A study of millerite from Cu-Ni-PGE footwall veins, Sudbury, ON: crystal-chemistry, morphology, & geological implications
Date
2020-05-20
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Abstract
Millerite (-NiS) is a common accessory mineral in magmatic Cu-Ni-PGE deposits, such
as those occurring in Sudbury, Ontario, Canada. In such localities, millerite develops as platy
crystals, often forming large, cleavable masses. This is in stark contrast to the more widespread
occurrence of millerite as a hydrothermal precipitate within vugs in carbonate basins, where it
develops an acicular morphology, with crystals often exhibiting exaggerated aspect ratios (l:w >
20). The reason for these contrasting morphologies is attributed directly to the genesis of
magmatic millerite that initially crystallizes as the high-temperature mineral crowningshieldite
(-NiS), later inverting to millerite upon further cooling. The morphology exhibited by platy
millerite is thus representative of crowningshieldite via a paramorphic relationship. Millerite of
magmatic origin also frequently exhibits secondary twinning on {011̅2}, resulting from a postcrystallization increase in pressure. Development of these twins produces a variable response
under BSE microscopy that is attributed to orientation contrast.
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Keywords
Millerite, NiS, magmatic sulphide, crystal structure, morphology, polymorph, crystal growth, twinning, backscatter coefficient, electron channelling