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

Original paper

Kechid, Sid-Ali; Parodi, Gian Carlo; Pont, Sylvain; Oberti, Roberta

Abstract

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, ω = 1.538(2), ε = 1.535(2). No cleavage was observed. Davidsmithite is hexagonal, space group P63 and has unit-cell dimensions: a = 9.982(1) Å; c = 8.364(2) Å; V = 721.74 Å3; 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: ([Ca0.636□0.636]□0.414K0.165Na0.149)Σ2.000Na6.000(Al7.863Fe3+ 0.019)Σ7.882Si8.192O32 (where □ = vacancy). Davidsmithite completes the compositional space of the nepheline-structure group by providing a new root-composition, (Ca□)2Na6Al8Si8O32. It is the Ca-analogue of classical nepheline, to which it is related by the heterovalent substitution of K+ 2 by [Ca2+□]. 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 Å 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 □. 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.

Keywords

davidsmithite • new mineral • crystal structure • EMP analysis • Raman spectroscopy • Ca-silicate • nepheline group • feldspathoid • Liset eclogite pod • Norway