GEMSTONES WITH TOURMALINE AS AN ACCESSORY

Tourmaline may form as a gemstone itself, but also as an accessory mineral with other gemstones. Because gemstone formation typically requires unusual petrochemical conditions, the ability of the tourmaline structure to accommodate a wide range of chemical constituents and to form over a wide P-T-X range make it a potentially valuable indicator of formation conditions for the associated gemstones. Selected examples demonstrate this potential. Emerald, the Cr (±V)-bearing variety of beryl, is a rare mineral that requires superimposing dichotomous geochemical reservoirs i.e. those enriched in Be and Cr (±V) and lacking Fe. Associated tourmalines typically reflect this hybrid chemical environment. For example, tourmalines associated with Swat (Pakistan) emeralds are Cr-rich dravites and oxy-dravites that develop in carbonate-talc altered ultramafic rocks (providing Cr) invaded by quartz veins sourced from younger granitoid intrusives (providing Be). Sapphire and ruby varieties of corundum require a combinaton of a silica-undersaturated aluminous environment with sources of Cr, Fe and Ti (for coloration). One example in which both ruby and sapphire develop with tourmaline, is in ultramafic enclaves in aluminous schists from Westland (New Zealand). Here, coexisting tourmalines are Al- and Cr-rich dravites consistent with the gemstone compositions. Diamonds develop at high pressures (>3 GPa) most commonly associated with mantle-like lithologies, but they have been identified in crustal lithologies that have subducted to depth by tectonic processes. Tourmalines associated with UHP terranes are usually dravitic to schorlitic. However, tourmalines with diamond inclusions from a UHP quartzofeldspathic gneiss from the Kokchetav Massif (Kazakhstan), has an unusual K-dominant X-site, oxy-magnesian composition, the new species maruyamaite. Gemmy amazonite is a variety of microcline in granitic pegmatites where Pb incorporated into the structure imparts a green-blue color. In an example from Luc Yen district (Vietnam), alteration veins within an amazonite (0.7 wt% PbO) develop Li-rich tourmaline with as much 17.5 w% PbO. Consequently, tourmaline is not only an accessory mineral to gemstones but it may also be used as an exploratory tool for these precious minerals.