NO ES & NEW TECHNIQUES A NEW DEPOSIT OF GEM-QUALITY GRANDIDIERITE IN MADAGASCAR Delphine Bruyère, Claude Delor, Julien Raoul, Rufin Rakotondranaivo, Guillaume Wille, Nicolas Maubec, and Abdeltif Lahfid Since its discovery, grandidierite has been found as A new deposit of grandidierite, considered one a rare accessory mineral in aluminous boron-rich peg- of the world’s rarest gems, has been discovered matite; in aplite, gneiss, and crystalline rock associ- in southern Madagascar. The new deposit is out- ated with charnockite; and in rock subjected to local side the town of Tranomaro, near the original lo- high-temperature, low-pressure metamorphism (con- cality of Andrahomana. It occurs in the form of tact aureoles and xenoliths). In addition to Madagas- strong bluish green to greenish blue translucent car, it has been reported from New Zealand (Black, to transparent crystals measuring up to 15 × 7 × 1970), Norway (Krogh, 1975), Suriname (de Roever and 3 cm. Grandidierite is the magnesium end mem- Kieft, 1976), Algeria (Fabriès et al., 1976), Italy (van ber in the solid-solution series with ominelite as Bergen, 1980), Malawi (Haslam, 1980), India (Grew, the iron end member. The studied samples have 1983), the United States (Rowley, 1987; Grew et al., a very low Fe/(Mg + Fe) ratio. This confirms that 1991), Canada (Lonker, 1988), Antarctica (Carson et the Tranomaro deposit, together with Johnsburg al., 1995), the Czech Republic (Cempírek et al., 2010), in New York State, provides the purest grandi- and other localities. Yet gem-quality grandidierite is dierite ever found. The crystals host inclusions of extremely rare. Facetable gem material larger than a Cl-apatite, zircon, and monazite.The paragenesis millimeter has only been found in Madagascar and Sri also includes plagioclase, phlogopite, enstatite, Lanka; the latter is the source of a clean faceted spec- diopside, dravite, and sapphirine (locally as gem- imen weighing 0.29 ct (Schmetzer et al., 2003). quality crystals). Transparent crystals have been faceted, yielding small but eye-clean jewelry- quality gems. Figure 1. This 0.11 ct round brilliant grandidierite is from a new deposit near the town of Tranomaro, in southern Madagascar. Photo by Delphine Bruyère. amed after French naturalist Alfred Grandidier (1836–1912), grandidierite is an extremely rare orthorhombicN Mg-Fe aluminous borosilicate with the formula (Mg,Fe)Al (BO )(SiO )O (Lacroix, 1902, 3 3 4 2 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 transpar- ent to translucent with a pale yellow to colorless, greenish blue, and blue trichroism. The larger elon- gate euhedral orthorhombic crystals, which may measure up to 8 cm, are often strongly corroded. See end of article for About the Authors and Acknowledgments. GEMS &G EMOLOGY, Vol. 52, No. 3, pp. 266–275, http://dx.doi.org/10.5741/GEMS.52.3.266 © 2016 Gemological Institute of America 266 NOTES & NEW TECHNIQUES GEMS & GEMOLOGY FALL 2016 D Sedimentary rocks N Vohibory Domain -D Androyan Domain Anosyan Domain D MADAGASCAR D Magmatic Androy Massif Ihosy Magmatic Ambalavao suite D 1 RANOTSARA Shear zones • ~ • Grandidierite-bearing localities Figure 2. A map of SHEAR ZONE Madagascar south of the Ranotsara shear zone showing the loca- tion of grandidierite- bearing deposits. 1: Ihosy, 2: Esira, 3: Vohi- bola, 4: Nampoana, 5: BETROKA BELT Andrahomana, 6: Tra- nomaro (this study). Modified after Razaka- BEKILY BELT manana et al. (2010) TRANOMARO BELT and Roig et al. (2012). Androy 2 VOHIBORY BELT Massif • Mahafaly Plateau Anosy Mountains 6 Ampanihy • FORT DAUPHIN BELT 3 • 4 Androhondroho Plateau • Tolanaro •5 Cape Andrahomana Karimbola Plateau 0 50 km The type locality described by Lacroix (1902) was GEOLOGICAL BACKGROUND visited in 1960 by C. Mignot, who was unable to find The crystalline bedrock of Madagascar is described additional grandidierite at the site (Béhier, 1960a,b), in terms of six geodynamic blocks that define the as the small pegmatite was depleted. Since then, constituent elements of the Precambrian Shield; grandidierite has been reported in a few other locali- these are, from southwest to northeast, the Vohibory, ties in southern Madagascar that have also since been Androyan-Anosyan, Ikalamavony, Antananarivo, depleted. The present study focuses on a new deposit Antongil-Masora, and Bemarivo domains. As the re- discovered in May 2014 near Tranomaro, close to the sult of a major five-year (2007–2012) study funded by original locality of Andrahomana (figure 2). The aim the World Bank, the Precambrian geology of Mada- of this study is to determine the material’s properties gascar has been reinterpreted. The Antongil-Masora and to evaluate its gem potential. and Antananarivo domains are now considered part NOTES & NEW TECHNIQUES GEMS & GEMOLOGY FALL 2016 267 of an even larger craton—the Greater Dharwar cra- Tolanaro (Farges, 2001) and at Sakatelo, Ampamatoa, ton—that encompassed part of present-day India and Marotrana, and Sahakondra, near Esira (McKie, 1965; Madagascar