IMA 2016-070 = davidsmithite

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Marco E. Ciriotti
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IMA 2016-070 = davidsmithite

Messaggio da Marco E. Ciriotti » ven 08 set, 2017 11:05

Prossima pubblicazione.

▪ Kechid, S.-A., Parodi, G., Pont, S., Oberti, R. (2017): Davidsmithite, (Ca,□)2Na6Al8Si8O32: a new, Ca-bearing nepheline-group mineral from the Western Gneiss Region, Norway. European Journal of Mineralogy, 29, (in press).

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 growth of a (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.

NdR: Questa specie ha relazioni con il gruppo della nefelina, come già precisato nel tiolo del lavoro, ma anche con la kaliophilite (vedasi gli studi presentati da Enrico Mugnaioli al recente Convegno congiunto di Pisa).
Marco E. Ciriotti

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