Synthesis of phosphorus- and arsenic-bearing framework silicates similar to feldspar

Abstract

The finds of phosphorusand arsenic-bearing feldspars in natural complexes are described in literature [London, et al., 1990; Vergasova, et al., 2004]. Phosphorus-bearing feldspars synthesis has been successfully produced in the works [Simpson, 1977; Bychkov, et al., 1989]. But the problems of isomorphic substitution and synthesis of solid solutions (Na,K)(Al,Si,P)4O8 и (Na,K)(Al,Si, As)4O8 remain insufficiently studied by experiment. So, we have carried out the synthesis of phosphorusand arsenic-bearing sodium (potassium) feldspars in hydrothermal conditions at T=400÷600C, P=1.5 kbar. The mixtures of salts and gels: NaPO3 + Al2SiO5 (sillimanite gel); Na3AsO4 (or NaOH + As2O5) + Al2SiO5 (sillimanite gel) + SiO2 were the initial mixtures. Sometimes glass of NaAl2SiPO8 composition obtained by melting of Al2SiO5 gel and sodium metaphosphate (NaPO3) and water (10 wt %) at T=1200C and pressure 2 kbar was used. The analyses of synthesized phases were produced by microanalysis and X-ray analysis. It is shown that synthesized phases may be related to feldspars based on the composition and X-ray properties. Isomorphism in synthesized feldspars exercises according to scheme of two silicon atoms substitution to aluminum and phosphorus (arsenic): 2Si ↔ Al + P(As). 1. Phosphorus-bearing phases. Apparently, synthesized phases don’t form continuous sequences of solid solutions in the NaAlSi3O8 – NaAl2SiPO8 systems. The figure 1 shows that in experiments of phosphorus-bearing feldspars synthesis practically pure albites coexist with phosphorus-bearing feldspars in which NaAlSi3O8 ↔ NaAl2SiPO8 substitution is up to 50 or more mol% of NaAl2SiPO8 minal. Moreover, in some experiments phosphorus practically entirely replaces silicon and phase Na0.76Al2.33P1.65O8 is formed. Substitution of 2Si ↔ Al + P in synthesized phases is described by following regression equation: (Al+P)= 3.956 – 0.488*(2Si); (n=40; r=0.998; Sx=0.07; Ex=0.02).