Lepidolite

Lepidolite
Lepidolite, Virgem da Lapa, Minas Gerais, Brazil (size 2.4 x 2.1 x 0.7 cm)
General
Category	Silicate mineral
Formula (repeating unit)	K(Li,Al)3(Al,Si,Rb)4O10(F,OH)2
Strunz classification	09.EC.20
Crystal symmetry	Monoclinic H-M symbol: 2/m Space group: C 2/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
Crystal system	Monoclinic
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	[1][2]

Lepidolite is a lilac-gray or rose-colored member of the mica group with formula K(Li,Al,Rb)₃(Al,Si)₄O₁₀(F,OH)₂.^[1][2] It is a secondary source of lithium. It is a phyllosilicate mineral^[3] and a member of the polylithionite-trilithionite series.^[4]

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.^[5] In 1861 Robert Bunsen and Gustav Kirchhoff extracted 150 kg of lepidolite and yielded a few grams of rubidium salts for analysis, and therefore discovered the new element rubidium.^[6]

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.^[1]

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

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  Yellow lepidolite from Itinga, Minas Gerais, Brazil. Size: 6.1 x 4.9 x 3.1 cm

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  Lavender lepidolite "books" from Himalaya Mine, Mesa Grande District, San Diego County, California, USA. Size: 4.8 x 3.9 x 3.5 cm

References

1. ^ ^{a b c} Handbook of Mineralogy
2. ^ ^{a b} Webmineral
3. ^ Hurlbut, Cornelius S.; Klein, Cornelis (1985), Manual of Mineralogy, Wiley, (20th ed.) ISBN 0-471-80580-7
4. ^ Lepidolite on Mindat.org
5. ^ H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968.
6. ^ G. Kirchhoff, R. Bunsen (1861). "Chemische Analyse durch Spectralbeobachtungen".