Cation ordering, valence states, and symmetry breaking in the crystal-chemically complex mineral chevkinite-(Ce): X-ray diffraction and photoelectron spectroscopy studies and mechanisms of Nb enrichment.

The crystal structures of natural Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, crystallizing in space groups C2/m and P21/a, were solved and refined to R1 = 0.03 and R1= 0.07, respectively, from data collected with a single-crystal diffractometer. X-ray photoelectron spectroscopy was used to determine the oxidation states of the following ions: Ce3+, Fe2+, Fe3+, Ti4+, and Ti3+. Typically, Nb-rich chevkinite-(Ce) has space group C2/m, but specimens with space group P21/a were also identified. While XPS shows that both C2/m and P21/a crystal structures contain Ti4+ and Ti3+, it also indicates that Ti2+ may occur in the P21/a phases. The general formula for the Nb-rich chevkinite-(Ce) having space group C2/m is (Ce3+,La3+,Nd3+,Pr3+,Ca2+,Na+)4Fe2+(Fe3+,Nb5+,Al3+)2 (Ti4+,Ti3+,Nb5+,Mg2+)2(Si2O7)2O8 and with P21/a is (Ce3+,La3+,Nd3+,Pr3+,Ca2+,Na+)4(Fe2+,Mg2+,Ti2+)1 (Fe3+,Nb5+,Al3+)2(Ti4+,Ti3+,Nb5+)2(Si2O7)2O8. It is proposed that, in addition to the substitution CFe3+ + DTi4+ CFe2+ + DNb5+, niobium can also be incorporated into chevkinite-(Ce) by the substitution 2DTi4+ DNb5+ + DTi3+, leading to substantial Nb-enrichment. The study has revealed that the distribution of cations between the various structural sites is considerably more complex than that envisaged in standard models of the chevkinite group, and that cations such as Ti can have more than one valency in the structure. Lighter elements with larger ionic radii (Mg2+) tend to occupy the D site in the C2/m crystal structure, whereas in the P21/a structure these elements (Mg2+, Ti2+) tend to enter the B site. Niobium is distributed between the C and D sites. The techniques employed provide a comprehensive representation of the distribution of cations and can be used to examine similar complexity in other mineral groups. The present study has shown that Nb can also be fractionated from residual melts.