Cryolite mineral: photo and formula, chemical properties and application

Cryolite was known to the Eskimos long before its discovery by Europeans. The name of the mineral comes from the Greek “krios” - ice and lithos - stone (ice stone) (d'Andrada, 1800); first reliably studied by Abilgard (1799).

The English name of the mineral is Cryolite

Synonyms: Ice stone - Eisstein (Glocker, 1831), orsugisat - Orsugisat - local designation in Greenland, meaning “fat salt” (according to Hinze); corresponds to artificial α-cryolite.

Cryolite

Formula

Na3AlF6

Chemical composition

Chemical theoretical composition: Na - 32.86; Al - 12.84; F - 54.30%. There are no chemical analyzes of cryolite that fully meet modern requirements; the limits of fluctuations in composition and the possibility of isomorphic occurrence of various elements in the crystal lattice of the mineral have not been established. Most analyzes detect a discrepancy between the amounts of cations and anions. More often a deficiency of F is detected, less often - its excess. The Na:Al ratio never corresponds to the theoretical one and in most analyzes ranges from 2.92 to 2.99 (0.36 - 0.04 wt.% Na below theoretical). These results are likely due to imperfect analysis methods. The discrepancies between the theoretical composition and analyzes of synthetic and natural minerals performed during various physicochemical and technological studies are especially large. Various variations of the cryolite formula have been proposed. Isomorphism is possible between high-temperature β-cryolite and isostructural compounds K3AlF6 and elpasolite (K2NaAlF6); in cryolite from the Ilmen Mountains, 0.08% K was discovered by flame photometry, and in cryolite from Tuva - 0.0028 Li2O, 0.0008 Rb2O and 0.0008 Cs2O, 0.07% K, 0.007 Li2O, 0.0003 Rb2O , Seine discovered. The often observed Ca is probably due to the presence of admixtures of other minerals, although Nöllner and Lemberg assumed an isomorphic replacement of sodium with calcium. Often observed in the mineral, Si and Mg are due to impurities of other minerals, and increased amounts of Li are due to the presence of cryolithionite.

Crystallographic characteristics

Syngony. Monoclinic. L2PC

Symmetry class . Prismatic - 2/m. Axis relationship. 0.973: 1: 1.391; р=90°11′.

Crystal structure

In the cryolite structure, discrete, slightly deformed AlF6 octahedra are located at the vertices and in the center of the cubic unit cell. Between them are Na atoms, 1/3 of which have coordination 6, and 2/3 - 12. NaF6 octahedra are located in the middle of the vertical edges and in the centers of the basal planes of the unit cell, and have common faces with cuboctahedra occupying the spaces between the AlF6 and NaF6 octahedra , in the centers of which the remaining 2/3 of the Na atoms are located. In another interpretation, the cryolite structure consists of chain NaAlF6 groups parallel to the c axis, with channels containing NaF groups.

Distances between atoms in AlF6 -Al octahedra - F = 1.79 - 1.83; in NaF6 octahedra Na - F = 2.42 - 2.32; for other Na atoms, the Na - F distance ranges from 2.21 to 2.68 A (Naray-Szabo and Sasvari).

High-temperature β-cryolite—cubic. The structure is of the (NH4)3AlF6 type, close to the structure of elpasolite. Upon cooling, due to the insufficient size of Na ions located in the centers of the cuboctahedra, the structure turns into a monoclinic one—the inclination of the polyhedra changes, and differences in the length of their edges appear. The (110) plane of monoclinic α-Na3AlF6 cryolite corresponds to the plane of a pseudocube, very close in size to the cell of elpasolite and (NH4)3AlF6. During the polymorphic transformation, several systems of polysynthetic twins are formed.

Cryolite: what kind of mineral is it?

Natural and artificial cryolite are very popular minerals nowadays.

For example, molten cryolite is actively used in various industries.

Cryolite is a fluoride, scientifically it is called sodium hexafluoroaluminate.

The fact is that the natural mineral is very rare, and its benefits are incredibly high.

The name means “frost stone” from Greek. Indeed, the mineral resembles a piece of snow-white ice.

Form of being in nature

The appearance of crystals. Crystals are pseudotetragonal and pseudocubic in appearance; crystals formed by m(110) and c(001) sharply predominate (compose pseudocubes); the r(110), v(101) and k(101) faces are less developed, which sometimes reach significant development with the formation pseudocubooctahedra. Rarely observed crystals of the second generation are richer in shape and sometimes form crystals of unusual habit, flattened along (001). The m(110) faces often have shading parallel to the edges. There are parallel stepwise intergrowths of crystals.

Mineral twins. Doubles are very common; Granular cryolite is always polysynthetically twinned. There are at least 14 laws of twinning, a significant part of which is established only in granular cryolite and is explained, according to most authors, by the influence of mechanical stresses that arose during the cooling of high-temperature cubic β-cryolite and during its transformation into a monoclinic α-modification. Such twins were reproduced experimentally. Twins usually appear immediately in each grain according to several laws. The general geometric patterns of this phenomenon have been considered by many authors. Various twinning laws, manifesting themselves together, increase the overall symmetry of the intergrowth to rhombic, tetragonal or cubic. Diagnosis of various types of twins presents great methodological difficulties; The laws indicated by various researchers are not accepted by everyone. The following types of twins were observed (according to Dana, 1951):

