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NATROMONTEBRASITE, A DISCREDITED MINERAL SPECIES

André-Mathieu Fransolet^1,, François Fontan² and Philippe de Parseval²

¹ Laboratoire de Minéralogie, Université de Liège, Bâtiment B18, Domaine du Sart Tilman, B–4000 Liège, Belgium
² LMTG 5563, Observatoire Midi-Pyrénées, 14, avenue Edouard Belin, F–31400 Toulouse, France

E-mail address: amfransolet{at}ulg.ac.be

Two samples of "natromontebrasite", among which a fragment of the type material, were re-investigated by X-ray-diffraction techniques and chemically analyzed. The X-ray powder-diffraction patterns clearly show the presence of lacroixite, whereas additional weak peaks are tentatively attributed to wardite. Electron-microprobe measurements confirm that Na is heterogeneously distributed within the samples, and essentially located in domains of lacroixite, itself scattered in a mineral of the montebrasite–amblygonite solid-solution series. "Natromontebrasite", originally described as (Na,Li)AlPO₄(OH,F), must thus be considered as a mixture of an OH-rich amblygonite and lacroixite, with subordinate amounts of wardite. The discreditation of "natromontebrasite" has been approved by the IMA Commission on New Minerals and Mineral Names.

Keywords: "natromontebrasite", lacroixite, amblygonite, montebrasite, Canon City, Colorado.