Images
Formula: Na2NaAlF6
Fluoride, cryolite subgroup,
perovskite supergroup. Epitaxial mineral:
thénardite (Mindat)
Crystal System: Monoclinic
Specific gravity: 2.96 to 2.98 measured, 2.973 calculated
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).
Common impurities: Fe,Ca
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)
Back to Minerals