THE NEW NICKEL SULFIDE MINERAL CROWNINGSHIELDITE FOUND WITHIN A SUPERDEEP DIAMOND FROM LETSENG

By virtue of their exceptionally deep origin and unique geological histories, diamonds have been found to contain a variety of minerals that are otherwise rare or absent at Earth’s surface. During examination of a type IIa diamond sample from the Letseng mine, Lesotho, a new nickel sulfide mineral was found as part of a fine-grained multiphase inclusion. This nickel sulfide is the natural analogue of the synthetic nickel monosulfide known as α-NiS, with a NiAs-type structure. This new mineral has been named crowningshieldite, after G.R. Crowningshield (1919 - 2006), whose many years of innovation at the Gemological Institute of America produced countless advances in the understanding and identification of natural, treated and synthetic diamonds and other gem materials.

Crowningshieldite is the high-temperature polymorph of millerite, with an inversion temperature of 379 °C from previous experiments on pure NiS. The observed sample of crowningshieldite has an empirical formula of (Ni_0.90Fe_0.10)S. The host diamond is gemmy, colorless, and relatively pure (type IIa, containing less than ~5 ppm nitrogen). It belongs to a variety of sublithospheric/superdeep diamonds named CLIPPIR diamonds that are notable for their metallic Fe-Ni-C-S melt inclusions. In this case, the inclusion is interpreted to be an alteration assemblage produced when a primary metallic Fe-Ni-C-S inclusion with a surface-reaching fracture reacted with hot fluids, likely associated with kimberlite emplacement. Other phases identified in association with crowningshieldite in this fine-grained alteration assemblage are magnetite-magnesioferrite, hematite, and graphite. Unexposed inclusions within the same diamond are of original, unaltered Fe-Ni-C-S composition. The new mineral crowningshieldite is a good example of the complexity and breadth of geological processes recorded within diamonds and their inclusions.