Crystal-chemistry of Ni-bearing spinel-group minerals

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Marco E. Ciriotti
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Crystal-chemistry of Ni-bearing spinel-group minerals

Messaggio da Marco E. Ciriotti » gio 01 mar, 2018 10:14

Prossima pubblicazione.

▪ Beckett-Brown, C.E. & McDonald, A.M. (2018): The crystal-chemistry of Ni-bearing spinel-group minerals: Chemical, geological, and exploration implications. Canadian Mineralogist, 56, (in press).

Spinels with Ni contents ranging from 196 ppm to 12.85 wt.%, obtained from intermediate-mafic to ultramafic environments (Morokweng crater and Bon Accord, South Africa; Puddy Lake, Ontario, Canada), have been studied using textural observations and in situ chemical analyses (e.g., energy and wavelength dispersive spectroscopy as well as laser-ablation inductively-coupled mass-spectrometry) to document and highlight the significance of Ni incorporation into spinel-group minerals. Those from the Puddy Lake ultramafic intrusion have the lowest Ni concentrations, ranging from 196 to 7880 ppm (avg. = 3326, n = 37). They develop in heavily serpentinized areas of the intrusion as isolated, optically zoned subhedral to anhedral crystals up to 1 mm in diameter. Chemically, they range from Ni-bearing chromite to Ni-bearing magnetite with an average composition of (based on 4 anions): (Fe2+0.80Mg2+0.13Mn2+0.05Ni2+0.02Zn2+0.01)Σ1.01(Cr3+1.28Fe3+0.38Al3+0.32Ti0.01)Σ1.99O4 and (Fe2+0.88Mg2+0.08Ni2+0.02Mn2+0.01Zn2+0.01)Σ1.00(Fe3+1.84Cr3+0.16)Σ2.00O4, respectively. Spinels from the quartz-norite melt sheet of the Morokweng impact crater have intermediate Ni concentrations ranging from 0.41 to 7.29 wt.% (avg. = 5.00, n = 81). They develop as anhedral rounded grains (∼25 μm in diameter) that surround disseminated sulfide patches (∼5 cm in diameter), the latter composed of millerite, siegenite, and chalcopyrite. Chemically, these spinels are Ni-bearing (<5.00 wt.% Ni) to Ni-rich (<12.52–5.00 wt.% Ni) magnetite, with an average composition of (Fe2+0.77Ni2+0.23)Σ1.00(Fe3+2.00)Σ2.00O4. Spinel from the Bon Accord ultramafic hosted nickel-oxide showing contain the highest Ni concentrations, ranging from 10.19 to 12.85 wt.% (avg. = 11.54, n = 50). They form as solitary subhedral grains (∼50 to 100 μm in diameter) or as aggregates of multiple grains (ranging from 0.5 to 1.0 cm), some containing inclusions of Ni-rich silicates (e.g., willemseite) and sulfides (e.g., millerite). Chemically, they are compositionally intermediate between magnetite and trevorite, with an average composition of (Fe2+0.54Ni2+0.46)Σ1.00(Fe3+1.99)Σ1.99O4. Nickel incorporation in spinel-group minerals appears to be process dependent, with no universal control on its incorporation. In general, the greatest Ni enrichment is observed in inverse spinel-group minerals (i.e., magnetite) due to site-preference energy constraints.
Marco E. Ciriotti

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