Davidsmithite a new Ca-bearing nepheline-group mineral

Restricted access Davidsmithite
Representative Image: Aquamarine Crystal

Davidsmithite, a newly approved feldspathoid mineral (IMA 2016-070), occurs as a rock-forming mineral in the Liset eclogite pod (Norwegian Caledonides).

The approved electron-microprobe analysis gave the crystal–chemical formula: ([Ca0.6360.636]◰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.

Davidsmithite

Formula: (Ca◰)2Na6Al8Si8O32
Colour: Colourless
Specific Gravity: 2.597 (Calculated)
Crystal System: Hexagonal

Locality: Liset eclogite pod, Liset, Selje, Sogn og Fjordane, Norway

Physical Properties of Davidsmithite

Transparency: Transparent
Colour: Colourless
Cleavage: None Observed
Density: 2.597 g/cm3

Reference:

  1. Sid-Ali Kechid, Gian Carlo Parodi, Sylvain Pont, Roberta Oberti. Davidsmithite, (Ca,◰)2Na6Al8Si8O32: a new, Ca-bearing nepheline-group mineral from the Western Gneiss Region, Norway. DOI: 10.1127/ejm/2017/0029-2667 Published on June 2017, First Published on June 28, 2017
  2. Smith, D.C., Kechid, S-A. and Rossi, G. (1986): Occurence and properties of lisetite, CaNa2Al4Si4O16, a new tectosilicate in the system Ca-Na-Al-Si-O. American Mineralogist 71, 1372-1377. [as K-poor, Ca-rich nepheline structure mineral]
  3. Kechid, S.-A., Oberti, R., Rossi, G., Parodi, G., Pont, P. (2016): Davidsmithite, IMA 2016-070. CNMNC Newsletter No. 34, December 2016, page 1317; Mineralogical Magazine: 80: 1315–1321
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