Trinepheline and fabriesite: two new mineral species from the jadeite deposit of Tawmaw (Myanmar)
Ferraris, Cristiano; Parodi, Gian Carlo; Pont, Sylvain; Rondeau, Benjamin; Lorand, Jean-Pierre
European Journal of Mineralogy Volume 26 Number 2 (2014), p. 257 - 265
published: Mar 1, 2014
ArtNo. ESP147052602006, Price: 29.00 €
Two new mineral species, trinepheline (NaAlSiO4) and fabriesite (Na3Al3Si3O122H2O), are described from late-stage metamorphic veins of the jadeite deposit of Tawmaw-Hpakant (Myanmar). Both minerals and their names were approved by the IMA Commission on New Minerals and Mineral Names (IMA 2012–024 and IMA 2012–080). The name trinepheline is known in literature for the polymorphs of synthetic NaAlSiO4 with a value of the c parameter that is three times that of nepheline. Fabriesite is named in memory of Jacques Fabriès (1932–2000), former professor of the “Muséum National d'Histoire Naturelle” in Paris (France). Fabriesite and trinepheline occur intimately intergrown together with nepheline, more rarely with albite and other feldspar-group phases such as banalsite and stronalsite; other associated minerals are jadeite and secondary products like natrolite and harmotome. All phases have been identified via electron backscatter diffraction (EBSD) patterns. Both fabriesite and trinepheline are pseudomorph after jadeite and occur as skeletal allotriomorphic crystals up to 15–20 μm long and 5–10 μm wide. They are white to yellowish in hand specimen, colourless in thin section; the streak is white and the lustre appears vitreous to greasy; they are non-fluorescent; Mohs' hardness is 5–5½. Empirical formulae (EMPA analysis) are very close to the ideal compositions with traces of Ca and K for trinepheline, and of Ca, K, Ba, Mg, Fe, and Mn for fabriesite. Calculated densities are 2.642 g cm3 for trinepheline (space group P61, a = 9.995 Å, c = 24.797 Å) and 2.386 g cm3 for fabriesite (space group Pna21, a = 16.426 Å, b = 15.014 Å, c = 5.223 Å), respectively. The strongest five lines in the calculated X-ray powder diffraction patterns [d (Å) (I)(hkl)] are: 3.163(100)(122), 3.834(81)(023), 4.133(49)(006), 3.272(40)(120) and 2.403(31)(127) for trinepheline; 3.41(100)(240), 4.41(77)(201), 2.97(70)(421), 2.61(40)(002) and 8.21(36)(200) for fabriesite.