Alluaivite

Alluaivite is a rare mineral of the eudialyte group,^[3] with complex formula written as Na₁₉(Ca,Mn)₆(Ti,Nb)₃Si₂₆O₇₄Cl·2H₂O.^[4][3] It is unique among the eudialyte group as the only titanosilicate (other representatives of the group are usually zirconosilicates). The two dual-nature minerals of the group, being both titano- and zirconosilicates, are labyrinthite and dualite. They both contain alluaivite module in their structures.^[5][6] Alluaivite is named after Mt. Alluaiv in Lovozero Tundry massif, Kola Peninsula, Russia, where it is found in ultra-agpaitic, hyperalkaline pegmatites.^[2][3][4]

Alluaivite
General
Category	Silicate mineral, Cyclosilicate
Formula	Na19(Ca,Mn)6(Ti,Nb)3Si26O74Cl·2H2O
IMA symbol	Aav[1]
Strunz classification	9.CO.10 (10 ed) 8/E.25-40 (8 ed)
Dana classification	64.1b.1.1
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3m
Unit cell	a = 14.04, c = 60.6 [Å]; Z = 6
Identification
Color	Colorless to weak brownish pink
Crystal habit	irregular accumulations
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Density	2.76 (measured)
Optical properties	Uniaxial (+)
Refractive index	nω = 1.62, nε = 1.63 (approximated)
Pleochroism	Colorless to pink (W), pink (E)
Ultraviolet fluorescence	Orange-red
Common impurities	Sr, REE, K, Ba, Zr
References	[2][3]

Notes on chemistry

Alluaivite contains relatively high amounts of admixing strontium, cerium, potassium, and barium, with lesser amounts of substituting lanthanum and zirconium.^[2]

Occurrence and association

Alluaivite was found in ultra-agpaitic (highly alkaline) pegmatites on Mt. Alluaiv, Lovozero massif, Kola Peninsula, Russia – hence its name. Associating minerals are aegirine, arfvedsonite, eudialyte, nepheline, potassic feldspar, and sodalite.^[2]