Abstract

Seven clinopyroxene compositions along the join M2LiM1AlTSi2O6 (spodumene) to M2LiM1Fe3+TSi2O6 (ferri-spodumene) were synthesized at 2 GPa, 800 °C under highly oxidizing conditions (using H2O2 fluid) in an end-loaded piston cylinder. In addition, the LiFe3+Si2O6 composition was also synthesized under the intrinsically reducing conditions in a piston cylinder, to check the effect of fO2 on iron speciation. The run products were characterized by field emission scanning electron microscope (FE-SEM), Rietveld refinements on XRPD synchrotron data, and space groups were assigned using SAED-TEM patterns. Run products are composed mainly of lithium clinopyroxene (Li-Cpx), plus minor amounts of hematite (magnetite under reducing condition) and corundum, as independently detected by image analysis (area%) and Rietveld refinements (wt%); moreover, Rietveld results were used to derive cell parameters, M1-site occupancy (Al vs. Fe3+), atomic positions, and average bond lengths of all these Li-Cpx indexed in the C2/c space groups according to SAED-TEM.

Li-Cpx with Al and Fe3+ amounts close to 50:50 are actually slightly richer in Al apfu than nominal; the LiFe3+Si2O6 grown under very oxidized and reducing conditions have very similar cell parameters, indicating that fO2 is unable to induce a significant incorporation of Fe2+ in these Li-Cpx. The replacement of Al with Fe3+ induces a linear (%) increase of the cell edges following b > a > c, whereas β is roughly constant and the cell volume increases linearly. Furthermore, the substitution of Al with Fe3+ only weakly affects the T-O average length (<1%), whereas M2-O and M1-O bonds increase linearly of 2.3 and 5.0%, respectively.

These new experimental data have been compared with other available on Li-, Na-, and Ca-Cpx, i.e., M2(Li,Na,Ca,Mg,Fe2+)M1(Mg,Fe2+Al,Ni,Cr,Ga,V,Fe3+,Mn,Sc,In)TSi2O6, to model lattice strain, bond lengths, steric effects, and phase transitions behaviors. The replacement of Al with progressively larger cations in LiM3+Si2O6 Cpx (M3+: Ni, Cr, Ga, V, Fe3+, Ti, Sc, and In) results in a linear increase following V > b > a > c, whereas β is roughly constant except for Ti-end-member and P21/c compositions. Lattice strains induced by X, T, and P for Li-Cpx in the C2/c stability field show that when the M1 site is progressively filled with a large cation, ε1 axis (ε1 > ε2 > ε3) increases along b, whereas ε2 and ε3 are nearly parallel to a and at about 30° from c. Conversely, T will provoke a similar enlargement of ε1 and ε2 along b and a edges, respectively, whereas ε3 is again oriented at about 30° from c; the increasing of P will instead shorten all strain tensor components (ε1, ε2, and ε3) with a similar percentage amount; notably, high-P is the only stress that induces a strain component to be almost parallel to c edge. Moreover, finite lattice strains and orientation in C2/c LiMe3+Si2O6 Li-Cpx induced by Me3+: Al-Fe3+, Fe3+-Sc, Sc-In are slightly different, with ε1 invariably lying along b; conversely, Li-Na cation substitution is completely different with the highest and lowest deformations on the ac plane and ε2 along b; ε3 vector is negative and oriented at about 30° from T-chains. The ideal replacement of Al with larger cations up to In in Li-Cpx induces the M1-O, M2-O, and T-O average bond lengths to increase by 10.6, 4.3, and <0.5%. Steric effects in LiM1Me3+Si2O6 and NaM1Me3+Si2O6 Cpx are significant and very similar, whereas several other Me1+ and Me2+ substitutions in Cpx at both the M1 and M2 site, keeping fixed the other site, display less or even the absence of steric effects. Our new data also better elucidate relationships between Li-Cpx composition, symmetry at room and non-ambient conditions and Tc. The aggregate cation radii at the M1 site does not exclusively control the stability of C2/c and P21/c polymorphs; instead valence electrons can profoundly favor the stabilization of a polymorph.

Finally, Li can be easily and accurately detected (0.1/0.2 apfu) in natural clinopyroxenes by cell parameters, especially using the β angle.

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