| Oneillite | |
|---|---|
| General | |
| Category | Silicate mineral, cyclosilicate |
| Formula (repeating unit) | Na15Ca3Mn3Fe3Zr3Nb(Si25O73)(O,OH,H2O)3(OH,Cl)2 (original form) |
| IMA symbol | One[1] |
| Strunz classification | 9.CO.10 |
| Crystal system | Trigonal |
| Crystal class | Pyramidal (3) H-M symbol: (3) |
| Space group | R3 |
| Unit cell | a = 14.19, c = 29.98 [Å], Z = 3 (approximated) |
| Identification | |
| Color | Yellowish brown |
| Crystal habit | anhedral grains |
| Cleavage | None |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 5–6 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent or translucent |
| Density | 3.20 g/cm3 (measured) |
| Optical properties | Uniaxial (−) |
| Refractive index | nω = 1.65 nε = 1.64 (approximated) |
| Pleochroism | None |
| References | [2][3] |
Oneillite is a rare mineral of the eudialyte group with the chemical formula Na15Ca3Mn3Fe2+3Zr3NbSiO(Si3O9)2(Si9O27)2(O,OH,H2O)3(OH,Cl)2.[2][3] 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.[3] Similar feature is displayed by some other eudialyte-group members: aqualite, labyrinthite, raslakite, and voronkovite.[2] Oneillite is strongly enriched in rare earth elements (REE, mainly cerium), but REE do not dominate any of its sites.[3]