Bjorn O. Mysen Phosphorus solubility mechanisms in haplogranitic aluminosilicate glass and melt: effect of temperature and aluminum content Received: 7 October 1997 / Accepted: 11 May 1998 Abstract The solubility behavior of phosphorus in glasses and melts in the system Na 2 O-Al 2 O 3 -SiO 2 -P 2 O 5 has been examined as a function of temperature and Al 2 O 3 content with microRaman spectroscopy. The Al 2 O 3 was added (2, 4, 5, 6, and 8 mol% Al 2 O 3 ) to melts with 80 mol% SiO 2 and 2 mol% P 2 O 5 . The composi- tions range from peralkaline, via meta-aluminous to peraluminous. Raman spectra were obtained of both the phosphorus-free and phosphorous-bearing glasses and melts between 25 and 1218 °C. The Raman spectrum of Al-free, P-bearing glass exhibits a characteristic strong band near 940 cm )1 assigned to PAO stretching in or- thophosphate complexes together with a weaker band near 1000 cm )1 assigned P 2 O 7 complexes. With in- creasing Al content, the proportion of P 2 O 7 initially increases relative to PO 4 and is joined by AlPO 4 com- plexes which exhibit a characteristic P-O stretch mode slightly above 1100 cm )1 . The latter complex appears to dominate in meta-aluminosilicate glass and is the only phosphate complex in peraluminous glasses. When P- bearing peralkaline silicate and aluminosilicate glasses are transformed to supercooled melts, there is a rapid decrease in PO 4 /P 2 O 7 so that in the molten state, PO 4 units are barely discernible. The P 2 O 7 /AlPO 4 abundance ratio in peralkaline compositions increases with in- creasing temperature. This decrease in PO 4 /P 2 O 7 with increasing temperature results in depolymerization of the silicate melts. Dissolved P 2 O 5 in peraluminous glass and melts forms AlPO 4 complexes only. This solution mechanism has no discernible in¯uence on the alumi- nosilicate melt structure. There is no eect of tempera- ture on this solution mechanism. Introduction Although generally present in comparatively low con- centrations in natural magmas (less than 2±3 wt% P 2 O 5 ), even in this abundance range phosphorus strongly modi®es both physical and chemical properties of silicate liquids (Kushiro 1975; Watson 1976; Visser and Koster Van Groos 1979; Ryerson and Hess 1980; London 1987; Mysen 1992; Dingwell et al. 1993; Lon- don et al. 1993; Toplis et al. 1994). These properties include viscosity, liquidus phase relations, redox equi- libria of iron, and element partitioning between melts, ¯uids, and minerals. The solubility behavior of phos- phorus in magmatic liquids is needed in order to des- cribe how their melt properties change with phosphorus content. Experimental studies on the structural role of phos- phorus in silicate melts have relied mostly on the use of quenched melts (glass). Even so, these studies reveal that in addition to silica, there is a complex compositional dependence of phosphate speciation with P 2 O 5 content and alkali/alumina ratio being important variables (e.g., Mysen et al. 1981, 1997; Dupree et al. 1989; Gan and Hess 1992; Mysen et al. 1997; Reynard and Toplis 1997). Results of in-situ examination of silicate melt and glass structure at high temperature indicate that above the glass transition, with increasing temperature there is a gradual change of the proportion of the silicate structural units (Q-species) for melts in the polymeriza- tion range relevant to ma®c, intermediate, and felsic magmatic liquids (see review of such data by Mysen, 1995a, and references therein). In a preliminary study of Na-silicate melts and glasses with about 2 mol% P 2 O 5 in solution, Mysen (1996) found that phosphate speciation is profoundly temperature dependent. In that study, the P-bearing glass showed essentially all the phosphorus in the form of orthophosphate (PO 4 ), but with perhaps as much as 50% of the phosphate species converted to pyrophosphate (P 2 O 7 ) as the temperature is increased across that of the glass transition range. Contrib Mineral Petrol (1998) 133: 38±50 Ó Springer-Verlag 1998 B.O. Mysen 1 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW, Washington, DC 20015, USA 1 Also at: NSF-sponsored Center for High Pressure Research (CHiPR) at the Geophysical Laboratory Editorial responsibility: T.L. Grove