Nepheline [KNa₃(Al₄ Si₄O₁₆)]

Structure Hexagonal
Space Group P6₃ (No. 173)
a=10.05 Å, c=8.38 Å
a=90.0, b=90.0, g=120.0
Z=2

Atomic Positional Parameters

Al1 2b 0.333 0.667 0.188
Al2 6c 0.090 0.328 0.686
K   2a 0.000 0.000 0.998
Na  6c 0.000 0.444 0.989
O1  2b 0.333 0.667 0.990

O2  6c 0.020 0.312 0.499
O3  6c 0.178 0.516 0.738
O4  6c 0.156 0.512 0.260
O5  6c 0.215 0.264 0.314
O6  6c 0.215 0.264 0.685

Si1 2b 0.333 0.667 0.812
Si2 6c 0.090 0.328 0.313

Reference
T Hahn and MJ Buerger (1955) Zeits. Krist., 106, 308

Mineral Chemistry
Nepheline is a member of the felsphathoid group of minerals. These crystallise from silica deficient, alkali (Na,K) rich melts, forming anhydrous framework alumino-silicates which are richer in interstitial cations than feldspars. Due to 4- and 6-membered tetrahedral linkages in these frameworks, felspathoids tend to be less dense than feldspars.

The framework of nepheline is derived from high-tridymite (b-SiO₂) - aluminium replaces silicon in half the tetrahedra, with sodium and potassium located in interstitial voids. Cavities in which potassiums are located are nearly hexagonal in shape, whereas cavities occupied by sodiums are irregularly shaped. The ratio of sodium to potassium, ideally 3:1, does vary; above 1000^oC, there is complete solid solution between nepheline and kalsilite (KAlSiO₄), a rare mineral formed in potassic lavas [1]. Nepheline is found in silica-deficient igneous rocks including nepheline-syenites and phonolites. It is also found in rocks formed from the metasomatic alteration of calcium-rich sediments by basic intrusives.

[1] J Ferry and JG Blencoe (1978) Subsolidus phase relations in the nepheline-kalsilite system at 0.5, 2.0 and 5.0 kBar. American Mineralogist, 63, 1225-1240.