3.3 e. The compressional behaviour of the columbite-group minerals along the FeNb₂O₆-MnNb₂O₆ solid solution (M. Pistorino/Pavia, F. Nestola and T. Boffa Ballaran, in collaboration with M.C. Domeneghetti/Pavia)

Columbite-tantalite minerals are the predominant Nb-Ta phases found in rare-earth granite pegmatites. These minerals have general formula AB₂O₆ (where A = Fe²⁺, Mn²⁺ and B = Nb⁵⁺, Ta⁵⁺, with other cations such as Mg²⁺, Ca²⁺, Sc³⁺, Fe³⁺, Ti⁴⁺, Sn⁴⁺, W⁶⁺ and rare-earth elements as minor substitutions). Columbites (samples with Nb as the dominant B cation) and tantalites (samples with Ta as the dominant B cation) crystallise in the orthorhombic system, space group Pbcn, with an α-PbO₂-type structure. A convergent order-disorder process occurs for samples at intermediate composition with Fe/Mn preferentially occupying the A site and Nb/Ta the B site. Minerals of the columbite group show a wide range of degrees of order; complete order can be experimentally achieved by annealing at high temperature, whereas completely disordered samples have never been found or synthesized.

The aim of this work is to study the high-pressure behaviour of the columbite-group minerals as a function of chemical composition and degree of order. In a first series of high-pressure experiments the effect of cation ordering on compressibility has been investigated. Two single-crystals from a natural sample of columbite from Raode, Africa, with same composition, X_Nb = Nb/(Nb+Ta) = 0.973 and X_Fe = Fe/(Fe+Mn) = 0.800, but different degrees of order, Q _{Raode 17} = 25 % and Q _{Raode 15} = 100 %, have been loaded together in a diamond anvil cell with a quartz single-crystal as internal pressure standard and a 4:1 methanol:ethanol mixture as pressure medium. Unit-cell parameters were measured at room temperature as a function of pressure up to 7.1 GPa (Fig. 3.3-6). The reversibility of the experiments was checked by measuring lattice parameters releasing pressure at 5.9 and 3.1 GPa.

Fig. 3.3-6: Volume variation of two samples having the same composition, but different degrees of order as a function of increasing pressure (crosses and circles) and decreasing pressure (squares)

In a second series of experiments the compressibility as a function of the Fe/Mn substitution alone has been measured. Two totally ordered single-crystals of columbite from Ivigtuut, Greenland, (MM-13) and from Kragerve, Norway, (Kra 11), with composition X_Nb = 0.993, X_Fe = 0.750 and X_Nb =0.923, X_Fe = 0.155 respectively, have been loaded together in a diamond anvil cell and studied by in situ high-pressure X-ray diffraction.

Cell parameters were measured at room temperature as a function of pressure up to 6.2 GPa and the reversibility of the experiments was checked by two measures of the lattice parameters releasing pressure at 4.2 and 1.7 GPa (Fig. 3.3-7).

Fig. 3.3-7: Volume variation of two samples having different Fe/Mn ratios with increasing pressure (crosses and circles) and with decreasing pressure (squares)

No phase transitions were observed in the two series of experiments, as indicated by the continuous decrease of unit cell parameters.

EoS parameters calculated using a third order Birch-Murnaghan equation of state are V₀ = 413.89(2), K₀ = 152(1), K’ = 4.8(3) for Raode 15, V₀ = 414.56(1), K₀ = 148.9(6), K’ = 4.1(2) for Raode 17, V₀ = 415.03(5), K₀ = 152(3), K’ = 4.9(8) for MM-13 and V₀ = 421.18(2), K₀ = 146(1), K’ = 5.0(4) for Kra 11.

It can be noticed that there is a slight increase of compressibility with Mn substitution into the Fe columbite. Although the difference in K₀ values is minimal, the high accuracy of the unit-cell data suggests that ordered and relatively disordered samples (Raode) have a different compressional behaviour.