TRACE ELEMENT CHEMISTRY OF EMERALDS

Schwarz and Giuliani (2001) divided emerald deposits into two groups. Group 1 includes deposits related to pegmatites, with or without associated schist, and Group 2 includes deposits related to tectonic structures. Occurrences in Group 2 include schist without pegmatite and black shales. Thirty-one individual emerald crystals, from emerald deposits in Brazil, Colombia, Pakistan and other locations, representing the range of deposit types, have been analyzed for 35+ elements using Instrumental Neutron Activation Analysis (INAA).

Chromium, Cs, and Ga abundances can be used to distinguish three emerald types. Emeralds associated with pegmatite deposits (Group 1) are characterized by high Cs (1300-1600 ppm) and low Cr (500-2000 ppm) contents. Emeralds associated with schist (Group 2a) are characterized by low Cs (100-600 ppm) and high Cr (4,000-9,000 ppm). Emeralds associated with black shales (Group 2b) have low Cr (<2,000 ppm) and Cs (<200 ppm) content. Emeralds associated with pegmatites generally have lower Ga content (<10 ppm versus >10 ppm). Group 2a emeralds have higher Sc content compared to Group 1 (>500 ppm versus <500 ppm). Antimony, similar to Cs, effectively discriminates between the Group 1 and Groups 2a and 2b emeralds. Group 1 has high Sb (8-14 ppm), Group 2a has lower Sb (0.5–5 ppm), and Group 2b has very low Sb (<0.5 ppm) content. The difference in Cr, Cs, Ga, Sc, and Sb content allows us to discriminate between emeralds from different mineral districts and geologic settings.

Preliminary rare earth data (REE) gives very low concentrations, with a few exceptions, of the REEs (generally <1x chondrites) in emeralds from all localities. The REE chondrite-normalized patterns tend to be flat. The REE data thus support a model for emerald formation involving saline brines.