Chemistry Reference
In-Depth Information
3.2.1.5 Lithium-bearing micas
The composition of lithium-bearing micas are deduced from those of the
common micas by substitutions:
Al
VI
2 Li
Al
VI
Li M
2+
2 R
2+
Al
VI
Li
where R
2+
refers to a divalent cation and a vacant site
Introduction of lithium in the lattice is accompanied by the introduc-
tion of fluoride. There are miscibility gaps, yet insufficiently explored, and
discontinuities in the crystallization, the results of which are discontinuous
zonations observable under the microscope (cross polarized light).
The general term lepidolite describes purely aluminous lithium-bearing
micas intermediate between di- and tri-octahedral micas. Zinnwaldite are
trioctahedral micas; they are practically devoid of magnesium.
Lithium-bearing micas are rare minerals that occur in lithic pegmatites
where they are associated with spodumene, amblygonite (LiAlPO
4
(F, OH)),
petalite, tourmaline, topaz, beryl, albite, quartz, cassiterite, fluorite. They
are also found in hydrothermal veins (and greisen).
3.2.1.6 Paragonite
Paragonite Na
2
Al
4
Si
6
Al
2
O
20
(OH)
4
is the sodic equivalent of muscovite,
which is indistinguishable under the microscope. There is a solvus between
muscovite and paragonite and the proportion of potassium entering para-
gonite, as with sodium entering muscovite, increases with temperature. The
natural paragonite may contain up to 4 wt% of K
2
O.
Paragonite is a metamorphic mineral. Its occurrences are similar to
those of muscovite: meta-pelites from low to medium temperature (with
kyanite, staurolite, garnet) from low to high pressure. It occurs particularly
in blueschist and eclogite facies.
3.2.1.7 Brittle micas
Margarite Ca
2
Al
4
Si
6
Al
2
O
20
(OH)
4
is a dioctahedral brittle mica, calciuc
equivalent to muscovite.
Clintonite and xanthophyllite Ca
2
(Mg, Al)
6
Si
1,5
Al
5,5
O
20
(OH)
4
are trioc-
tahedral micas. They are the equivalent of phlogopite. The iron only sub-
stitutes very little to magnesium. These two minerals are distinguished by
their optical properties:
α
is perpendicular to (010) in clintonite, parallel in
xanthophyllite.
Brittle micas are rare minerals.
