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We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H 2 O &lt; 3 ppm) single rutile crystals. They were equilibrated with pure H 2 O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and fO2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm3) were analyzed for H +  by FTIR. Hydration occurs almost spontaneously and the H 2 O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H 2 O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H 2 O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H 2 O, and rutile with 2000 ppm V does not incorporate H 2 O. During stepwise heating at atmospheric pressure of a reacted Nbdoped rutile, H +  is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm -1  wavenumber attributed to intervalence charge transfer transition Ti 3+ +Ti 4+  → Ti 4+ +Ti 3+ . The reduction of Ti 4+  to Ti 3+  is charge balanced by the incorporation of H + . The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm -1  indicates that Nb 5+  is charge balanced by Ti 3+ . In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H+ incorporation is lower by the equivalent of Ti 3+  reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti 3+ / Ti 4+  IVCT band at ~6400 cm -1  with moderate intensity considering the high-H 2 O contents of ~900 ppm. The high-H +  contents are best explained by the reduction of Cr 4+  to Cr 2+ . The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti 4+  to Ti 3+  for charge balance of V 5+ . This forms a deep narrow window of transmittance in the range 25 000-20 000 cm -1 , which causes the dark-blue color.

Coupled H and Nb, Cr, and V trace element behavior in synthetic rutile at 600 °C, 400 MPa and possible geological application