The noun “eutaxy” and the adjective “eutactic”

The words eutaxy and eutactic have Greek roots. The noun eutaxy means “good or established order or arrangement” [1].

The two terms are mostly found in the literature of chemistry, geology and crystallography. In an article on ion packing in crystal structures, M. O'Keeffe explains that the terms were suggested (via private communication) by Mrs. M. Hyde [2]. O'Keeffe proposed to call the arrangements of spheres (atoms, ions) in a periodic lattice eutactic, when the spheres are configured in a close-packing geometry, but without being in contact. G. S. Rohrer uses the term eutactic arrangement as a synonym for pseudo-close packed framework [3]. Identification of the packing type and eutactic framework is critical in naming and classifying crystal structures.

Keywords: sphere packing, structural concepts, density and volume of ionic compounds

References
[1] The Free Dictionary: www.thefreedictionary.com/Eutaxy.
[2] M. O'Keeffe: On the Arrangement of Ions in Crystals. Acta Cryst. A 1977, 33, pp. 924-927. DOI: 10.1107/S056773947700223X.
[3] G. S. Rohrer: Structure and Bonding in Crystalline Materials. Cambridge University Press, Cambridge, UK, 2001; page 145.