Leifite

Leifite
Leifite from Mont Saint-Hilaire, Canada
General
Category	Silicate mineral
Formula (repeating unit)	Na7Be2(Si15Al3)O39(F,OH)2
Strunz classification	9.EH.25 (10 ed) 8/J.10-10 (8 ed)
Dana classification	78.07.10.01
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	P3m1
Identification
Formula mass	425.47 g/mol
Colour	white, colourless to pale violet
Crystal habit	Typically radiating aggregates of fine needles
Twinning	None
Cleavage	Distinct on {1010}
Fracture	Uneven to splintery
Tenacity	Brittle
Mohs scale hardness	6
Lustre	Silky to vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.6
Optical properties	Uniaxial (+)
Refractive index	nω = 1.516 and nε = 1.520.
Birefringence	δ = 0. 011
References

Leifite is a rare tectosilicate. Tectosilicates are built on a framework of tetrahedra with silicon or aluminium at the centre and oxygen at the vertices; they include feldspars and zeolites, but leifite does not belong in either of these categories. It is a member of the leifite group, which includes telyushenkoite (Cs,Na,K)Na₆(Be₂Al₃Si₁₅O₃₉) and eirikite KNa₆Be₂(Si₁₅Al₃)O₃₉F₂. Leifite was discovered in 1915, and named after Leif Ericson who was a Norse explorer who lived around 1000 AD, and was probably the first European to land in North America, nearly 500 years before Christopher Columbus. Eirikite was named in 2007 after Eirik Raude, or Erik the Red, (950–1003), who discovered Greenland and who was the father of Leif Ericson. The third mineral in the group, telyushenkoite, was discovered in 2001. It was not named after any of Leif Ericson's family members, but after a professor of geology in Turkmenistan.

Structure

Leifite is a trigonal mineral, class 3 2/m, space group P3m1. There are 3 formula units in the unit cell (Z = 3), and cell dimensions are 14.4 Å in the a direction and 4.9 Å in the c direction. It contains OH groups, but no water of crystallization as was previously assumed. Tetrahedrons of silicon or aluminium atoms surrounded by four oxygen atoms link to form six-membered rings stacked along the c direction to form channels, similar to those in zeolites.

Appearance

Leifite is generally white or colourless, with a white streak and a silky or vitreous lustre. It occurs as fine needles making up radiating aggregates and rosettes. Individual crystals are deeply striated hexagonal prisms that are transparent to translucent.

Physical Properties

The mineral is hard, with Mohs hardness 6, the same as that of feldspar, and specific gravity 2.6, again like the feldspars. It is brittle, with an uneven to splintery fracture.

Optical Properties

Leifite is uniaxial(+) with refractive indices n_ω = 1.516 and n_ε = 1.520. The maximum birefringence δ = 0. 011. Leifite is not luminescent.

Occurrence

The type locality is the Narsaarsuk pegmatite on the Narsaarsuk Plateau, Igaliku, Narsaq, Kujalleq, Greenland and type material is kept at the University of Copenhagen, Denmark, and at the National Museum of Natural History in Washington DC, US. Leifite occurs in cavities in alkali-pegmatite veins. At Mont Saint-Hilaire it occurs as crystals up to 5 cm long and as disk-shaped aggregates and radially fibrous spheres associated with albite, natrolite, serandite and catapleite. In Greenland it occurs in a pegmatite with microcline, calcite, zinnwaldite and acmite. In Russia it occurs in the Lovozero Massif associated with albite and natrolite. In Norway it occurs in a nepheline syenite pegmatite on the south eastern part of the island of Vesle Aroya in the Langesundsfjord District, Oslo Region, as white to colourless fibrous masses and radiating bundles.