Kentbrooksite
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CategorySilicate mineral, Cyclosilicate
Formula(Na,REE)15(Ca,REE)6Mn3Zr3NbSi25O74F2·2H2O (original form)
Strunz classification9.CO.10 (10 ed)
8/E.23-20 (8 ed)
8/E.23-20 (8 ed)
| Kentbrooksite | |
|---|---|
 | |
| General | |
| Category | Silicate mineral, Cyclosilicate |
| Formula | (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] |
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]
