Nepheline Solid Solutions

Gabrielle Donnay, J. F. Schaires and J. D. H. Donnay
Geophysical Laboratory, Carnegie Instituteion of Washington, Washington, D.C., U.S.A.
The Johns Hopkins University, Baltimore, Maryland, U.S.A.

Summary: Published chemical analyses demonstrate that the nepheline formula should be written KxNayCazs-(x+y+z)Alx+y+2zSiSi16-(x+y+2z)OO32, where □ stands for vacant sites. X-ray data are presented for the nepheline phase in four binary systems: Ne-CaAl2O4, Ne-An, Ne-Ab, Ne-Kp. Only in two of these systems do the cell-dimensions change with composition. In the first one, the cell-volume V increases linearly with increasing calcium content; in the last one, two singularities in the curve of V against x divide the phase Na8-xKxAl8Si8O32 into three subphases: subpotassic (0 < x < 0·25), mediopotassic (0·25 < x < 2·00), and perpotassic (2·00 < x < 4·73). Only in the subpotassic range are both high- and low-temperature forms found. Twenty-eight natural nephelines, for which chemical analyses and X-ray data are available in the literature, show that only the potassium content affects cell-dimensions. Although all analysed natural nephelines fall outside the subpotassic range, the re-examination of a Monte Somma specimen studied by Bannister (1931) reveals a few euhedral crystals of subpotassic nepheline in a mediopotassic phase.

Mineralogical Magazine; July 1959 v. 32; no. 245; p. 93-109; DOI: 10.1180/minmag.1959.32.245.02
© 1959, The Mineralogical Society
Mineralogical Society (