Origin of Bluish Pectolite Aka Larimar from the Dominican Republic: Constraints from Mineralogy and Geochemistry

The commonly known larimar is usually a gem-quality, bluish pectolite (NaCa2Si3O8OH), intergrown with other associated minerals such as natrolite, prehnite, calcite, chlorite, found only in the Dominican Republic. Less frequently it shows consecutive and/or sequential green, blue, and white coloration within single amygdales/veins developed in altered (chloritized, serpentinized) Cretaceous basalts of the Dumisseau Formation. The origin of larimar is linked to the circulation of low-temperature (<170 °C) alkali-rich deuteric fluids that were later mixed with meteoric waters. Variations of CL colors, ranging from weak brown in green pectolite to strong yellow (Mn2+-activated) in blue and white species, were ascribed to the transition from oxic to suboxic crystallization environment within the single larimar bodies. These changes could be induced by the burial of magmatic rocks and resulted in the emergence of early zeolite (natrolite), and subsequent formation of native copper in a sulphur-deficient hydrothermal environment. Pectolite-forming elements (e.g. Ca and Na), followed by such transition metals as Cu, Cr, Co, and Ni, were released via the alterations of magmatic minerals (diopside, aegirine-augite, and chromite-magnetite) and/or leached from basaltic groundmass. Decoupling between δ18O values of blue (up to 10.67‰), green (up to 9.66‰), and white pectolite (up to 15‰) was maintained by a progressive temperature drop during ongoing pectolitization and/or presence of the boiling-related environment. Meanwhile, anomalously low δD values (from –390.51‰ to –294.02‰) indicate that the crystal structure of pectolite tends to incorporate light hydrogen isotope regardless of the original δD values in mineralizing fluids.