Zirsilite-(Ce)

Zirsilite-(Ce) is a very rare mineral of the eudialyte group, with formula (Na,□)₁₂(Ce,Na)₃Ca₆Mn₃Zr₃NbSi(Si₉O₂₇)₂(Si₃O₉)₂O(OH)₃(CO₃)·H₂O.^[2][3] The original formula was extended to show the presence of cyclic silicate groups and the presence of silicon at the M4 site, according to the nomenclature of the eudialyte group.^[4] Zirsilite-(Ce) differs from carbokentbrooksite in cerium-dominance over sodium only. Both minerals are intimately associated.^[2][3] The only other currently known representative of the eudialyte group having rare earth elements (in particular cerium, as suggested by the "-Ce)" Levinson suffix in the name^[5]) in dominance is johnsenite-(Ce).^[2]

Zirsilite-(Ce)
General
Category	Cyclosilicate
Formula	(Na,□)12(Ce,Na)3Ca6Mn3Zr3Nb(Si25O73)(OH)3(CO3)·H2O (original form)
IMA symbol	Zir-Ce[1]
Strunz classification	9.CO.10
Dana classification	64.1.5
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	R3m
Unit cell	a = 14.25, c = 30.08 [Å] (approximated); Z = 3
Identification
Color	Creamy white
Crystal habit	rhombohedra (rims of zoned crystals)
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Density	3.15 (measured)
Optical properties	Uniaxial (−)
Refractive index	nω = 1.65, nε = 1.64 (approximated)
Pleochroism	None
Ultraviolet fluorescence	No
References	[2][3]

Occurrence and association

Zirsilite-(Ce) and carbokentbrooksite occur in pegmatites of Darai-Pioz alkaline massif, Tajikistan – a locality known for many rare minerals.^[6] They are found as replacements of grains and crystals of eudialyte.^[2] The minerals are associated with aegirine, ekanite, microcline, polylithionite, quartz, stillwellite-(Ce) (silicates), pyrochlore-group mineral, fluorite, calcite, and galena.^[3]

Notes on chemistry

Beside the elements given in the formula, zirsilite-(Ce) contains admixtures of lanthanum, strontium, neodymium, iron, yttrium, titanium, potassium, chlorine, and praseodymium.^[3]

Notes on structure

Zirsilite-(Ce) is isostructural with kentbrooksite.^[3]