Tourmaline is the most significant borosilicate mineral in the Earth's crust due to its exceptional stability and its capacity to incorporate a wide range of elements from its surrounding environment. The general chemical formula of tourmaline is X Y₃ Z₆ T₆ (BO₃)₃ O₁₈ (V)₃ (W), where the most common substituents are X = Na, Ca, K or [  ] (X-site vacancy); Y = Al, Li, Fe²⁺, Mg, Mn²⁺, Fe³⁺, V³⁺, Cr³⁺, Ti⁴⁺; Z = Al, Mg, Cr³⁺, V³⁺, Fe²⁺ and Fe³⁺; T = Si, Al, B; V = OH^1-, O^2- and W = F^1-, O^2-, OH^1- [1]. As a result, it serves as a robust recorder of the geochemical conditions under which it forms. Structurally, tourmaline is an asymmetric cyclosilicate that exhibits both pyroelectric and piezoelectric properties. It occurs in a diverse range of geological and geochemical settings. The temperature and pressure stability range of tourmaline extends from below ~150°C to over 900°C and from ~1 MPa to over 4 GPa, encompassing nearly the entire range of conditions found in the Earth's crust [2]. In addition to its thermal and baric stability, it has extreme mechanical durability and is an important detrital heavy mineral in clastic sedimentary environments for provenance. Once buried and heated in a metamorphic setting, this sedimentary tourmaline grain commonly serves as a nucleus for further tourmaline. As it develops in the metamorphic environment, the new metamorphic tourmaline changes its composition in response to the chemical environment. Further, once it is crystallized, it holds that compositions without homogenizing at elevated temperatures i.e. it is an exceptional record of the evolving chemical environment during metamorphism.

Two case studies showcase how tourmaline's chemistry and textures can be used to trace the geochemical and mineralogical evolution of metamorphic rocks, using imaging and micro-analytical data from the LSU Electron Microprobe. Case study 1 – Detrital tourmaline grains and their associated tourmaline overgrowths in 380 Ma low-grade clastic metasedimentary rocks from Maine, USA [3]. The chlorite-zone metasedimentary rocks contain tourmaline with three well-defined zones: a detrital core with a metamorphic overgrowth consisting of an inner mantle and an outer rim of an overgrowth.  The detrital cores were derived from a variety of source rocks, including low-grade siltstone, Al-poor metasandstone, medium-grade aluminous metapelite, low-Li granite, Li-rich granite, and calcareous metasediment. This diversity suggests a heterogeneous sedimentary input and complex provenance history. Metamorphic overgrowths reflect diagenetic to low-grade metamorphic processes. A diagnostic chemical trend is Mg replaces Fe²⁺ in the metamorphic overgrowths at a 1:1 ratio reflecting changes in the metamorphic environment with progressive metamorphism. Case study 2 – Tourmaline from a 550-500 Ma metamorphosed evaporite from Namibia [4]. Tourmaline from meta-evaporitic tourmalinites of central Namibia share compositional similarities with tourmalines from other meta-evaporite localities worldwide, suggesting a common geochemical process. The meta-evaporitic tourmalines are generally sodic, magnesian, moderately-to-highly depleted in Al, and enriched in Fe³⁺ with a diagnostic substitution of Fe³⁺ replacing Al at a 1:1 ratio. These latter tourmaline compositions reflect metasomatic processes that produced these unusual bulk compositions found in evaporite deposits and/or the influx of a reactive fluid that eliminated any earlier chemical signatures of meta-evaporitic fluids or protoliths. This chemical feature is attributed to the influence of oxidizing, highly saline, boron-bearing fluids that are associated with these meta-evaporite lithologies. These studies demonstrate the unparalleled geological history embedded in a crystalline solid.