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

In situ high-pressure synchrotron X-ray powder diffraction study of three tunnel manganese oxide minerals (hollandite with 2 × 2 MnO₆octahedra tunnels, romanechite with 2 × 3 tunnels, and todorokite with 3 × 3 tunnels) was performed using a diamond anvil cell and nominally penetrating alcohol and water mixture as a pressure-transmitting medium up to ~8 GPa. Bulk moduli (B₀)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., $$ \beta_{0}^{a} $$^β0a(a/a₀)=−0.00066(3) GPa⁻¹, $$ \beta_{0}^{b} $$^β0b (b/b₀)=0.00179(8) GPa⁻¹,$$ \beta_{0}^{c} $$^β0c(c/c₀)=0.00637(4) GPa⁻¹[c > b > a]for hollandite;$$\beta_{0}^{a} $$^β0a(a/a₀)=0.00485(4) GPa⁻¹,$$ \beta_{0}^{b} $$^β0b (b/b₀)=0.0016(1) GPa⁻¹, $$ \beta_{0}^{c} $$^β0c(c/c₀)=0.00199(8) GPa⁻¹[a > c > b] for romanechite; and $$ \beta_{0}^{a} $$^β0a(a/a₀)=0.00826(9)GPa⁻¹,$$ \beta_{0}^{b} $$^β0b(b/b₀)=0.0054(1)GPa⁻¹, $$ \beta_{0}^{c}$$^β0c(c/c₀)=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 β angles up to 3 GPa of 0.62°,0.8°, and 1.15° in hollandite, romanechite, and todorokite, respectively. The compression of tunnel manganese oxides is related to the tunnel distortion and the size of the tunnel.