CRYOLITE

Cryolite – Na3[AlF6] – is a very rare halide that was historically used as an Al ore and later as a flux in the electrolytic processing of bauxite, the main Al ore. It was mined for over a century from an occurrence in Ivigtut, Greenland. The world’s largest reserve, of more than 9 million tons, is in Pitinga (state of Amazonas, Brazil), but at a depth of 150 m, making its exploitation economically unviable. Nowadays, cryolite is produced through a synthetic process (Bayer and Hall-Héroult) from fluorite and is called “chemical cryolite”. Due to its rarity, pieces with well-formed crystals fetch high prices in the collector’s mineral market.

It is slightly soluble in water and has a refractive index almost identical to that of water. Therefore, if a very clear sample is immersed in water, it “disappears”, more or less like ice, except it does not float. Due to this characteristic, cryolite received the nickname “Eisstein” (ice stone) in German.

It is classified in the Perovskite Supergroup. It typically contains Fe and Ca. Under short UV wavelengths, it can exhibit intense fluorescence in yellow or blue-white. Under long UV wavelengths, it can exhibit yellow to weakly yellow phosphorescence. It is weakly thermoluminescent and melts at 1020º.

1. Characteristics

Crystal system: Monoclinic prismatic.  

Color: Colorless, white, brown, gray, pink, rarely black.  

Habit: Typically massive or coarse-grained. Pseudocubic crystals, rare, up to 3 cm.

Cleavage: No, but several partitions simulate pseudo-cubic cleavage. Various striations!       

Tenacity: Brittle.        

Twinning: Twins are very common. There are 13 different types of twins: repeating, penetration, and polysynthetic. Crystals can simultaneously exhibit several twinning laws.       

Fracture: Irregular.       

Mohs Hardness: 2.5

Parting: On {001} and {110}.         

Streak: White.         

Lustre: Vitreous, greasy, pearly.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.98

           

2. Geology and Deposits

Cryolite occurs as a late mineral in some granitic pegmatites, in Sn-containing alkaline granites, and as a vapor-phase mineral in some fluorine-rich, topaz-bearing rhyolites.

It is also found in pockets within a carbonatite vein that runs through a fenitized biotite gneiss.

Rarely authigenic in marls and shales of the Green River Formation.

 

3. Mineral Associations

It occurs associated with gangue minerals typical of granitic pegmatites: quartz (including smoky quartz variety), microcline (including amazonite variety), albite, micas (biotite, muscovite, lithionite-polylithionite) and fluorite. In these pegmatites it also associates with sulfides (pyrite, galena, chalcopyrite, sphalerite, arsenopyrite, molybdenite), oxides (cassiterite, wolframite, columbite), carbonates (siderite), “limonite” (goethite), phosphates (triplite), zircon, topaz and kaolinite.

Also in pegmatites and other parageneses, with rare minerals such as chiolite, cryolithionite, gearksutite, prososite, thomsenolite, weberite, jarlite, villiaumite, eudialyte, hydrokenoralstonite, sabinaite, gaidonnayite, fluocerite-(Ce), gagarinite-(Y), genthelvite, fluorannite, weloganite, lovozerite, zeolites (natrolite, chabazite), ellingsenite, villiaumite, aegirine, sodalite, eudialyte, pachnolite and others.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.339     nβ: 1.339      nγ: 1.340

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. 

Relief: Moderate.           

Cleavage: The partitions {001} and {110} simulate pseudocubic cleavage.

Habits: Usually massive or coarse-grained.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.001, very low, resulting in black to dark gray colors and simulating isotropy. 

Extinction: It is not possible to determine this due to pseudo-isotropy. 

Elongation sign: No information available.            

Twins: Very common, 13 different types, including polysynthetic ones. 

Zoning: No information available.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 43º         

Alterations: No information available.           

May be confused with: Other medium-relief and cubic minerals (isotropic).

Fluorite may be very similar, but it does not provide the B(+) interference figure because it is isotropic.

Caption.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for identifying the mineral, as its characteristics under reflected light are identical to those of many other minerals that occur in the paragenesis.

Preparing a polished section may be useful for determining associated sulfides and oxides.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption.