Mineral chemistry of tourmalines from the Bottino mining district, Apuane Alps (Italy)

Tourmalines from Bottino essentially preserve primary pre-metamorphic features. In particular, some tourmaline crystals show sharp, concentrical colour zones, interpreted to represent primary growth structures. Post-growth chemical exchange with fluids of probable metamorphic origin have led to the local development of discordant palecoloured tourmaline rims. Microprobe analyses reveal that growth and discordant zonings mainly depend upon Fe/(Fe+Mg) and Na/(Na+Ca) variations, respectively. Colour zoning of the crystals appears to be related to Ti content rather than to Fe/(Fe+Mg) variations. Except for (rare) discordant zoning, the effect of greenschist-facies metamorphism on tourmaline composition appears limited, as shown by the lack of systematic cation partitioning between coexisting tourmaline and metamorphic chlorite.