Lepidolite

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.[1][2] It is the most abundant lithium-bearing mineral[3] and is a secondary source of this metal. It is a phyllosilicate mineral[4] and a member of the polylithionite-trilithionite series.[5] Lepidolite is part of a three-part series consisting of polylithionite, lepidolite, and trilithionite. All three minerals share similar properties and are caused because of varying ratios of lithium and aluminum in their chemical formulas. The Li:Al ratio varies from 2:1 in polylithionite up to 1.5:1.5 in trilithionite. [6]

Lepidolite
General
CategoryPhyllosilicate
Formula
(repeating unit)		K(Li,Al)3(Al,Si,Rb)4O10(F,OH)2
Strunz classification9.EC.20
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m, Cm
Unit cella = 5.209(2) Å, b = 9.011(5) Å,
c = 10.149(5) Å;
β = 100:77(4)°; Z = 2
Identification
ColorPink, light purple, purple, rose-red, violet-gray, yellowish, white, colorless other colors possible but are rare.
Crystal habitTabular to prismatic pseudohexagonal crystals, scaly aggregates and massive
TwinningRare, composition plane {001}
Cleavage{001} perfect
FractureUneven
Mohs scale hardness2.5–3
LusterVitreous to pearly
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.8–2.9
Optical propertiesBiaxial (-)
Refractive indexnα=1.525–1.548, nβ=1.551–1.58, nγ=1.554–1.586
Birefringence0.0290–0.0380
PleochroismX = almost colorless; Y = Z = pink, pale violet
2V angle0° - 58° measured
References[1][2]

Lepidolite is found naturally in a variety of colors, mainly pink, purple, and red, but also gray and, rarely, yellow and colorless. Because lepidolite is a lithium-bearing mica, it is often wrongly assumed that lithium is what causes the pink hues that are so characteristic of this mineral. Instead, it is trace amounts of manganese that cause the pink, purple, and red colors. [7]

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.[8] 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.[9]

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.

References
  1. Handbook of Mineralogy
  2. Webmineral
  3. Deer, W.A.; Howie, R.A.; Zussman, J. (1966). An Introduction to the Rock Forming Minerals. London: Longman. p. 218. ISBN 0-582-44210-9.
  4. Hurlbut, Cornelius S.; Klein, Cornelis (1985), Manual of Mineralogy, Wiley, (20th ed.) ISBN 0-471-80580-7
  5. Lepidolite on Mindat.org
  8. H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968.
  9. G. Kirchhoff, R. Bunsen (1861). "Chemische Analyse durch Spectralbeobachtungen" (PDF). Annalen der Physik und Chemie. 189 (7): 337–381. Bibcode:1861AnP...189..337K. doi:10.1002/andp.18611890702.
The 3D structure of lepidolite consists of alumina-silicate layers (blue, red) with the green (lithium) and the purple (various alkali metals, such as potassium). Image generated by the VESTA (Visualization for Electronic and Structural analysis) software.http://jp-minerals.org/vesta/en/
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