Kentbrooksite

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	(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

Kentbrooksite was found in alkaline pegmatites within pulaskites of the Kangerdlugssuaq intrusion in East Greenland.^[3]

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.