and was consolidated approximately Farges, 2001). It was also found in association with 2.45–2.5 billion years ago (Tucker et al., 2012). The serendibite and tourmaline in anatectic gneiss near Ranotsara shear zone marks the approximate bound- Ihosy (Nicollet, 1990). These reports relate only mi- ary between the Archean crust to the north and a croscopic crystals of grandidierite, and the Lacroix more recent basement to the south. The Androyen- finding from a century ago remains the only one of Anosyen domain of southern Madagascar is indeed a gemological interest. Grandidierite was recently re- continental terrane of Paleoproterozoic age (1.8–2.0 ported from a mine near Tolanaro, with no further in- billion years ago). It was accreted to the Vohibory do- formation yet (Vertriest et al., 2015). main, an oceanic terrane of early Neoproterozoic age, in Cryogenian time (about 0.62–0.64 billion years LOCATION AND MINING ago). A first deformation (600–650 Ma) gave rise to A new deposit of grandidierite was discovered in May recumbent isoclinal folds and a regional shallow-dip- 2014 by two of the authors and their mining team, ping foliation, with a second deformation (500–530 about 15 km from the village of Tranomaro. The area Ma) producing steep north-south-trending shear is located in the Amboasary district of southern zones that separate five gneissic tectonic belts. The Madagascar’s Anosy region (again, see figure 2), 60 last episode of Madagascar’s Neoproterozoic geody- km northwest of Cape Andrahomana. Access from namic evolution is marked by the alkaline magma- Tolanaro is via a 60 km paved road west to Am- tism of the Ambalavao suite, which forms major boasary Atsimo, followed by a rough, unpaved 50 km batholitic intrusions (up to 1400 km²). Numerous road north to Tranomaro that requires a four-wheel- pegmatitic vein intrusions associated with this drive vehicle. Final access from the village of Tra- episode are directly related to gem and rare earth el- nomaro to the deposit is half a day by foot only. ement (REE) mineralization. A final magmatic activ- Security is a problem because bandits operate throughout the region. Mining is done by hand due to the locality’s re- mote location, its limited production, and its irregu- In Brief lar, discontinuous veins in opposition to large linear • A new deposit of gem-quality grandidierite was discov- veins. The deposit itself extends over a few acres. ered in May 2014 in southern Madagascar. The weathered pegmatite is exploited by near-surface • The very low Fe/(Mg + Fe) ratio makes these crystals, artisanal and small-scale mining (figure 3). Using together with samples from Johnsburg in New York spades and pickaxes, about 12 miners dig holes up to State, the purest grandidierite ever found. a depth of 15 meters. These open-air corridors cross- • The ratio of gem-quality crystals is about 1 in 10,000. cut two grandidierite-bearing veins separated by 30 • A few dozen rough transparent crystals have been cm to a few meters. The uppermost tunnel is called faceted, yielding small but eye-clean jewelry-quality “Vein 1,” while the deeper one is “Vein 2.” The gems, including fewer than 10 that are above 1 carat. rough blue crystals are manually extracted and sorted on-site. The workers carefully remove the valuable mineral specimens to avoid damaging them. Be- ity during the Cretaceous period is expressed by the tween May 2014 and March 2016, 800 kg of rough megavolcanic Androy Massif. specimens were produced. Mining is still in progress A grandidierite-bearing pegmatite is found in the as of this writing. Anosyan domain (again, see figure 2). At Cape Andra- homana, where grandidierite was first described, it MATERIALS AND METHODS was associated with quartz, microcline, red alman- Using standard gemological methods, we tested a dine, pleonaste, andalusite, and biotite in a pegmatite dozen grandidierite crystals from the two veins near and an aplite (Lacroix, 1922a,b). Grandidierite was also Tranomaro for pleochroism, fluorescence reaction, found 30 km to the north, near Vohibola, in associa- and specific gravity at the French Geological Survey tion with serendibite and sinhalite in a diopsidite- (BRGM) in Orléans, France. After the samples were bearing paragneiss (Razakamanana et al., 2010). It was prepared as polished thin sections, we observed them associated with quartz in the Nampoana quarry near under a polarizing microscope. The crystals’ chemi- 268 NOTES & NEW TECHNIQUES GEMS & GEMOLOGY FALL 2016 Figure 3. At the new deposit in Madagascar, grandidierite is exploited by artisanal and small-scale mining (left), and rough crystals are sorted on-site (right). Photos by Rufin Rakotondranaivo. cal homogeneity was investigated with a scanning 6° to 75° 2 on a Bruker D8 Advance diffractometer θ electron microscope (SEM) on sections that had been with Cu-K radiation ( =1.5418 Å, 40 kV, and 40 α λ carbon coated under vacuum (approximately 20 nm mA) and a Lynx-Eye 1D detector, using a step size of thick).