Thematic Articles

With their multitude of colors, gem tourmalines are among the most popular colored gemstones. Spectacular color-zoned tourmalines are valued as gems and crystal specimens, and some complexly zoned crystals contain nearly the entire spectrum of color variation found in the mineral world. The top-quality “neon” blue-to-green, copper-bearing tourmaline, the Paraíba-type, is one of the highest-priced colored gemstones, with values comparable to those of some diamonds. The wide variety and intensity of colors are related primarily to color-producing ions in the structure and to exposure to natural radiation. Gem tourmalines that form in magmatic, pegmatitic environments are most commonly elbaite and fluorliddicoatite species, and the rarer gem tourmalines that develop in metamorphic rocks are generally dravite–uvite species.

Tourmaline is nature’s perfect forensic mineral. From a single grain, the full geological past of its host rock can be reconstructed, including the pressure–temperature path it has taken through the Earth and the changing fluid compositions it has encountered. Tourmaline is able to provide this record owing to its compositional and textural sensitivity to the environment in which it grows, and is able to preserve this record because element diffusion in its structure is negligible. Furthermore, tourmaline has an exceptionally broad stability range, allowing it to record conditions in igneous, sedimentary, metamorphic, and hydrothermal settings. As our mineralogical and geochemical tools advance, we are able to interrogate tourmaline’s memory with increasing precision, making tourmaline a truly powerful indicator of conditions in the Earth.

Tourmaline occurs in diverse types of hydrothermal mineral deposits and can be used to constrain the nature and evolution of ore-forming fluids. Because of its broad range in composition and retention of chemical and isotopic signatures, tourmaline may be the only robust recorder of original mineralizing processes in some deposits. Microtextures and in situ analysis of compositional and isotopic variations in ore-related tourmaline provide valuable insights into hydrothermal systems in seafloor, sedimentary, magmatic, and metamorphic environments. Deciphering the hydrothermal record in tourmaline also holds promise for aiding exploration programs in the search for new ore deposits.

Tourmaline typically forms where crustal rocks interact with migrating hydrous fluids or silicate melts, and its isotopic composition provides a reliable record of the isotopic composition of the fluids and melts from which it crystallized. Minerals of the tourmaline supergroup are exceptional in their physical robustness and chemical variability, and they allow us to extract a uniquely broad range of isotopic information from a single mineral. The chemical variability of tourmaline confronts us with the difficulty of deciphering an extremely complex mineral system, but it also presents us with a geochemical recorder of half the periodic table, a breadth of representation that is unparalleled among minerals. Plate tectonic–scale geochemical cycles, local and regional fluid–rock interactions, magmatic–hydrothermal systems, ore-forming processes, and ages of tourmaline formation have all been reconstructed using this unique isotopic broadband recorder.

Tourmaline sensulato has been known for at least two thousand years, and its unique combination of physical properties has ensured its importance to human society, from technical devices (such as a possible Viking navigational aid and early piezoelectric gauges in the 20th century) to attractive and popular gemstones. The chemical diversity and accommodating nature of its structure combine to make tourmaline a petrogenetic indicator for the wide range of rocks in which it occurs. Recent advances in understanding the structure, site assignments, and substitution mechanisms have led to a new nomenclature for the tourmaline supergroup minerals. Eighteen species have been described to encapsulate the chemical variety found in this intriguing structure.

Tourmaline is an eye-catching mineral, but even more importantly, it has played a significant role in the evolution of scientific thought and, more recently, has been recognized as a medium for recording geologic information, not unlike a DVD. With its plethora of chemical constituents, its wide range of stability from conditions near the Earth’s surface to the pressures and temperatures of the upper mantle, and its extremely low rates of volume diffusion, tourmaline can acquire a chemical signature from the rock in which it develops and can retain that signature through geologic time. As a source as well as a sink for boron, tourmaline is nature’s boron recorder. Tourmaline can be used as a geothermometer, provenance indicator, fluid-composition recorder, and geochronometer. Although long prized as a gemstone, tourmaline is clearly more than meets the eye.