Lepidolite

Lepidolite
Lepidolite, Virgem da Lapa, Minas Gerais, Brazil (size 2.4 x 2.1 x 0.7 cm)
General
Category	Phyllosilicate
Formula (repeating unit)	K(Li,Al)3(Al,Si,Rb)4O10(F,OH)2
Strunz classification	9.EC.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m, Cm
Unit cell	a = 5.209(2) Å, b = 9.011(5) Å, c = 10.149(5) Å; β = 100:77(4)°; Z = 2
Identification
Color	Pink, purple, rose-red, violet-gray, yellowish, white, colorless
Crystal habit	Tabular to prismatic pseudohexagonal crystals, scaly aggregates and massive
Twinning	Rare, composition plane {001}
Cleavage	{001} perfect
Fracture	Uneven
Mohs scale hardness	2.5–3
Luster	Vitreous to pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.8–2.9
Optical properties	Biaxial (-)
Refractive index	nα=1.525–1.548, nβ=1.551–1.58, nγ=1.554–1.586
Birefringence	0.0290–0.0380
Pleochroism	X = almost colorless; Y = Z = pink, pale violet
2V angle	0° - 58° measured
References

Lepidolite is a lilac-gray or rose-colored member of the mica group of minerals with formula K(Li,Al,Rb)₂(Al,Si)₄O₁₀(F,OH)₂. It is the most abundant lithium-bearing mineral and is a secondary source of this metal. It is a phyllosilicate mineral and a member of the polylithionite-trilithionite series.

It is associated with other lithium-bearing minerals like spodumene in pegmatite bodies. It is one of the major sources of the rare alkali metals rubidium and caesium. In 1861 Robert Bunsen and Gustav Kirchhoff extracted 150 kg (330 lb) of lepidolite and yielded a few grams of rubidium salts for analysis, and therefore discovered the new element rubidium.

It occurs in granite pegmatites, in some high-temperature quartz veins, greisens and granites. Associated minerals include quartz, feldspar, spodumene, amblygonite, tourmaline, columbite, cassiterite, topaz and beryl.

Notable occurrences include Brazil; Ural Mountains, Russia; California, United States; Tanco Mine, Bernic Lake, Manitoba, Canada; and Madagascar.