Kentbrooksite is a moderately rare mineral of the eudialyte group, with formula (Na,REE)15(Ca,REE)6Mn3Zr3NbSi[(Si9O27)2(Si3O9)2O2]F2·2H2O. It is a trigonal-ditrigonal pyramidal yellowish-brown mineral containing calcium, fluorine, hydrogen, la, ce, pr, nd, sm, manganese, niobium, oxygen, silicon, sodium, and zirconium. 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.

General Information

  • Category: Silicate mineral, Cyclosilicate
  • Formula: (Na, REE)15(Ca, REE)6Mn3Zr3NbSi[(Si9O27)2(Si3O9)2O2]F22H2O
  • Crystal system: Trigonal
  • Crystal class: Ditrigonal pyramidal (3m)


The characteristic features of kentbrooksite, that make it different from eudialyte are (1) dominance of fluorine (the only currently known example among the whole group), (2) dominance of manganese, and (3) dominance of niobium. Trace hafnium and magnesium are also reported.

  • Color: Yellow-brown
  • Crystal habit: aggregates (anhedral to subhedral)
  • Fracture: Uneven
  • Tenacity: Brittle
  • Mohs scale hardness: 5-6
  • Luster: Vitreous
  • Streak: White
  • Diaphaneity: Transparent
  • Density: 3.10 (measured)


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

Kentbrooksite is relatively common when compared to most other species of the group.


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