Hydrothermal Synthesis of Columbite-(mn), Tantalite-(mn), Hafnon, and Zircon At 800–850 °c and 200 Mpa

Fully ordered columbite-(Mn), tantalite-(Mn), hafnon, and zircon were successfully synthesized using an acid solution of 6N:1N (N = normal) HF:H₂SO₄ and stoichiometric oxide mixtures. The mixtures and solutions were sealed and placed in rapid-quench cold-seal pressure vessels (CSPV) at 800–850 °C and 200 MPa. Mineral structures and compositions have been confirmed by scanning electron microscope (SEM), micro X-ray diffraction (μXRD), and electron probe microanalysis (EPMA). The unit cell parameters of the minerals determined by μXRD: Orthorhombic (Pbcn) columbite-(Mn) a 14.434(2), b 5.768(1), c 5.084(1) Å; orthorhombic (Pbcn) tantalite-(Mn) a 14.457(3), b 5.773(1), c 5.096(1) Å; tetragonal hafnon (I4₁/amd) a 6.582(2), c 5.9702(1) Å; and tetragonal zircon (I4₁/amd) a 6.607(4), c 5.9898(2) Å. The chemical formulae as determined by EPMA are very close to stoichiometric: Mn_0.96Nb_2.02O₆, Mn_0.99Ta_2.01O₆, Hf_0.99Si_1.01O₄, and Zr_1.00Si_1.00O₄, respectively.