Lamellar microstructure of amorphous silica from leached labradorite feldspar
Müller, Wolfgang Friedrich; Pentinghaus, Horst; Kronimus, Berndt
published: May 1, 1998
ArtNo. ESP154017202002, Price: 29.00 €
The reaction of 5 to 10 mm sized labradorite feldspar single crystals (composition An51) with H2O-HCl solutions at 200 °C and a pressure of 1.6 Mbar for 5 hours to five days produced solid reaction layers of amorphous silica around a core of labradorite or a solid residual of completely amorphous silica. The dissolution process was non-stoichiometric. Transmission electron microscopy revealed that this silica material has a submicroscopic microstructure characterized by alternating lamellae of different porosity; their periodicity is 123 ± 10 nm. The two kinds of lamellae differ in porosity by a factor of about two. The bulk density of the amorphous silica residual is about 1.5 g/cm3, corresponding to a porosity of about 68 %. The lamellar microstructure of the residual has its origin in the lamellar Boggild microstructure formed by structurally and chemically different plagioclase lamellae in the starting labradorite. The periodicity of the lamellae (124 ± 10 nm) in labradorite is the same as in the amorphous silica residual. The difference in porosity of the lamellae of the residual arises because the dissolution of the Ca- and Al-richer plagioclase lamellae of labradorite, those with the pronounced superstructure, is faster than that of the lamellae poorer in An. A corrugated boundary was observed at the reaction front between the crystal and the amorphous layer, where the more porous lamellae indent into the Anricher plagioclase lamellae richer.