{{Short description|Mineral of the eudialyte group}} {{infobox mineral | name = Kentbrooksite | category = Silicate mineral, Cyclosilicate | image = Terskite & Kentbrooksite.jpg | imagesize = | alt = | caption = | formula = {{chem2|(Na,REE)15(Ca,REE)6Mn3Zr3NbSi25O74F2*2H2O}} (original form) | IMAsymbol = Ktb<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 (10 ed) <br/><small>8/E.23-20 (8 ed)</small> | dana = 64.1.2.1 | system = Trigonal | class = Ditrigonal pyramidal (3m) <br/>H-M symbol: (3m) | symmetry = ''R3m'' | unit cell = a = 14.24, c = 30.03 [Å] (approximated); Z = 3 | color = Yellow-brown | colour = | habit = aggregates (anhedral to subhedral) | twinning = | cleavage = None | fracture = Uneven | tenacity = Brittle | mohs = 5–6 | luster = Vitreous | streak = White | diaphaneity = Transparent | gravity = | density = 3.10 (measured) | polish = | opticalprop = Uniaxial (−) | refractive = nω = 1.63, nε = 1.62 (approximated) | birefringence = | pleochroism = None | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = Pyroelectric | references = <ref name=Mindat /><ref name=J /> }} '''Kentbrooksite''' is a moderately rare mineral of the eudialyte group,<ref name=Mindat>Mindat, Kentbrooksite, http://www.mindat.org/min-7132.html</ref> with chemical formula {{chem2|(Na,REE)15(Ca,REE)6Mn3Zr3NbSi[(Si9O27)2(Si3O9)2O2]F2*2H2O}}.<ref name=J>Johnsen, O., Grice, J.D., and Gault, R.A., 1998. Kentbrooksite from the Kangerdlugssuaq intrusion, East Greenland, a new Mn-REE-Nb-F end-member in a series within the eudialyte group: Description and crystal structure. European Journal of Mineralogy 10(2), 207–220.</ref> This extended formula shows the presence of cyclic silicate groups and dominance of Si at the M4 site, according to the nomenclature of the eudialyte group.<ref>Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785–794</ref> The characteristic features of kentbrooksite, that make it different from eudialyte are: (1) dominancy of fluorine (the only currently known example among the whole group<ref name=Mindat />), (2) dominancy of manganese, and (3) dominancy of niobium. Trace hafnium and magnesium are also reported.<ref name=J /> Kentbrooksite is relatively common when compared to most other species of the group.<ref name=Mindat />
==Occurrence== Kentbrooksite was found in alkaline pegmatites within pulaskites of the Kangerdlugssuaq intrusion in East Greenland.<ref name=J />
==Notes on chemistry== Rare earth elements (REE) in kentbrooksite are dominated by cerium and yttrium. Potassium, strontium, iron, aluminium, titanium, magnesium are present as other admixtures. An important fraction of fluorine is substituted by chlorine and hydroxyl groups.
==References== {{reflist}}
Category:Cyclosilicates Category:Sodium minerals Category:Calcium minerals Category:Manganese minerals Category:Zirconium minerals Category:Niobium minerals Category:Trigonal minerals Category:Minerals in space group 160
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