Raslakite

Raslakite is a rare mineral of the eudialyte group with the chemical formula Na₁₅Ca₃Fe₃(Na,Zr)₃Zr₃(Si,Nb)SiO(Si₉O₂₇)₂(Si₃O₉)₂(OH,H₂O)₃(Cl,OH). This formula is based on the original one, and is extended to show the presence of cyclic silicate groups. The additional silicon and oxygen shown in separation from the cyclic groups (in parentheses) are in fact connected with two 9-fold rings.^[2] The mineral has lowered symmetry (space group R3, instead of more specific for the group R3m one),^[3] similarly to some other eudialyte-group members: aqualite, labyrinthite, oneillite and voronkovite.^[2] The specific feature of raslakite is, among others, the presence of sodium and zirconium at the M2 site. Raslakite was named after Raslak Cirques located nearby the type locality.^[3]

Raslakite
Red glassy raslakite in nepheline-syenite from the type locality: Karnasurt Mountain, Lovozero Massif, Northern Region, Russian Federation.
General
Category	Silicate mineral
Formula	Na15Ca3Fe3(Na,Zr)3Zr3(Si,Nb)(Si25O73)(OH,H2O)3(Cl,OH) (original form)
IMA symbol	Rsk[1]
Strunz classification	9.CO.10
Crystal system	Trigonal
Crystal class	Pyramidal (3) H-M symbol: (3)
Space group	R3
Unit cell	a = 14.23, c = 30.02 [Å]; Z = 3
Identification
Color	Brownish-red
Crystal habit	Grains
Cleavage	Imperfect
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.95 (measured)
Optical properties	Uniaxial
Refractive index	nω = 1.61, nε = 1.61 (approximated)
References	[2][3]

Occurrence and association

Raslakite was discovered, together with ikranite, in peralkaline pegmatites of the Mt. Karnasurt, Lovozero massif, Kola Peninsula, Russia. Raslakite is associated with aegirine, fluorcaphite, kazakovite, lamprophyllite, microcline, nepheline, and terskite.^[3]

Notes on chemistry

Beside main elements given in the formula, raslakite contains some potassium, strontium, manganese, magnesium, cerium, titanium, and aluminium, with minor amounts of lanthanum and hafnium.^[3]

Notes on crystal structure

The M(1) site in raslakite is split into two sub-sites, where Fe and Ca are located. The M(2) site is occupied by sodium, manganese (both with coordination number 5), and zirconium (tetrahedral coordination).^[3]