The name volkonskoite was first used in 1830 to describe a bright blue-green, chromiumbearing clay material from the Okhansk region, west of the Ural Mountains, U.S.S.R. Since that time, the name has been applied to numerous members of the smectite group of clay minerals, although the reported chromium content has ranged from 1% to about 30% Cr2O3. The name has also been applied to some chromian chlorites. Because volkonskoite has been used for materials that differ not only in their chromium content but also in their basic structure, the species status of the mineral has been unclear.

To resolve this uncertainty, two specimens of volkonskoite from (1) Mount Efimiatsk, the type locality in the Soviet Union (USNM 16308) and (2) the Okhansk region in the Perm Basin, U.S.S.R. (USNM R4820), were examined by several mineralogical techniques. Neotype sample 16308 has the following structural formula:

$({\text{Ca}}_{0.11}{\text{Mg}}_{0.11}{\text{Fe}}^{2+}{\hspace{0.17em}}_{0.03}{\text{K}}_{0.02})({\text{Cr}}_{1.18}{\text{Mg}}_{0.78}{\text{Fe}}^{3+}{\hspace{0.17em}}_{0.29}\text{Ca}{}_{0.02})({\text{Si}}_{3.50}{\text{Al}}_{0.51}){\text{O}}_{10}{(\text{OH})}_2\cdot 3.64{\text{H}}_2\text{O}.$

Sample R4820 has the following structural formula:

$({\text{Ca}}_{0.25}{\text{Mg}}_{0.05}{\text{Fe}}^{2+}{\hspace{0.17em}}_{0.01}{\text{K}}_{0.03}{\text{Mn}}_{0.01})({\text{Cr}}_{1.07}{\text{Mg}}_{0.75}{\text{Fe}}^{3+}{\hspace{0.17em}}_{0.35}({\text{Si}}_{3.59}{\text{Al}}_{0.43}){\text{O}}_{10}{(\text{OH})}_2\cdot 4.22{\text{H}}_2\text{O}.$

Mössbauer spectroscopy indicates that 91% and 98% of the iron is present as Fe3+ in samples 16308 and R4820, respectively. X-ray powder diffraction patterns of both samples have broad lines corresponding to minerals of the smectite group.

On the basis of these data, volkonskoite appears to be a dioctahedral member of the smectite group that contains chromium as the dominant cation in the octahedral layer. Smectites containing less than this amount of octahedral chromium should not be called volkonskoite, but should be named by chemical element adjectives, e.g., chromian montmorillonite, chromian nontronite.

Ten montmorillonites of different origins with variable Fe contents were analysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octahedral cations. For low to medium Fe contents (≤0.56 per Si8 formula unit), a linear relationship between the position of the δAlFeOH band and Fe content was observed. Such correlation might be explained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three components (δAlAlOH, δAlMgOH and δAlFeOH) which are correlated with cation abundances derived from chemical analysis. The relative area of each band can then be compared with theoretical areas calculated assuming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearly randomized octahedral distribution. The two samples from Wyoming were clearly different as they exhibited a strong ordering tendency.