Demantoid Garnet from Antetezambato, Northern Madagascar—Internal Characteristics and Their Use in Deciphering Geographic Origin

April 29, 2022

Demantoid garnets from Antetezambato, northern Madagascar, contain a characteristic inclusion suite typical of a skarn occurrence. Unlike demantoid from the classic deposits in the Ural Mountains, Russia, and from other serpentinite-related occurrences, the stones from Madagascar do not contain ‘horsetail’ inclusions, but instead host groups of small, rounded diopside grains, fluorapatite, dolomite, calcite, quartz, pyrite and wollastonite, as well as rare native bismuth. Partially healed fissures contain two-phase fluid inclusions (with H2O and a gas bubble) of varying salinity. Negative crystals, commonly large and empty, sometimes also contain H2O. Pronounced growth features and parallel hollow tubes are also present. Demantoid from Namibia likewise has a skarn origin and shows some similar internal features, as well as some that are different, such as the common presence of fluorapatite and small multiphase fluid inclusions, along with the absence of large negative crystals and native bismuth.

Learn more about this topic by J.C. (Hanco) Zwann, published in The Journal of Gemmology, including a review of advocated methods for geographic origin determination.

Read the complete article here.

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