Original paper

Feldspars with trivalent non-tetrahedral cations: Experimental studies in the system NaAlSi3O8 - CaAl2Si2O8 - LaAl3SiO8

Kneip, Hans-Jürgen; Liebau, Friedrich

European Journal of Mineralogy Volume 6 Number 1 (1994), p. 87 - 98

36 references

published: Feb 4, 1994
manuscript accepted: Oct 18, 1993
manuscript received: Jul 22, 1993

DOI: 10.1127/ejm/6/1/0087

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Abstract

Abstract Feldspars with up to about 45 % trivalent cations on M positions were synthesized by heating gels of composition Ca8-2xNaxLaxAl16Si16O64 at 1300ºC and 1 bar. In the range O Na+ + La3+; at x(gel) ≳ 3 this substitution is accompanied by the substitution Ca2+ + Al3+ -> Na+ + Si4+. As x increases, the triclinic anorthite structure gradually becomes monoclinic at x(gel) = 3.05(3). Electron diffraction indicates transformation from P1¯ through I1¯ to I2/c with increasing x. 29Si MAS NMR spectra indicate considerable Al/Si disorder, as well as low long- and medium-range order of the Q4 (4A1) units. The latter is due to distortion of the [SiO4] tetrahedra and considerable variation of the Si-O-T angles caused by rotation of the [TO4] tetrahedra as they adjust to the different M cations

Keywords

lanthanum sodium feldsparsodium lanthanum feldsparfeldsparanorthitealbite