Alkali sulfates with aphthitalite-like structures from fumaroles of the tolbachik volcano, kamchatka, russia. iv. aphthitalite-palmierite regular intergrowths: crystallography, chemistry, and genesis

The first discovery of regular intergrowths of aphthitalite K3Na(SO4)(2) and palmierite K2Pb(SO4)(2) are reported. Crystals of aphthitalite, intergrown with lamellae of palmierite (up to 15 lm thick), along with grains of arcanite, occur in encrustations recovered from the active Arsenatnaya fumarole (Tolbachik volcano, Kamchatka, Russia). These were studied using a combination of scanning electron microscopy, electron probe microanalysis, powder X-ray diffraction, and electron backscatter diffraction techniques. Three types of grain boundaries involving intergrowths of aphthitalite and palmierite were observed: (1) those with a misorientation angle of 608 relative to [001] (twinning), (2) those parallel to the (001) plane, and (3) those with a misorientation angle of 608 relative to [001]. The twinned aphthitalite domains are related by a two-fold rotation about [001] (Dauphine twin law). The heating of aphthitalite crystals containing palmierite lamellae at 400, 600, and 750 degrees C shows a nearly complete redistribution and gradual dissolution of palmierite in the aphthitalite matrix. The character of solid-state transformations in the K2SO4-Na2SO4-PbSO4 system during cooling is controlled by the structural similarity of aphthitalite-type sulfates and palmierite, which contain topologically identical 2 [M(SO4)(2)] (M = Na, Pb) layers.