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: ([Ca 0.636 ◰ 0.636 ]◰ 0.414 K 0.165 Na 0.149 ) Σ2.000 Na 6.000 (Al 7.863 Fe3+ 0.019 ) Σ7.882 Si 8.192 O 32 (where ◰ = vacancy)

Davidsmithite completes the compositional space of the nepheline-structure group by providing a new root-composition, (Ca◰) 2 Na 6 Al 8 Si 8 O 32 . 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◰) 2 Na 6 Al 8 Si 8 O 32

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

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