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1.3 cm crystal of narsarsukite from Mont Saint-Hilaire
Category Silicate mineral
(repeating unit)
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
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 [2][1][3]

Narsarsukite is a rare silicate mineral with chemical formula Na2(Ti,Fe3+)Si4(O,F)11[2] or Na4(Ti,Fe)4[Si8O20](O,OH,F)4.[1]

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.[2] It occurs associated with aegirine, microcline, albite, elpidite, epididymite, taeniolite, pectolite, calcite, galena and quartz.[2]

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