A Cobalt-Rich Spinel Inclusion in a Sapphire from Bo Ploi, Thailand

Jingfeng Guo*, W. L. Griffin+ and S. Y. O'Reilly*
* School of Earth Sciences, Macquarie University, NSW 2109, Australia
+ CSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde, NSW 2113, Australia

Abstract: Cobalt-rich spinel is found as a ∼200 µm inclusion, together with a glassy phase, in a gem-quality blue sapphire from Bo Ploi, Thailand. This is the first reported natural occurrence of such a spinel. Its composition is directly analogous with that of cochromite, a previously reported rare cobalt-rich chromite. The compositional ranges for the cobalt-rich spinel, obtained using electron microprobe and proton microprobe (methods described below), are Al2O3 48.18–61.27 %, CoO 19.7–22.84 %, Cr2O3 0–12.28 %, FeO 8.64–9.67 %, MgO 6.04–6.89, TiO2 0.49–0.73 %, Ni 2251–2532 p.p.m., Zn 335–371 p.p.m., Mn < 177–849 p.p.m., Ga 113–153 p.p.m., Nb 24–1252 p.p.m., Zr <4–167 p.p.m., Sn 22–428 p.p.m., As <4–56 p.p.m., Sr <4–59 p.p.m., Ag 13–64 p.p.m. Transition elements decrease in abundance from core to rim of the spinel while the other elements increase. Crystal chemical considerations suggest that a vacancy-creating substitution mechanism may be operative in the cobalt-rich spinel despite the small scale, i.e. 3Co2+ = 2Al3+ + [4]□. The glassy phase coexisting with the spinel is likely to be the product of heating by the host basaltic magma. The mode of occurrence of the cobalt-rich spinel prevents further physical investigation. This unusual spinel is considered to be the result of a complex magma mixing process in the lower crust.

Keywords: cobalt • spinel • corundum • sapphire • megacryst • alkali basalt • Thailand

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 247-258; DOI: 10.1180/minmag.1994.058.391.07
© 1994, The Mineralogical Society
Mineralogical Society (www.minersoc.org)