Behaviour of potassium feldspar at high water pressures

• Main Author: Thompson, Pauline
• Published: University of Edinburgh 1995
• Subjects: 549
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.662876

Some natural potassium feldspars have been shown to contain traces of structural "water" which may have been incorporated into the structure at high partial pressures of water. At even higher partial pressures of water potassium feldspar is known to break down to form a hydrated phase (referred to as "sanidine hydrate") which is structurally comparable to cymrite which forms from barium feldspar (celsian) at high pressures. This study investigates the capacity for potassium feldspar and "sanidine hydrate" to act as reservoirs for hydrous fluids in the Earth's crust and upper mantle, and establishes the pressure-temperature range of stability of the hydrate phase. Sanidine and "sanidine hydrate" have been synthesised from gels at a variety of pressures and temperatures in a piston cylinder apparatus and cold seal apparatus. These run products have been used to determine the equilibrium position of the reaction between sanidine and water to form "sanidine hydrate". The reaction was found to lie between four brackets of 2.35 and 2.5 GPa at 450°C, 2.4 and 2.59 GPa at 550°C, 2.67 and 2.74 GPa at 650°C and 2.70 and 2.72 GPa at 680°C. Infrared spectroscopy showed that the dominant water species in "sanidine hydrate" was structural H<SUB>2</SUB>O. The quantity of this structural H<SUB>2</SUB>O, measured by thermogravimetric analysis, was found to vary between 4.42 and 5.85 wt% over the pressure range of 2.7 to 3.2 GPa and the temperature range of 450 to 680°C. Systematic variation in water content over the experimental range studies was not clearly established. The samples may have been contaminated with platinum from the capsule and the resultant thermogravimetric analyses would be too low. The maximum value was below 6.07 wt% which would be equivalent to 1 molecule of H<SUB>2</SUB>O per formula unit. It was possible to remove the water entirely by heating at atmospheric pressure to produce anhydrous hexagonal KAlSi<SUB>3</SUB>O<SUB>8</SUB> ("hexasanidine") implying that the structural "water" content of "sanidine hydrate" is variable, perhaps as a continuous solid solution between the end members where n = 1 and n = 0 in the formula KAlSi<SUB>3</SUB>O<SUB>8</SUB>.nH<SUB>2</SUB>O. The unit cell parameters measured by powder X-ray diffraction, for "sanidine hydrate" were <I>a</I> = 0.5337 nm and <I>c</I> = 0.7714 nm, and those for "hexasanidine" were <I>a</I> = 0.5288 nm and <I>c</I> = 0.7818 nm. This change in the unit cell parameters on dehydration was of a similar magnitude and direction to those for cymrite and hexacelsian, which would also suggest a solid solution in the water content as was found in the barium analogue. The water content data was not of a sufficient quality to calculate the average enthalpy and entropy of the dehydration of "sanidine hydrate". If the structural "water" content of "sanidine hydrate" is presumed constant over the pressure-temperature range of the study the average values for the enthalpy is 18000 (±5000)Jmol<SUP>-1</SUP> and for the entropy is 92.75 (±5.80) JK<SUP>-1</SUP>mol<SUP>-1</SUP>.