Kentbrooksite

Kentbrooksite is a moderately rare mineral of the eudialyte group,[1] with formula (Na,REE)15(Ca,REE)6Mn3Zr3NbSi[(Si9O27)2(Si3O9)2O2]F2·2H2O.[2] 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.[3] 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[1]), (2) dominancy of manganese, and (3) dominancy of niobium. Trace hafnium and magnesium are also reported.[2] Kentbrooksite is relatively common when compared to most other species of the group.[1]

Kentbrooksite
General
CategorySilicate mineral, Cyclosilicate
Formula
(repeating unit)		(Na,REE)15(Ca,REE)6Mn3Zr3NbSi25O74F2·2H2O (original form)
Strunz classification9.CO.10 (10 ed)
8/E.23-20 (8 ed)
Dana classification64.1.2.1
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupR3m
Unit cella = 14.24, c = 30.03 [Å] (approximated); Z = 3
Identification
ColorYellow-brown
Crystal habitaggregates (anhedral to subhedral)
CleavageNone
FractureUneven
TenacityBrittle
Mohs scale hardness5-6
LusterVitreous
StreakWhite
DiaphaneityTransparent
Density3.10 (measured)
Optical propertiesUniaxial (-)
Refractive indexnω=1.63, nε=1.62 (approximated)
PleochroismNone
Other characteristicsPyroelectric
References[1][2]

Occurrence

Kentbrooksite was found in alkaline pegmatites within pulaskites of the Kangerdlugssuaq intrusion in East Greenland.[2]

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.

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References
  1. Mindat, Kentbrooksite, http://www.mindat.org/min-7132.html
  2. 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.
  3. 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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