Discovery of a rare earth mineral in an archaeological artefact

Abstract

The chemical and mineral composition of an archaeological artefact — Bronze Age ceramics from a settlement near the city of Kaniv was studied. It was established that red pottery clay mixed with sand was used for its formation, most likely from the deposits of the Kaniv Paleogene suite. An increased concentration of phosphorus (1—3 wt. % P₂O₅), a reduced alkalis content, and the absence of REE characterised the clay. In one of the ceramic samples, a solid-phase cluster ~100 μm in size was found, which, in reflected electrons, stands out with a bright glow against the background of a grey clay matrix and a contrasting chemical composition. The dominant role in the cluster belongs to cerium (Ce₂O₃ 36—49 wt. %), subordinated to oxides of Si, Al and P (on average 23, 9 and 6 %, respectively). Other REE, except for a small amount of La oxide (1 %), are practically absent. Within the cluster, at least two phases with blurred hexagonal outlines are distinguished, which visually appear homogeneous, which also confirms the relatively uniform distribution of other components — oxides of Si, Al, P, Fe and Ca. Identified as a Ce and Al phosphosilicate with the empirical formula Ce_1.78La_0.03Ca_0.24Al_1.29Fe_0.43Si_2.65P_0.58O_12.3, the phases have no analogues among known mineral species. In terms of the ratio of components, the phases are simultaneously close to species in both the allanite and brytholite groups, and with such a ratio of components, they can only be an amorphous mineral close to brytholite. Various possible sources of Ce for mineral formation and the reason for its purely cerium composition are considered. Most likely, the phosphosilicate was formed in the deposits of the Kaniv suite due to Ce ions physically adsorbed on clay minerals. The ions easily passed into solution and were precipitated by phosphorus compounds according to the chelation scheme. The migration of the element under oxidising conditions in the Ce⁴⁺ form caused its separation from other trivalent rare earth elements.