Boron-rich tourmalines and their application in geothermobarometry: a review

Tetravalent Si, typically the predominant cation occupying the tetrahedral (T) site of the tourmaline structure, can be partially replaced by B 3+ in Al-rich tourmalines. It is relatively common for Al-rich and Li-containing tourmalines (without significant amounts of Mg) to also contain some [4] B when the Al 2 O 3 content is > 38 wt%. The highest boron content (~ 2 apfu [4] B) in a natural tourmaline was found in the Al-rich tourmaline ertlite from Mogok Township, Mandalay Region, Myanmar, whereas even higher contents of up to ~ 3 apfu have been reported for synthetic Al-rich tourmaline. An updated formula for calculating the < T-O> distance using different T-site occupants has been provided. To calculate the content of [4] B, a second-degree polynomial equation ( r ² = 0.98) using the unit cell volume was proposed. This formula can also be used for relatively high amounts of [4] B. Formulae for low-pressure and high-pressure conditions have been proposed for calculating the crystallization temperature of Al-rich tourmalines when the content of [4] B is known. Occurrence of Al-rich tourmalines with > 2.2 apfu [4] B is only expected in rocks that have reached high-pressure or ultra-high pressure conditions.