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| Formula | K(Li,Al)3(Si,Al)4O10(F,OH)2 - K(Li,Fe2+,Al)3(Si,Al)4O10(F,OH)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X near c, Y = b, Z near a |
| | Optical plane | (010) |
| | Relief | Low-negative to low-positive |
| | Refractive indices | nx = 1.528 -1.557
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ny = 1.548 -1.585
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nz = 1.551 -1.588
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n increases with increasing Fe and Mn; n decreases with increasing substitution of F for OH
Lpd: nx = 1.528 - 1.540, ny = 1.548 - 1.559, nz = 1.551 - 1.567
Znw: nx = 1.542 - 1.557, ny = 1.567 - 1.585, nz = 1.570 - 1.588 |
| | Birefringence (max.) | 0.022 - 0.032 |
| | | Lpd: 0.022 - 0.032; Znw: 0.029 - 0.032 |
| | Optic Angle
| 2Vx
= 2M: 27 - 50°, 3T varieties: 0° |
| | 2Vz
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| | Sign of elongation | Length-slow, l(+) |
| | Interference figure | (001) sections yield centred acute bisectrix figures (or uniaxial figures if 2V = 0) with first- to second-order isochromes |
| | Colour / pleochroism | Lpd: Colourless; strongly coloured varieties may show pleochroism to pale violet or pale green. Znw: colourless to pale grey-brown. Absorption Z ≥ Y > X. |
| | Zoning | |
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| Form | Habit | Hexagonal-platy, fine-grained scaly aggregates |
| | | Surface | Typically subhedral |
| | Cleavage | {001} |
| | Twinning | {001} rare |
| | Extinction | Lpd: max 0 – 7°, up to 10° in Znw |
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| Reaction textures | Replacement by muscovite |
| | Alteration / decomposition | Relatively stable |
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| Occurence | Ign | Granitic pegmatites, also in granites |
| | | Met | |
| | | Sed | |
| | | Hyd | Veins |
| | | Other | |
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| Distinctive properties | Low relief; colour (if visible), other properties are very similar to muscovite; restricted occurrence. |
| | Additional comments | Stoichiometry: The octahedral positions are commonly not completely filled.
The pink colour of lepidolite is due to Mn2+.
In the n-Δn chart, reference data for polylithionite (Pln), KLi2Al(Si4O10)F2, trilithionite (Trl), KLi1.5Al1.5(AlSi3O10)F2, and muscovite (Ms) are shown. A second n-Δn chart shows data for the more exotic Li micas ephesite, NaLiAl2(Al2Si2O10)(OH)2, Masutomilite KLiAlMn(AlSi3O10)F2, and tainiolite, KLiMg2(Si4O10)F2, with the lepiidolite (Lpd) and zinnwaldite (Znw) fields for reference. |
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