corundum, properties.3, gems

The relationship between crystal structure and splitting behaviour in gem minerals is examined through discussion of spinel, corundum and chrysoberyl, with particular attention to the distinction between cleavage and parting. Cleavage is attributed to the atomic arrangement and bonding of a crystal and is theoretically reproducible down to atomic dimensions, whereas parting occurs along comparatively coarse discontinuities such as twin planes, structural faults or defects. Corundum illustrates the difficulty of applying this distinction. Its rhombohedral splitting is described, following Miers, as associated with lamellar twinning, potentially produced by pressure and movement along glide planes. Basal parting is attributed to incipient decomposition and microscopic cavities along planes susceptible to solution, while a less distinct prismatic parting is also recognised. Corundum has an approximately hexagonal close-packed oxygen arrangement, with aluminium occupying octahedral sites, and its three parting directions parallel principal directions of atomic layering. Strong aluminium–oxygen bonding helps explain why splitting is difficult to produce in most gem specimens; crystals exhibiting pronounced twinning or decomposition are generally unsuitable for gem use. Synthetic spinel is noted to show a tendency towards cubic cleavage, tentatively associated with excess gamma alumina in its structure. Gamma alumina, a synthetic polymorph possessing the spinel structure, is also discussed in connection with wartime manufacture of industrial cutting and abrasive tools. Spinel boules containing excess alumina in solid solution could be shaped and subsequently hardened through a process converting that alumina to the corundum structure, producing tough material with little tendency to cleave. Chrysoberyl is compared with olivine because of their similar crystal habits, dimensions and fundamental structures, as established by X-ray analysis. Beryllium occupies tetrahedral coordination sites corresponding to silicon in olivine, while aluminium occupies octahedral sites corresponding to magnesium. Although beryllium–oxygen and aluminium–oxygen bonds are considered similarly covalent, the smaller beryllium atom forms stronger bonds distributed among four oxygen neighbours rather than six. Cleavage is consequently expected along planes involving the relatively weaker aluminium-linked portions of the structure, an interpretation consistent with chrysoberyl’s imperfect (010) cleavage. Its second imperfect cleavage, (011), is less readily explained, particularly because the corresponding cleavage apparently does not occur in olivine, and the discrepancy is left for subsequent discussion.