digonaladj.
/ˈdɪɡənəl//dʌɪˈɡəʊnəl/ Crystallography.
Denoting an axis of two-fold symmetry.
ΚΠ
1890 G. H. Williams Elem. Crystallogr. (ed. 2) iii. 47 Two other sets of axes are of use in the isometric system. One is the set of intersection-lines between the principal and secondary planes of symmetry... The first..are called the digonal.
1898 Nature 27 Jan. 309 Only digonal, trigonal, [tetragonal,] and hexagonal axes [of symmetry] are possible with crystals.
1902 H. A. Miers Mineral. i. 62 The crystals of this class [sc. sulphur]..possess three digonal axes coinciding with the crystal axes.
1911 A. E. H. Tutton Crystallogr. x. 136 A digonal axis..is such that half a complete revolution about it restores the original appearance of the crystal.
This entry has not yet been fully updated (first published 1933; most recently modified version published online June 2018).
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adj.1890