Green andradite (demantoid variety) from Muslim Bagh, Zhob District, Balochistan, Pakistan: gemological and mineralogical characterization

Andradite garnet (Ca3Fe2Si3O12) is a rock-forming mineral, typically found in metamorphic rocks, such as serpentinites and skarns. Green andradite, often described with the varietal name "demantoid", represents one of the most appreciate and precious gemstones among the garnet group minerals, due to its color, brilliance, and rarity. The traditional green andradite sources are Russia and Italy (Val Malenco, Central Alps), whereas recently new commercially important deposits were found in Madagascar, Iran, Pakistan (Kaghan Valley), and Namibia (Adamo et al., 2011). Recently, gem-quality green andradite crystals indicated to come from Muslim Bagh, Balochistan (Pakistan) are occasionally available in the Peshawar gem market. The transparent and fine gem-quality dodecahedral crystals show an attractive fancy green color, although they are very small, rarely exceeding 1 ct in weight. The Muslim Bagh area shows a nearly complete ophiolite sequence, mainly composed by peridotites (harzbugite and dunite), partially to completely serpentinized. Many outcrops of dunites contain podiform chromite deposits (Kakar et al., 2013). A suite of gem-quality samples from this locality, provided by the collector S. Hanken (Germany), has been investigated by means of standard gemological methods, SEM-EDS, and EMP chemical analyses. All the samples (0.18-0.98 ct) are transparent, with a fancy medium green color. They are all single refractive with moderate to strong anomalous birefringence and refractive indices >1.79. The measured density ranges from 3.80 to 3.90 g/cm3 and the gems are inert under ultraviolet radiation. These gemological properties are characteristic of andradite, demantoid variety. The samples contain "horse-tail" inclusions and opaque whitish crystals, chemically identified as chrysotile and antigorite, respectively. Opaque blackish crystalline inclusions of chromium-bearing magnetite are also present. The specimens show various growth structures and, in a few cases, dodecahedral twinning. The chemical composition of garnet results in almost pure andradite (Adr>=96 mol%, generally ~98-99 mol%), with minor amounts of Mg, Al, Cr, Ti, V, and Mn. In particular, chromium ranges from below detection limit to maximum 0.92 wt% as Cr2O3, this latter measured near the Cr-rich magnetite inclusions. Chromium causes, along with the major constituent Fe3+, the bright green color of this andradite, such as typical for demantoid associated with serpentinites (Bocchio et al., 2010).