The crystal chemistry of silica-rich,alkali-deficient nepheline

Coarse crystals of an extremely silica-rich, potassium-deficient nepheline have been synthesized hydrothermally with albite. Electron microprobe analysis yielded the formula: □_1.76K_o.24Na_6.00Al_6.24-Si_9.76O₃₂. A crystal structure analysis of this nepheline has revealed (1) a disordered Si-Al distribution, (2) full occupancy of the smaller cavity site by sodium atoms, and (3) the larger cavity is vacant except for the minor potassium content, with the size of this cavity being the same as when largely occupied with potassium atoms. In addition, the crystal structure of another nepheline prepared by alkali exchange in molten NaCl showed that when Na atoms replace the K atoms of the large cavity: (1) this cavity does not collapse around the smaller atoms, and therefore, (2) the sodium atoms occupy an offcenter position displaced by about 0.3 Å from the cavity center in order to form a rather one-sided bonding configuration with some of the cavity wall oxygen atoms. These structures further support the strong site preference indicated earlier by Buerger and coworkers and restated by Barth: that for the large cavity K > □ ? Na and for the small cavity site Na>Ca(?)?K, □. Perfect compliance with this site preference scheme would tend to restrict nephelines to the Barth compositional join: □₂Na₆Al₆Si₁₀O₃₂-K₂Na₆Al₈Si₈O₃₂, neglecting the minor Ca component usually present. Thirteen new electron microprobe analyses of nephelines from a variety of occurrences and sixteen additional microprobe analyses from the literature which comply with nepheline-structure allowed stoichiometry, are plotted showing that (1) natural nephelines closely conform to this site preference scheme with less than 10% of the large cavity sites containing sodium atoms, that is, the analyses cluster near the Barth join, and (2) examples of natural nephelines can be found at nearly all intermediate compositions along this join.