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Formula: NaF
Normal halide, halite group, very poisonous
Crystal System: Isometric
Specific gravity: 2.79 measured, 2.81 calculated
Hardness: 2 to 2½
Streak: White
Colour: Dark carmine-red, dark red, light orange, pink
Solubility: Soluble in water
Environments
Plutonic igneous environments
Pegmatites
Chemical sedimentary environments
Villiaumite is formed in nepheline syenite and
nepheline syenite pegmatites, and also in lake-bed deposits, associated
with aegirine, sodalite,
nepheline, neptunite,
lamprophyllite, pectolite,
serandite, eudialyte,
ussingite, chkalovite and
zeolites
(HOM).
There are three principal fluoride minerals in igneous rocks, villiaumite, cryolite
and fluorite. Villiaumite has only been found in
nepheline syenites with a higher molecular content of sodium and
potassium combined than of aluminium. It occurs in association with nepheline,
analcime, K-feldspar,
aegirine or alkali-rich amphibole, and never
with quartz. Thermodynamic calculations show that assemblages deficient in both calcium
and silica favour the formation of villiaumite
(AM55.126-134).
At the type locality, Rouma Island, Los Archipelago, Guinea, villiaumite occurs in small cavities in
nepheline syenite
(Mindat).
At the Palitra Pegmatite, Kola Peninsula, Russia, villiaumite of at least three generations is widespread in all assemblages.
It forms inclusions in large crystals of other early minerals including eudialyte,
lorenzenite, lomonosovite and
especially microcline. First stage villiaumite forms intergrowths with
microcline; second stage forms inclusions up to 3 cm in
analcime and sodalite and third generation
forms aggregates up to 30 cm in the core of the pegmatite, filling cavities together with simultaneously crystallised
natrosilite. Perfect crystals of earlier formed
analcime, serandite,
nordite-(Ce), ferronordite-(Ce),
vuonnemite, bario-olgite and other
minerals are commonly present inside third stage villiaumite aggregates
(Minrec 36.5.412).
Alteration
aegirine, nepheline and F2
to villiaumite, albite, magnetite
and O2
6NaFe3+Si2O6 + 6NaAlSiO4 + 3F2 = 6NaF + 6Na(AlSi3O8)
+ 2Fe2+Fe3+2O4 + 2O2
The reaction above represents the typical occurrence of villiaumite in an assemblage of
aegirine, nepheline,
albite and magnetite
(AM55.126-134).
villiaumite, quartz and anorthite to
albite and fluorite
2NaF + 4SiO2 + CaAl2Si2O8 = 2NaAlSi3O8 + CaF2
Villiaumite and quartz cannot co-exist because they combine with
anorthite present in the assemblage to form
albite and fluorite, according to the
above equation
(AM55.126-134).
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