|Category||Silicate mineral, Cyclosilicate|
|(Na,REE)15(Ca,REE)6Mn3Zr3NbSi25O74F2·2H2O (original form)|
9.CO.10 (10 ed) |
8/E.23-20 (8 ed)
Ditrigonal pyramidal (3m) |
H-M symbol: (3m)
|Unit cell||a = 14.24, c = 30.03 [Å] (approximated); Z = 3|
|Crystal habit||aggregates (anhedral to subhedral)|
|Mohs scale hardness||5-6|
|Optical properties||Uniaxial (-)|
|Refractive index||nω=1.63, nε=1.62 (approximated)|
Kentbrooksite is a moderately rare mineral of the eudialyte group, with formula (Na,REE)15(Ca,REE)6Mn3Zr3NbSi[(Si9O27)2(Si3O9)2O2]F2·2H2O. 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. 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), (2) dominancy of manganese, and (3) dominancy of niobium. Trace hafnium and magnesium are also reported. Kentbrooksite is relatively common when compared to most other species of the group.
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.
- Mindat, Kentbrooksite, http://www.mindat.org/min-7132.html
- 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.
- 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
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