral plane has the intercepts This is the only closed form in this the others are open forms and can only in combination. Sulphur often tallizes in simple bipyramids.
Prism.—This consists of four faces par allel to the vertical axis and intercepting the horizontal axes in the lengths a and b or in any multiples or sub-multiples of these ; the indices are therefore { I I o 1, { 2 I o } , or Macro-prism.—This consists of four faces parallel to the macro-axis, and has the indices{ IoI } , 201 } . . . or { hol } Brachy-prism.—This consists of four faces parallel to the brachy-axis, and has the indices { oI I } , { on } . . . { okl } . The macro- and brachy-prisms are often called "domes." Pinacoids.—The basal pinacoid consists of a pair of parallel faces perpendicular to the vertical axis; the indices are { ooi } . The macro-pinacoid { ioo } and the brachy pinacoid { oio } each consist of a pair of parallel faces respectively parallel to the macro- and the brachy-axis.
Figs. 56-58 show combinations of these six open forms, and fig. 59 a combination of the macro pinacoid (a), brachy-pinacoid (b), a prism (m), a macro-prism (d), a brachy-prism (k), and a bipyramid (a).
Examples of substances crys tallizing in this class are ex tremely numerous; amongst naturally occurring minerals are sulphur, stibnite, cerussite, chry soberyl, topaz, olivine, nitre, barytes, columbite and many others; and amongst artificial products iodine, potassium per manganate, potassium sulphate, benzene, barium formate, etc.