Kentbrooksite is a moderately rare mineral of the eudialyte group,[2] with chemical formula (Na,REE)15(Ca,REE)6Mn3Zr3NbSi[(Si9O27)2(Si3O9)2O2]F2·2H2O.[3] 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.[4] 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]), (2) dominancy of manganese, and (3) dominancy of niobium. Trace hafnium and magnesium are also reported.[3] Kentbrooksite is relatively common when compared to most other species of the group.[2]
| Kentbrooksite | |
|---|---|
 | |
| General | |
| Category | Silicate mineral, Cyclosilicate |
| Formula (repeating unit) | (Na,REE)15(Ca,REE)6Mn3Zr3NbSi25O74F2·2H2O (original form) |
| IMA symbol | Ktb[1] |
| Strunz classification | 9.CO.10 (10 ed) 8/E.23-20 (8 ed) |
| Dana classification | 64.1.2.1 |
| Crystal system | Trigonal |
| Crystal class | Ditrigonal pyramidal (3m) H-M symbol: (3m) |
| Space group | R3m |
| Unit cell | a = 14.24, c = 30.03 [Å] (approximated); Z = 3 |
| Identification | |
| Color | Yellow-brown |
| Crystal habit | aggregates (anhedral to subhedral) |
| Cleavage | None |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 5–6 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent |
| Density | 3.10 (measured) |
| Optical properties | Uniaxial (−) |
| Refractive index | nω = 1.63, nε = 1.62 (approximated) |
| Pleochroism | None |
| Other characteristics | Pyroelectric |
| References | [2][3] |
Occurrence
editKentbrooksite was found in alkaline pegmatites within pulaskites of the Kangerdlugssuaq intrusion in East Greenland.[3]
Notes on chemistry
editRare 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
edit- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ a b c d Mindat, Kentbrooksite, http://www.mindat.org/min-7132.html
- ^ a b c d 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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