{{Short description|Mineral of the eudialyte group}} {{infobox mineral | name = Oneillite | category = Silicate mineral, cyclosilicate | image = | imagesize = | alt = | caption = | formula = {{chem2|Na15Ca3Mn3Fe3Zr3Nb(Si25O73)(O,OH,H2O)3(OH,Cl)2}} (original form) | IMAsymbol = One<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref> | strunz = 9.CO.10 | dana = | system = Trigonal | class = Pyramidal (3) <br/>H-M symbol: (3) | symmetry = ''R''3 | unit cell = a = 14.19, c = 29.98 [Å], Z = 3 (approximated) | color = Yellowish brown | colour = | habit = anhedral grains | twinning = | cleavage = None | fracture = Uneven | tenacity = Brittle | mohs = 5–6 | luster = Vitreous | streak = White | diaphaneity = Transparent or translucent | gravity = | density = 3.20 g/cm<sup>3</sup> (measured) | polish = | opticalprop = Uniaxial (−) | refractive = nω = 1.65 nε = 1.64 (approximated) | birefringence = | pleochroism = None | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = <ref name=Mindat /><ref name=J /> }} '''Oneillite''' is a rare mineral of the eudialyte group with the chemical formula {{chem2|Na15Ca3Mn3Fe(2+)3Zr3NbSiO(Si3O9)2(Si9O27)2(O,OH,H2O)3(OH,Cl)2}}.<ref name=Mindat>Mindat, Oneillite, http://www.mindat.org/min-11029.html</ref><ref name=J>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</ref> 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 ''R''3, instead of more specific for the group ''R''3''m'' one) due to Ca-Mn ordering.<ref name=J /> Similar feature is displayed by some other eudialyte-group members: aqualite, labyrinthite, raslakite, and voronkovite.<ref name=Mindat /> Oneillite is strongly enriched in rare earth elements (REE, mainly cerium), but REE do not dominate any of its sites.<ref name=J />
==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.<ref name=J />
==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.<ref name=J />
==References== {{reflist}}
Category:Cyclosilicates Category:Sodium minerals Category:Calcium minerals Category:Manganese minerals Category:Iron(II) minerals Category:Zirconium minerals Category:Niobium minerals Category:Trigonal minerals Category:Minerals in space group 146
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