Nagelschmidtite as a candidate host phase for actinides, rare earth and different waste elements

Abstract

AbstractNagelschmidtite, Ca7P2Si2O16, is an end-member of continuous solid solution Ca2SiO4 – Ca3(PO4)2⋅2Ca2SiO4 within the pseudo-binary system Ca3(PO4)2 – Ca2SiO4 (whitlockite – larnite). This phase is capable to wide isomorphic exchanges in Ca, P and Si sites: Ca2+ = Sr2+; Ca2+ = Eu2+; Ca2+ + P5+ = (RE,An)3+ + Si4+, 2Ca2+ = Na+ + (RE,An)3+; 2Ca2+ = An4+ + ☐; Ca2+ + Si4+ = (RE,An)3+ + (Al,Fe)3+; Ca2+ + Si4+ = Na+ + P5+; 2Ca2+ = Na+ + (Al,Fe)3+; Ca2+ + P5+ = Na+ + S6+. It was found in metallurgical slags and geological formations. We revealed nagelschmidtite-type phase in vitrified phosphorus-bearing radioactive incinerator slags. The materials were glass-crystalline and contained nano-sized nagelschmidtite crystals distributed in vitreous matrix phase. Average chemical composition of the largest (few microns) crystals was recalculated to formula Na1.21K1.05Ca2.22Al2.02Fe0.46Si2.69P1.26U0.08O15.76. Significant oxygen misbalance suggests higher than U(IV) oxidation state for uranium – U(V) or U(VI). Capability of nagelschmidtite to be crystallized from melt makes it promising phase for actinides, rare earths and some other fission and corrosion products at using a melting route to nuclear waste forms including cold crucible induction melting and self-propagating high-temperature synthesis.