Cryolite

halide

thenardite

Formula: Na2NaAlF6
Halide, perovskite group. Epitaxial mineral: thénardite (Mindat)
Specific gravity: 2.96 to 2.98
Hardness: 2½
Streak: White
Colour: Colourless, white, brown, grey, black; colourless in transmitted light.
Solubility: Soluble in sulphuric acid with evolution of HF. Slightly soluble in water. Readily soluble in AlCl3 solution (Mindat).
Melting Point: 1020°. Inverts to an isometric form at about 560° (Mindat).
Environments

Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Carbonatites
Sedimentary environments rare

Cryolite is a late-stage mineral in some granite pegmatites; in tin-bearing granite; a vapour-phase mineral along fractures and in the groundmass of some fluorine-rich, topaz-bearing rhyolite; in pods in a carbonatite vein cutting biotite gneiss. Also as a rare authigenic (formed in place) component of the marl and shale of the Green River Formation (HOM). There are three principal fluoride minerals in igneous rocks, villiaumite, cryolite and fluorite. Cryolite is usually accompanied by quartz, and occurs in silica-rich but calcium-poor rocks, such as granite (AM55.126-134).
Common associates include chiolite, feldspar, fluorite, pachnolite, quartz and topaz (Mindat).
Cryolite alters by weathering to a series of secondary fluorides, including pachnolite, thomsenolite, gearksutite, prosopite, cryolithionite, weberite and jarlite (Dana).

At Mont Saint-Hilaire, Quebec, Canada, cryolite is associated with sodalite, villiaumite, eudialyte, lovozerite, natrolite, chabazite and aegirine (HOM).

At the Francon quarry, Montreal, Qebec, Canada, cryolite is associated with calcite, dresserite, hydrodresserite, dawsonite and weloganite in the upper sill, and with siderite, calcite, gibbsite, quartz, doyleite and dawsonite in the lower sill. A unique assemblage in a very small zone of the lower sill contained cryolite, weloganite, elpidite, gibbsite, baryte, strontianite, calcite and marcasite. Perimorphs of gibbsite or fluorite mixed with doyleite after cryolite have been found. (Minrec 37.1.24-26)

At the type locality, the Ivigtut cryolite deposit, Greenland, Denmark, cryolite occurs as a pegmatitic body in a granite stock intruded into gneiss, associated with zircon, wolframite (material between hübnerite and ferberite), weberite, topaz, thomsenolite, siderite, quartz, pyrite, prosopite, pachnolite, molybdenite, microcline, mica, jarlite, gearksutite, galena, fluorite, cryolithionite, columbite, chiolite, cassiterite and arsenopyrite (Mindat, HOM).

Alteration

aegirine, albite and F2 to cryolite, quartz, magnetite and O2
6NaFe3+Si2O6 + 3albite + 9F2 = 3Na2NaAlF6 + 21SiO2 + 2Fe2+Fe3+2O4 + 5O2
(AM55.126-134)

aegirine, anorthite, quartz and cryolite to albite, fluorite, magnetite and O2
12NaFe3+Si2O6 + 9CaAl2Si2O8 + 21SiO2 + 3Na2NaAlF6 = 21NaAlSi3O8 + 9CaF2 + 4Fe2+Fe3+2O4 + O2
It is only in anorthite-poor siliceous rocks that cryolite will form in preference to fluorite.
(AM55.126-134)

aegirine and cryolite to villiaumite, magnetite, albite, quartz and O2
12NaFe3+Si2O6 + 3Na2NaAlF6 = 18NaF + 4Fe2+Fe3+2O4 + 3NaAlSi3O8 + 15SiO2 + O2
The forward reaction forming villiaumite occurs only in silica- and anorthite-poor rocks, such as phonolite.
(AM55.126-134)

albite, aegirine, F2 and CO2 to cryolite, siderite, quartz and O2
NaAlSi3O8 + 2NaFe3+Si2O6 + 3F2 + 2CO2 = Na2NaAlF6 + 2FeCO3 + 7SiO2 +2O2
Large quantities of siderite occur with cryolite at Ivigtut, Greenland, possibly formed by the above reaction.
(AM55.126-134)

Common impurities: Fe,Ca

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