Order and anti-order in Ge-substituted alkali feldspars
Kroll, Herbert; Flögel, Jοsef; Breit, Udo; Löns, Jürgen; Pentinghaus, Horst
European Journal of Mineralogy Volume 3 Number 5 (1991), p. 739 - 750
published: Oct 2, 1991
manuscript accepted: Apr 23, 1991
manuscript received: Dec 26, 1990
ArtNo. ESP147050305004, Price: 29.00 €
Abstract The crystal structures of four Ge-substituted alkali feldspars have been refined by conventional X-ray methods: monoclinic K[AlGe3O8] and triclinic Na[AlGe3O8], and two crystals of Na[AlSiGe2O8] synthesized at 1040°C and 600°C, respectively. The (Al, Si), Ge distribution was determined by site refinement, the Al, Si distribution was estimated from tetrahedral size. Like in natural alkali feldspars, the three-valent Al atoms preferentially occupy the T1 site (s) whereas the four-valent Ge and Si atoms prefer the T2 site (s). However, when differentiating between O- and m-subsites in the triclinic crystals, the distribution of T cations was fully unexpected. In all three Na-feldspar structures Ge prefers the T1O over T1m site. This is in contrast to all other natural and substituted triclinic alkali feldspars refined so far, where the three-valent T cation occupies the T1O site and the four-valent one the T1m site. This phenomenon of anti-order in the Ge-substituted albites is discussed in terms of T-O-T angle preferences.