Petrographic contribution on the larimar host rock (Dominican Republic) and some notes about its origen

Abstract

Larimar is an ornamental and semiprecious stone known worldwide as being iconic of the Dominican Republic. This rock is composed mainly of pectolite, usually seen associated with natrolite and calcite, as well as carbonaceous material, chalcocite and hematites. Sometimes, disseminations such as native copper, also occur amongst several other minerals. The rock crops out in a very small area (< 0.3 km2 , Los Checheses-Los Chupaderos mine or simply the Larimar mine) as a result of the transpressional movement of the Arroyo Seco fault which exposed the basement. The host rock is composed mainly of olivine basaltic lava flows (15 - 18% olivine, 6 - 8% pyroxene, 5 - 10% plagioclase and < 1% opaque minerals in a 64 to 72% groundmass), as well as related basaltic breccias. These volcanic rocks are strongly altered (by hydrothermal, tectonic and weathering processes) at different scales. Olivine phenocrysts were highly serpentinized and altered to iddingsite; pyroxene phenocrysts have some degrees of argillization and sericite; and carbonate has formed minerals from plagioclase crystals. Clays and quartz veinlets are also present as secondary minerals. Pectolite and natrolite minerals fill open spaces in the volcanic rock (vesicules, fractures, cavities, crustification and comb structure vugs) and substitute organic pieces (wood) by an epygenic process as a result of temperatures of 200°- 340°C, during a phyllic epithermal transition to mesothermal (quarz-sericitic) alteration. Larimar is not just a unique gemstone but it is also a very rare petrification process. Therefore we propose the term xilolarimar. The formation environment is suggested to be an oceanic island construction prior to the Eocene time, probably Late Cretaceous. Further studies are necessary to understand the blue pectolite (a colour mainly linked to Mn, V and Cu), its formation mode and its petrogenic environment.