Davidsmithite, (Ca,square)(2)Na6Al8Si8O32: a new, Ca-bearing nepheline-group mineral from the Western Gneiss Region, Norway

Davidsmithite, a newly approved feldspathoid mineral (IMA 2016-070), occurs as a rock-forming mineral in the Liset eclogite pod (Norwegian Caledonides). It is transparent, colourless, uniaxial negative, omega = 1.538(2), epsilon = 1.535(2). No cleavage was observed. Davidsmithite is hexagonal, space group P6(3) and has unit-cell dimensions: a = 9.982(1) angstrom ; c = 8.364(2) angstrom ; V = 721.74 angstrom(3); Z = 1; the c: a ratio is 0.8379; the calculated density is 2.597 g cm(-3). The approved electron-microprobe analysis gave the crystal-chemical formula: ([Ca-0.636 square(0.636)]square 0.414K0.165Na0.149)(Sigma 2.000)Na-6.000(Al7.863Fe0.0193+)Sigma 7.882Si8.192O32 (where square = vacancy). Davidsmithite completes the compositional space of the nepheline-structure group by providing a new root-composition, (Ca square)(2)Na6Al8Si8O32. It is the Ca-analogue of classical nepheline, to which it is related by the heterovalent substitution of K-2(+) by [Ca2+square.]. Most of the Ca2+ ions are situated in the same atomic position as K+ in nepheline, but some occur in a new and disordered (Ca) atomic position, whose centre is shifted by 2.18 angstrom along the 6-fold axis. The studied samples show some solid-solution towards the other two possible end-members of the nepheline compositional space, so that the channel site contains all of Ca and K in the unit formula, with some Na and square. In the Liset eclogite pod, davidsmithite occurs in retrogressed, formerly jadeite-rich zones; it commonly overgrows lisetite and is associated with albitic plagioclase and taramitic amphibole. This eclogite occurrence is noted for its bulk-rock compositions rich in (Na+Al) and poor in (K+Mg). The paucity in K prevented the growth of nepheline, and the paucity in Si in precursor jadeite led to the growth of a feldspathoid (davidsmithite) as well as of lisetite; a feldspar (albite or oligoclase) also occurs nearby.