What is "oneillite"

n. (context mineralogy English) A trigonal-pyramidal yellowish brown mineral containing calcium, chlorine, hydrogen, iron, manganese, niobium, oxygen, silicon, sodium, and zirconium.

Oneillite is a rare mineral of the eudialyte group with the formula NaCaMnFe3ZrNbSiO(SiO)(SiO)(O,OH,HO)(OH,Cl). The formula is based on the original one but extended to show the presence of cyclic silicate groups and domination of Si at the M4 site. The mineral has lowered symmetry (space group R3, instead of more specific for the group R3m one) due to Ca-Mn ordering. Similar feature is displayed by some other eudialyte-group members: aqualite, labyrinthite, raslakite, and voronkovite. Oneillite is strongly enriched in rare earth elements (REE, mainly cerium), but REE do not dominate any of its sites.