Narsarsukite

Narsarsukite is a rare silicate mineral with either the chemical formula Na₂(Ti,Fe³⁺)Si₄(O,F)₁₁[3] or Na₄(Ti,Fe)₄[Si₈O₂₀](O,OH,F)₄.[1]

Narsarsukite
1.3 cm crystal of narsarsukite from Mont Saint-Hilaire
General
Category	Silicate mineral
Formula (repeating unit)	Na4(Ti,Fe)4[Si8O20](O,OH,F)4[1]
IMA symbol	Nar[2]
Strunz classification	9.DJ.05
Crystal system	Tetragonal
Crystal class	Dipyramidal (4/m) H-M symbol: (4/m)
Space group	I4/m
Unit cell	a = 10.72, c = 7.95 [Å]; Z = 4
Identification
Color	Honey to lemon-yellow, reddish brown, brownish gray, tan, pink; may be green from inclusions; may show color zoning
Crystal habit	Occurs as flat tabular to equant, striated crystals In divergent, radiating groups; massive
Cleavage	Good on {100} and {110}
Fracture	Uneven to subconchoidal
Tenacity	Brittle
Mohs scale hardness	5.5 - 7
Luster	Vitreous, pearly on {110}
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.64-2.83
Optical properties	Uniaxial (+)
Refractive index	nω = 1.609 nε = 1.630
Birefringence	δ = 0.021
Pleochroism	Weak; O = colorless to yellow; E = colorless to honey-yellow
References	[3][1][4]

It was first described in 1900 for an occurrence in the Narsarsuk pegmatite in the Ilimaussaq intrusive complex of West Greenland.[1] It has also been reported from a syenite which intruded limestone in the Sweetgrass Hills, Montana, and within hornfels and marble xenoliths in the alkalic intrusive of Mont Saint-Hilaire, Quebec.[3] It occurs associated with aegirine, microcline, albite, elpidite, epididymite, taeniolite, pectolite, calcite, galena and quartz.[3]

Narsarsukite from Whitlash, Liberty County, Montana {size: 5.6 x 5.4 x 5.1 cm)