GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 136-7
Presentation Time: 3:30 PM


LUM, Jullieta Enone, Department of Geology, University of Johannesburg, P.O Box 524, Auckland Park, 2006, Johanneburg, 2006, South Africa, VILJOEN, Fanus, Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, South Africa, CAIRNCROSS, Bruce, Geology, University of Johannesburg, PO Box 524, Aukland Park, 2006, South Africa and FREI, Dirk, Central Analytical Facility, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch, 7602, South Africa; Department of Earth Sciences, University of the Western Cape, Private Bag X17, Bellville, 7530, South Africa,

A collection of beryls (emerald and aquamarine) from the Erongo Volcanic Complex in central Namibia as well as the Gravelotte-Leydsdorp area of South Africa, were examined, and their major and trace element chemistries determined.

The beryls are subhedral to euhedral, have a well-developed prismatic habit, and are characterized by abundant inclusions (clearly reflecting the host rock mineralogy), as well as numerous cracks and veins which greatly impact on their degree of transparency. Emeralds are characterised by higher contents of TiO2, Cr2O3, Sc2O3, MgO, and Na2O than aquamarines. Optically and chemically homogeneous specimens are common, although colour-zoned, optically heterogeneous crystals, are also encountered. These are characterized by variable Cr, Na, Mg, Fe and Al. Based on electron microprobe analysis, two main chromophores are encountered, being Cr in the case of the emeralds, and Fe in the case of the aquamarines. A well-defined positive correlation between Cr and Ti, coupled with an increase in green (emerald) colour saturation with increasing Cr and Ti content is consistent with Ti also acting as a chromophore in emerald.

Octahedral cation substitution is the dominant mechanism for the incorporation of a variety of minor and trace elements. The channel sites of emerald incorporate Na (over Cs) while Na and Cs are incorporated at the channel sites of aquamarine. For aquamarine, Cs, Sc, Ga and Mn are positively correlated with Rb, consistent with the incorporation of these elements at the octahedral sites (Sc, Mn and minor Ga) or the channel sites (Cs, Rb). A combination of geochemistry and inclusion species allow for possible source discrimination (‘fingerprinting’) of the emeralds investigated in the present study, but not for the aquamarines.