Oneillite

Oneillite is a rare mineral of the eudialyte group with the formula Na15Ca3Mn3Fe2+3Zr3NbSiO(Si3O9)2(Si9O27)2(O,OH,H2O)3(OH,Cl)2.[2][1] 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.[2] Similar feature is displayed by some other eudialyte-group members: aqualite, labyrinthite, raslakite, and voronkovite.[1] Oneillite is strongly enriched in rare earth elements (REE, mainly cerium), but REE do not dominate any of its sites.[2]

Oneillite
General
CategorySilicate mineral, Cyclosilicate
Formula
(repeating unit)		Na15Ca3Mn3Fe3Zr3Nb(Si25O73)(O,OH,H2O)3(OH,Cl)2 (original form)
Strunz classification9.CO.10
Crystal systemTrigonal
Crystal classPyramidal (3)
H-M symbol: (3)
Space groupR3
Unit cella = 14.19, c = 29.98  [Å], Z = 3 (approximated)
Identification
ColorYellowish brown
Crystal habitanhedral grains
CleavageNone
FractureUneven
TenacityBrittle
Mohs scale hardness5-6
LusterVitreous
StreakWhite
DiaphaneityTransparent or translucent
Density3.20 g/cm3 (measured)
Optical propertiesUniaxial (-)
Refractive indexnω=1.65 nε=1.64 (approximated)
PleochroismNone
References[1][2]

Notes on chemistry

Rare earth elements in oneillite include cerium, lanthanum, neodymium, and yttrium, with minor praseodymium and gadolinium. Other impurities are potassium, and minor amounts of aluminium, hafnium, strontium and tantalum.[2]

Occurrence and association

Oneillite is one of four eudialyte-group minerals discovered within alkaline rocks of Mont Saint-Hilaire, Quebec, Canada. It associates with aegirine, albite, sodalite, and pyrite.[2]

References
  1. Mindat, Oneillite, http://www.mindat.org/min-11029.html
  2. Johnsen, O., Grice, J.D., and Gault, R.A., 1999. Oneillite: a new Ca-deficient and REE-rich member of the eudialyte group from Mont Saint-Hilaire, Quebec, Canada. The Canadian Mineralogist 37, 1295-1301
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