The effect of temperature and bulk composition on the solution mechanism of phosphorus in peraluminous haplogranitic magma

Abstract

Solution mechanisms of P in peraluminous glasses and melts in the system CaO-Na 2 O-K 2 O-Al 2 O 3 - SiO 2 -P 2 O 5 have been examined with in-situ microRaman spectroscopy from ambient temperature to near 1200 °C. The principal aim was to examine the relative stabilities of phosphate complexes as functions of P content, peraluminosity, and temperature. Increasing peraluminosity was accomplished by increasing the proportions of Al 3+ and Ca 2+ of constant SiO 2 content. The molar ratio Al 2 O 3 / (CaO+Na 2 O+K 2 O) (A/CNK) ranged from ~1 to ~1.3. In all compositions, P 5+ is bonded to Al 3+ to form AlPO 4 complexes. In addition, there is evidence for pyrophosphate complexing (P 2 O 7 ). In melts with the highest (Ca+Na+K)/P, there is probably also a small fraction of orthophosphate complexes present. The relative importance of AlPO4-like complexes is correlated positively with peraluminosity (A/CNK), P 2 O 5 content, and increasing temperature at temperatures above that of the glass transition. These structural relationships among phosphate complexes are coupled with decreasing polymerization of the aluminosilicate melts.