The Synthesis of Mg3(PO4)2·8H2O and Its New Polymorphism

Abstract

Bobierrite, Mg3(PO4)2·8H2O, could be synthesized from mixed solutions of Na2HPO4 and MgSO4 at an initial pH of 6.4–7.0 with the addition of Ca(NO3)2, CuSO4, NiSO4, gelatin, or agar, by heating at 80–90 °C for 2–48 h. X-Ray studies indicated the coexistence of two kinds of lattice in each synthetic Mg3(PO4)2·8H2O crystal. One phase (Phase 1) is monoclinic, space group P21/m or P21 with a=10.08(2), b=27.86(3), c=4.656(7) Å, and β=105.0(2)°. The other phase (Phase 2) is monoclinic, space group C2/m, C2, or Cm, with a=10.07(2), b=13.38(2), c=4.656(7) Å, and β=105.0(2)°. If a centrosymmetric space group C2/m is assumed for the latter, Phase 2 may be considered to be a new polymorph of Mg3(PO4)2·8H2O, one belonging to the vivianite series. Newberyite, MgHPO4·3H2O, was also formed in the above synthetic procedure. This phosphate was precipitated under slightly more acidic conditions than those where Mg3(PO4)2·8H2O was obtained.