In situ high-pressure synchrotron X-ray powder diffraction study of tunnel manganese oxide minerals: hollandite, romanechite, and todorokite

摘要

In situ high-pressure synchrotron X-ray powder diffraction study of three tunnel manganese oxide minerals (hollandite with 2 x 2 MnO6 octahedra tunnels, romanechite with 2 x 3 tunnels, and todorokite with 3 x 3 tunnels) was performed using a diamond anvil cell and nominally penetrating alcohol and water mixture as a pressure-transmitting medium up to similar to 8 GPa. Bulk moduli (B (0)) calculated using Murnaghan's equation of state are inversely proportional to the size of the tunnel, i.e., 134(4) GPa for hollandite (I2/m), 108(2) GPa for romanechite (C2/m), and 67(5) GPa for todorokite (P2/m). On the other hand, axial compressibilities show different elastic anisotropies depending on the size of the tunnel, i.e., (a/a (0)) = -0.00066(3) GPa(-1), (b/b (0)) = 0.00179(8) GPa(-1), (c/c (0)) = 0.00637(4) GPa(-1) [c > b > a] for hollandite; (a/a (0)) = 0.00485(4) GPa(-1), (b/b (0)) = 0.0016(1) GPa(-1), (c/c (0)) = 0.00199(8) GPa(-1) [a > c > b] for romanechite; and (a/a (0)) = 0.00826(9) GPa(-1), (b/b (0)) = 0.0054(1) GPa(-1), (c/c (0)) = 0.00081(8) GPa(-1) [a > b > c] for todorokite. Overall, the degree of tunnel distortion increases with increasing pressure and correlates with the size of the tunnel, which is evidenced by the gradual increases in the monoclinic beta angles up to 3 GPa of 0.62A degrees, 0.8A degrees, and 1.15A degrees in hollandite, romanechite, and todorokite, respectively. The compression of tunnel manganese oxides is related to the tunnel distortion and the size of the tunnel.