Blue Amblygonite-Montebrasite from Rwanda
Amblygonite-montebrasite, with the formula LiAlPO4(F,OH), is a fluorophosphate mineral series found in granitic pegmatites and pegmatite-related environments. The Pala district in California and the Black Hills in South Dakota are historic U.S. sources of amblygonite-montebrasite for industrial use, but most of the world’s well-known pegmatite districts have also produced considerable quantities of amblygonite-montebrasite.
A relatively common rock-forming mineral in these specific environments, the mineral is typically opaque and contains abundant fractures. Its color is often an unremarkable creamy white, causing it to blend in with associated feldspars. All of these factors limit its use as a gemstone. When used as a gem material, it is often colorless to yellow-green.
Geologists are interested in this mineral because of its high lithium content (up to 10%), which often suggests the presence of other (and easier to process) lithium minerals nearby. With increasing demand for lithium ores, exploration teams around the world are paying closer attention to amblygonite-montebrasite as an indicator of high lithium content in geological environments. This was the case in western Rwanda, an area known for its pegmatite-related tin deposits, where local geologists discovered an unusual variety of amblygonite-montebrasite in multiple pegmatites and sent it to mineral traders in Bangkok.
The new material (see above) differs from classic varieties in that its color is bright blue, sometimes associated with creamy white patches in a mottled pattern. All other characteristics match the known properties of amblygonite-montebrasite.
Confocal Raman spectroscopy showed a perfect match with amblygonite for several of the whitish and all of the bright blue patches. The position of certain peaks is related to the fluorine concentration in the mineral and can help to estimate the distribution between both end members (B. Rondeau et al., “A Raman investigation of the amblygonite-montebrasite series,” Canadian Mineralogist, Vol. 44, No. 5, 2006, pp. 1109–1117). The position of the peak around 1060 cm–1 was in the lower range (1052 cm–1 or lower), which points to a very fluorine-poor composition, near pure montebrasite. Raman spectroscopy performed at GIA’s Bangkok laboratory also revealed the presence of other minerals in the creamy white matrix. This included a mix of complex phosphates that share mineralogical and chemical similarities (e.g., berlinite, souzalite, and trolleite, confirmed by Raman spectroscopy) as well as feldspar. The only exception in terms of coloration consisted of small dark blue spots of the mineral scorzalite, with the formula (Fe2+,Mg)Al2(OH,PO4)2, a common secondary phase in complex, phosphate-rich pegmatites.
Despite advanced techniques indicating that this newly discovered variety, as well as the majority of previously documented gems, are closer to montebrasite (the OH-rich end member), the gemological community keeps referring to these materials as amblygonite. Amblygonite-montebrasite remains a rare gem that is unfamiliar to the general consumer. It requires considerable care due to its perfect cleavage and hardness of approximately 6, making it less suitable for daily wear. With this new addition to its color range, amblygonite-montebrasite might gain more recognition.
