Villiaumite

halide

nepheline syenite

Formula: NaF
Normal halide, halite group, very poisonous
Specific gravity: 2.79
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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