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Lepidolite is a series between polylithionite and
trilithionite
Formulae:
Polylithionite: KLi2AlSi4O10F2
Trilithionite: KLi1.5Al1.5(Si3Al)O10F2
Both are phyllosilicates (sheet silicates) mica group
Crystal System: Monoclinic
Specific gravity: 2.8 to 2.9 measured, 2.83 calculated
Hardness: 2 to 2½
Streak: White
Colour: Pink, lilac, reddish; The lavender pink colour is due to Mn3+. The colour of the mineral is not
an indication of its lithium content.
Solubility: Slightly soluble in hydrochloric acid
Common impurities in polylithionite: Ti,Fe,Mn,Mg,Ca,Na,H2O
Environments:
Plutonic igneous environments
Pegmatites (almost exclusively)
Hydrothermal environments
Lepidolite is a comparatively rare mineral, found in almost exclusively in
lithium-rich
pegmatites,
(DHZ 3 p90)
usually associated with other lithium-bearing
minerals such as pink and green tourmaline,
amblygonite,
spodumene and zinnwaldite (a series between
siderophyllite and
polylithionite), as well as
topaz, cassiterite,
beryl and
quartz
(DHZ 3 p90).
It is often intergrown with
muscovite in parallel layers. It appears that lepidolite forms late in
the crystallisation of pegmatites, succeeding the more common muscovite and
biotite of the outer pegmatite zones.
Lepidolite has also been reported in granite and
aplite, and in high temperature veins which are often tin-bearing
(DHZ 3 p90).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, lepidolite is rare, but in some pockets
muscovite is overgrown by lepidolite. The Emmons pegmatite is an example of a
highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.510).
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