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Abstract
Ballas diamond is a rare form of the polycrystalline radial aggregate of diamonds with diverse internal structures. The morphological features of ballas diamonds have experienced repeated revision. The need that this paper presents for development of a crystal-genetic classification was determined by a rich variety of combined and transitional forms of ballas-like diamonds, which include aggregates, crystals, and intergrowths. The new crystal-genetic classification combines already-known and new morphological types of ballas as well as ballas-like diamonds discovered in the placers of Yakutia, the Urals, and Brazil. The ballas-like diamond forms include spherocrystals, aggregates with a single crystal core, split crystals, radial multiple twin intergrowths, and globular crystals. The crystal genetic scheme of the evolution of ballas and ballas-like diamonds is a sequence of the morphological types arranged in accordance with the conventional model of the dependence of the mechanism and diamond growth from carbon supersaturation developed by I. Sunagawa. The evolution of the growth forms of ballas and ballas-like diamonds was tracked based on the macrozonal structure of diamonds varying from a flat-faced octahedron to a fibrous cuboid with its transition forms to the radiating crystal aggregates. The morphological diversity of the ballas-like diamonds depends on the level of supersaturation, and abrupt changes of the level of supersaturation engender abrupt changes in a mechanism of crystal growth. The change in the rate of growth under the influence of adsorption and absorption of the mechanic impurities accompanied the sudden appearance of the autodeformation defects in the form of splitting and multiple radial twinning of crystals. The spherical shape of Yakutia ballas-like diamonds is due to the volumetric dissolution that results in the curved-face crystals of the “Urals” or “Brazilian” type associated with ballas diamonds in placers.
Highlights
Ballas diamond is a rare type of natural diamond aggregate that A.F
The classic ballas diamonds are absent in kimberlites and have been reliably established in the diamondiferous placers of the Urals [3,6], the Sayan region [7,8], and Yakutia [9]
The morphology of the UR specimen corresponds to the description of the shape of the ballas from the Krasnovishersky region [3]
Summary
Ballas diamond is a rare type of natural diamond aggregate that A.F. Williams [1] first described as “shot bort”, a term that emphasised the similarity of the globular shape and dark grey colour of ballas diamonds. Yu.L. Orlov [2,3] defined ballas diamonds as spherulites with a radial-fibrous structure. Ballas diamonds are well known in the placers of Brazil and South Africa [1,3,4,5], while location of ballas in Russia is rare and has to date received little attention in systematic studies. The classic ballas diamonds are absent in kimberlites and have been reliably established in the diamondiferous placers of the Urals [3,6], the Sayan region [7,8], and Yakutia [9]. Ballas-like spherical intergrowths of crystals in the Sytykanskaya and Yubileinaya kimberlite (Yakutia kimberlite province) pipes have been described [10]
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