Description: The crystal structure of three C 2/ c clinopyroxenes with composition (Ca 0.8 Co 0.2 )CoSi 2 O 6 , (Ca 0.6 Co 0.4 )CoSi 2 O 6 and (Ca 0.4 Co 0.6 )CoSi 2 O 6 was refined down to R 4 = 2.6% by single-crystal X-ray diffraction. The crystals were synthesized at P = 3 GPa by cooling from 1500 to 1200 °C in a piston-cylinder apparatus. At the end of the refinement cycles, electron density residuals (up to 2.1 e – ) were observed close to the M2 site and related to the site splitting of Ca and Co in the M2 polyhedron in the two subsites M2 and M2'. Split refinement significantly improved the agreement factor and decreased the uncertainty in the atomic coordinates. Similar features were found in (Ca,Mg)MgSi 2 O 6 and (Ca,Fe)FeSi 2 O 6 intermediate pyroxenes. The average structural changes related to the cation substitution at the M2 site in (Ca,Co)CoSi 2 O 6 , (Ca,Mg)MgSi 2 O 6 , and (Ca,Fe)FeSi 2 O 6 pyroxenes are similar: the T tetrahedron becomes more regular, the difference between M1-O bond lengths increases, and the M2-O3 bond lengths with the furthermost O3 oxygen atoms become longer. The changes in the M2-O bond distances are, however, not linear, and they are higher for more increased substitution. The largest structural deformation occurs on the (010) plane, with higher deformation at about 60° from the c axis for any composition. The orientation of the deformation ellipsoid is most related to a shift in tetrahedral chains. The scalar deformation for the cation substitution, s , is linearly related to the cation radius of the average M2 site (IR M2 ), i.e., the deformation is higher as the cation size decreases, following the equation: s = –0.0072(12)IR M2 + 0.0082(13), R 2 = 0.75. Increasing deformation with cation substitution is supported as the major limiting factor for solid solution. The displacement parameters for unsplit M2, O2, and O3 atoms increase up to the intermediate composition, indicating a local configuration for the M2 polyhedron centered by Ca and Co. However no significant change in U eq of the O3 atom is observed up to 20% substitution of the smaller cation in the M2 site. Comparison with Raman spectral data suggests that local chain structural configurations occur only for the substitution of the smaller cation in the M2 site higher than 20%, and that the substitution mechanism is different for C 2/ c clinopyroxenes with lower and higher Ca content.

Description: The effect of Zn substitution on a series of clinopyroxenes along the join CaZnSi 2 O 6 -Zn 2 Si 2 O 6 was studied. The pyroxenes were synthesized at P = 4–5 GPa and T = 1000–1200 °C by a multi-anvil apparatus. SEM-EDS and XRD analysis showed complete solid solution; all of the samples have the C 2 /c space group. No miscibility gap between clino- and orthopyroxene nor phase transition to the P 2 1 /c space group was found. Moreover, the cell volume of Ca-Zn pyroxenes decreases less than expected from the decrease of the average cation size for the substitution of Zn for Ca. The crystal structures of three synthetic pyroxenes of composition (Ca 0.5 Zn 0.5 )ZnSi 2 O 6 , (Ca 0.3 Zn 0.7 )ZnSi 2 O 6 , and (Ca 0.2 Zn 0.8 )ZnSi 2 O 6 were refined by single-crystal X-ray diffraction (R 4 between 3 and 4.5%). It was observed that the Ca-Zn substitution occurs in the M2 polyhedron, with a sub-site splitting of Zn in a position at approximately 0.7 Å from Ca. In this position, Zn retains a highly distorted fourfold coordination; moreover, the tetrahedral chain configuration is little changed along the series, and the M1 polyhedral size increases with Zn substitution in M2. An implication of the present work is the interpretation of the partitioning of Zn between clinopyroxene and melt. The distribution coefficients of Zn and Co are quite different in rocks of the same composition, despite their very similar ionic radius, and the difference is related to the preference of Zn for the M2 site, where Zn may find a suitable atomic coordination.

Description: The effect of the substitution of Ca with Co, on the phase transition and on the extension of the miscibility gap, was studied to model the general mechanism of phase transitions and solid solutions in (Ca,M 2+ )M 2+ Si 2 O 6 pyroxenes. Eleven pyroxenes with composition Ca 1–x Co 1+x Si 2 O 6 , (0 ≤ x ≤ 1) were therefore synthesized by piston cylinder at P = 3 GPa, and T between 1100 and 1350 °C. The samples were characterized by SEM-EDS, XRD powder diffraction, and TEM. The results were compared with those of Ca-Fe and Ca-Mg pyroxenes. The phase diagram of Ca-Co pyroxenes is similar to that of Ca-Fe and Ca-Mg ones, with a wide asymmetric miscibility gap, and higher solubility in the Ca-rich side of the gap. The solubility on the Ca-rich side of the gap is related to the radius of the cation substituting. The cell parameters of the Ca-Co pyroxenes undergo a sudden change at the composition of about 0.4 Ca apfu, due to the C 2/ c – P 2 1 / c phase transition. The change in volume with composition follows an ideal trend, in the C 2/ c phase, dictated by the ionic size of the substituting cation. Deviation from the C 2/ c behavior are instead observed in the P 2 1 / c field and ascribed to volume strain. The same turnover was found in Ca-Mg, Ca-Fe, and Ca-Mn pyroxenes. The C 2/ c – P 2 1 / c transition occurs with decreasing the M2 average cation radius, down to a critical value between 0.85 and 0.88 Å, depending on the series. A stabilization of the C 2/ c phase related to crystal field in Ca-Fe and Ca-Co pyroxenes is suggested by the analysis of the volume strain in the P 2 1 / c field. A key finding is that a miscibility gap may develop either by lattice strain related to cation substitution, within a series where all end-members have the same structure, or for the combined effect of lattice strain and a phase transition, as is the case for pyroxenes.

Description: The Raman spectra of eight clinopyroxenes synthesized along the join diopside-clinoenstatite (Di-Cen, CaMgSi 2 O 6 -Mg 2 Si 2 O 6 ) were measured. The splitting of the 670 cm –1 mode of the A g symmetry, observed in the composition Di 52 En 48 to clinoenstatite, was interpreted as evidence of a C 2/ c-P 2 1 / c phase transition. The transition was also revealed by deviation from the linear dependence of the peak position vs. composition and by the appearance of several new peaks in the samples richer in clinoenstatite. Analysis of peak positions vs. structural changes suggests that for the M2 polyhedron, in which Ca substitution for Mg occurs, a different deformation mechanism acts in Ca richer and poorer P 2 1 / c pyroxenes, and that Ca richer P 2 1 / c pyroxenes deform with the same mechanism as C 2/ c pyroxenes. The frequency of the peak at 670 cm –1 was found to change linearly with the kinking angle of the tetrahedral chains for C 2/ c and of the B chain for P 2 1 / c , whereas the position of the peak ascribed to the A chain was little affected by the kinking angle. Peak broadening in C 2/ c Ca-rich homogeneous pyroxenes was interpreted to be a consequence of the positional disorder of the Ca and Mg in the M2 cavity: peak broadening increases with increasing Mg content for peaks assigned to M2-O vibrations, but it changes little for peaks assigned to chain bending, which suggests that cation substitution in the M2 cavity occurs with little interaction with the silicate chain. Furthermore peak broadening was observed in intermediate pyroxenes as a consequence of mottled textures, antiphase domains, and compositional inhomogeneity.