Compositional Variation and Crystal-Chemical Characterization of a Watermelon Variety of Tourmaline from Anjanabonoina, Central Madagascar

Abstract

A polychrome tourmaline crystal from Anjanabonoina pegmatite (Madagascar) was characterized using a multi-analytical approach. The sample showed a complex concentric zoning and a wide range of colors typical of the variety known as “watermelon”. The sample was cut perpendicularly to the c axis. The basal slice exhibits a rim characterized by narrow, differently colored layers parallel to the prism faces and a relatively homogeneous triangular core. Four main pronounced color zones were identified from the rim to core: a dark green rim (M1RVS); a pale green rim (M1RVC); a pale pink rim (M1CR); and a brownish yellow core (M1CG). Compositional variations in the basal slice were studied by scanning electron microscopy and electron microprobe analyses (WDS mode). The Li content was determined via micro-laser-induced breakdown spectroscopy. To deeply characterize the sample, single crystal structure refinement was also performed on fragments extracted from the four zones. The results show that the polychrome tourmaline sample consists of two different species: the three outer zones are Mn-rich fluor-liddicoatite, whereas the inner zone is Mn-rich fluor-elbaite. The structural and compositional characterization of the color zoning shows that each step of the tourmaline growth is related to a change in the geological environment.