Zoned Hibonites from Punalur, South India

M. Santosh, M. Sandiford and S. J. B. Reed
Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558, Japan Centre for Earth Science Studies, Akkulam, Trivandrum 695 031, India
Department of Geology and Geophysics, University of Adelaide, Adelaide, G.P.O. Box 498, South Australia, Australia
Department of Earth Sciences, Bullard Laboratories, Cambridge CB3 0EZ, U.K.

Abstract: Zoned hibonites from a kalsilite, leucite, spinel, corundum, perovskite gneiss from the southern Indian granulite terrain near Punalur, Kerala, have rims that are the most Ti-rich yet recorded (0.83–0.87 Ti atoms per 19 O) and are essentially free of REE elements (ΣREE < 0.01 atoms per 19 O) while the cores are the most REE-rich compositions yet recorded (ΣREE = 0.55–0.65 atoms per 19 O). Within the limits of analytical uncertainty, the compositions of the hibonite can be related to the theoretical end-member CaAl12O19 by the substitutions REE R2+ ⇌ CaAl and R2+R4+ ⇌ Al2 with the REE-rich cores containing in excess of 50% of the REE R2+ Al11O19 end member. Minor substitution of Na for Ca occurs in the rims, while non-stoichiometry in both the cores and rims is indicated by partial 12-fold site occupancy. Ion-microprobe analysis of the REE-rich hibonites reveals strong enrichment in LREE with La/Lu c. 250 000.

Keywords: hibonite • rare earth elements • titanium • zoning • ion-microprobe analysis • India

Mineralogical Magazine; June 1991 v. 55; no. 379; p. 159-162; DOI: 10.1180/minmag.1991.055.379.02
© 1991, The Mineralogical Society
Mineralogical Society (