A new deposit of grandidierite, considered one of the world’s rarest gems, has been discovered in southern Madagascar. The new deposit is outside the town of Tranomaro, near the original locality of Andrahomana. It occurs in the form of strong bluish green to greenish blue translucent to transparent crystals measuring up to 15 × 7 × 3 cm. Grandidierite is the magnesium end member in the solid-solution series with ominelite as the iron end member. The studied samples have a very low Fe/(Mg + Fe) ratio. This confirms that the Tranomaro deposit, together with Johnsburg in New York State, provides the purest grandidierite ever found. The crystals host inclusions of Cl-apatite, zircon, and monazite. The paragenesis also includes plagioclase, phlogopite, enstatite, diopside, dravite, and sapphirine (locally as gem-quality crystals). Transparent crystals have been faceted, yielding small but eye-clean jewelry-quality gems.
Named after French naturalist Alfred Grandidier (1836–1912), grandidierite is an extremely rare orthorhombic Mg-Fe aluminous borosilicate with the formula (Mg,Fe)Al3(BO3)(SiO4)O2 (Lacroix, 1902, 1922a,b). The material was first discovered at the cliffs of Andrahomana, on the southern coast of Madagascar. Grandidierite is described as a bluish green to greenish blue mineral (figure 1); the blue component increases with Fe content. It is transparent to translucent with a pale yellow to colorless, greenish blue, and blue trichroism. The larger elongate euhedral orthorhombic crystals, which may measure up to 8 cm, are often strongly corroded.
Since its discovery, grandidierite has been found as a rare accessory mineral in aluminous boron-rich pegmatite; in aplite, gneiss, and crystalline rock associated with charnockite; and in rock subjected to local high-temperature, low-pressure metamorphism (contact aureoles and xenoliths). In addition to Madagascar, it has been reported from New Zealand (Black, 1970), Norway (Krogh, 1975), Suriname (de Roever and Kieft, 1976), Algeria (Fabriès et al., 1976), Italy (van Bergen, 1980), Malawi (Haslam, 1980), India (Grew, 1983), the United States (Rowley, 1987; Grew et al., 1991), Canada (Lonker, 1988), Antarctica (Carson et al., 1995), the Czech Republic (Cempírek et al., 2010), and other localities. Yet gem-quality grandidierite is extremely rare. Facetable gem material larger than a millimeter has only been found in Madagascar and Sri Lanka; the latter is the source of a clean faceted specimen weighing 0.29 ct (Schmetzer et al., 2003).
The type locality described by Lacroix (1902) was visited in 1960 by C. Mignot, who was unable to find additional grandidierite at the site (Béhier, 1960a,b), as the small pegmatite was depleted. Since then, grandidierite has been reported in a few other localities in southern Madagascar that have also since been depleted. The present study focuses on a new deposit discovered in May 2014 near Tranomaro, close to the original locality of Andrahomana (figure 2). The aim of this study is to determine the material’s properties and to evaluate its gem potential.