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May-June 2018

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Who's Who in Mineral Names: Edgar Donald Mountain (1901–1985)

Mountainite, KNa2Ca2[Si8O19(OH)]·6H2O, is a hydrated monoclinic phyllosilicate. It has a hardness of 3, is white, transparent to translucent, and has a silky luster. It occurs as fibrous crystals elongate along [010] that may be matted together or clustered into millimeter-sized rosettes. The type locality is the Bultfontein diamond mine in Kimberley, South Africa, where it was found associated with rhodesite, another type species from the Bultfontein kimberlite pipe (Gard, Taylor, and Chalmers 1957; Mountain 1957). Type-specimen material is lodged at the Natural History Museum, London and the National Museum of Natural History, Washington D.C. Apart from the type locality, mountainite is currently known from four other localities: Höwenegg quarry, Baden-Württemberg, Germany; Comet Hill, Hune-doara, Romania; the Yubileinaya pegmatite, Lovozero Massif, Northern Region, Russia; and the Trinite mining claim, California (; accessed January 2017). A Lovozero locality specimen is featured here. At this locality, mountainite is associated with several other species including aegirine, chkalovite, halite, leucosphenite, natrolite, neptunite, polylithionite, raite, sérandite, and zorite (; accessed January 2017). Furthermore, it was the Yubileinaya mountainite that Zubkova et al. (2009) used to define the crystal structure of the species and refine its chemical formula.


Dr. Bruce Cairncross, a consulting editor of Rocks & Minerals, is a professor of geology at the University of Johannesburg.

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