1. twin axis [110], rotation 90°. Twins of fusion and germination are common, sometimes quadruples. Twinning according to this law is very typical for coarse-grained minerals (the so-called “Baumhauer’s law”);
2. twin axis [110], rotation 180°; repeat twins are observed in granular minerals, but are less common in individual crystals. Artificially obtained by cooling a heated stone;
3. twin axis [021], rotation 120°; the fusion surface is irregular. Artificially obtained by cooling a heated mineral. In the form of thin plates they are common in granular cryolite (the so-called “new law” of Böggild);
4. twin axis [111], rotation 180°; the rhombic section is close to (110). Doubles of repeated type. Rare, found only on crystals, unknown in granular cryolite;
5. twin axis [100], rotation 180°, fusion plane (001); found only in granular cryolite, for which they are common;
6. by (100), 180° rotation around [001], fusion plane (100); found only in granular cryolite; thin plates; common;
7. twin plane and fusion plane (112); found only in granular mineral; records; common;
8. twin plane and fusion plane (112); found only in granular mineral; records; common;
9. twin plane and fusion plane (110); records; found only in coarse-grained cryolite from the Urals;
10. twin axis [111], rotation 180°; the rhombic section is not a possible face, but is close to (110); were not established by Böggild, but it is possible that this is the “law d” of Kroes and Hillebrand; found only in granular cryolite;
11. but (211); one of Padurov's new laws; apparently very rare;
12. twin axis [001], rotation 90°; very close to 9;
13. twin axis [201], rotation 120°; very close to 8;
14. twin axis [201], rotation 120°; very close to 7.

For twin crystals, the 1st law is most common, less so the 2nd and 4th. Doubles according to other laws are found only in granular stone. There appear to be epitaxial intergrowths between high-temperature cubic p-cryolite and cryolithionite; the orientation of minerals in the intergrowths has not been established. Epitaxial intergrowths with thenardite were artificially obtained.

Aggregates . Granular discharge, individual grains, crystals, porcelain-like discharge mixed with opal.

Story

The origin of the name of the stone is as follows. At the end of the 18th century, a fairly large fragment of stone, very similar to a block of ice, was found in Greenland. The naturalist scientist from Denmark, Peter Christian Abildgaard, became interested in him. It was he who completely examined it and gave a comprehensive description. The beauty of the find was so amazing and impressive that the naturalist gave it the name “Ice Stone” or “Frost Stone” from two Greek words “krios”, which means “ice” and “lithos” - “stone”. The year of its opening is considered to be 1799.

Physical properties

Optical

• Color. Usually colorless to white, also grayish, yellowish-gray, dirty brown to almost black with a bluish tint, very rarely bright pink or pale greenish. When struck with a hammer and during explosions, it can acquire a red-violet color. Cryolite sometimes acquires a dark color near inclusions of green and red-brown fluorite, red feldspar and greisen. This color also occurs outside of inclusions. The intersection of white cryolite with dark-colored veins was observed. The radiation nature of the color of dark cryolite is likely. This color appears when cryolite is irradiated for a long time with cathode rays, but quickly disappears in the light.
• The streak is white, yellowish in dark brown varieties.
• The luster of transparent crystals is glassy, ​​white cryolite is glassy to slightly greasy, yellow and dark varieties are greasy; On the separation plane along (001) a pearlescent sheen is sometimes observed.
• Transparency. Transparent to translucent in bulk, dark varieties are translucent in thin fragments.

Mechanical

• Hardness 2.5-3. Crystals are much more fragile than granular aggregates.
• Density 2.96 (Ural, Tuva).
• Cleavage. As observed on water-transparent crystals, there is no cleavage. On large crystals and grains, separation is always observed. Most often, very perfect separateness appears at (001) and distinct at (110); gives almost cubic shapes. In rare cases, a more weakly expressed separation is observed at (011) and (101). In the Ural cryolite, a very pronounced separation was noted only along one of the planes (110). The separation of the mineral is caused by thin twin plates that promote the splitting of the mineral in certain directions.
• Kink. Spar-like. In other directions the fracture is uneven, on small water-transparent crystals it is conchoidal.

Chemical properties

Dissolves in HCl and H2SO4. 0.037% of cryolite dissolves in water at 12°, and 0.034% at 15°. Synthetic cryolite has the same solubility in water: at 0° - 0.0348; 25° - 0.04175; 50° - 0.07932; 75° - 0.09302; 100° - 0.13%. Cryolite easily dissolves in an aqueous solution of AlCl3 and an acidified aqueous solution of H3BO3; with the formation of complex compounds, it is easily dissolved in HCl, HNO3, more difficult - in oxalic acid. Up to 19% of cryolite dissolves in HF; easily decomposes H2SO4 releasing HF. Slowly decomposes by fusion with KHSO4 and boiling in an alkali solution.

Other properties of cryolite

Fine polysynthetic twinning causes a bluish iridescence of the mineral. The mineral is a non-conductor of electricity at room temperature, with a specific electrical conductivity of the order of 10-8 - 10-7 ohm-1•cm-1. As the temperature increases, the electrical conductivity increases and reaches approximately 1 ohm -1 • cm-1 at the melting temperature. On the smooth curve of electrical conductivity - temperature, inflections are observed at 565° and 880° due to a sharp increase in electrical conductivity at the moments of cryolite transformations. The increase in electrical conductivity is apparently caused by the decomposition of AlF6 groups into AlF4 with the formation of mobile F ions, which increase electrical conductivity. Cryolite does not glow in either cathode or ultraviolet rays. When heated, it glows yellowish; dark varieties glow more strongly, but when heated strongly, the glow disappears and the mineral becomes discolored. When crushed, cryolite from Ivigtut luminesces very strongly, producing bluish sparks. Floats with fatty acids and their soaps.

Melting point 1013°. Heat of fusion. 18.2 kcal/g-mol. Monoclinic α-Na3AlF6 (cryolite) at 565° transforms into cubic (β-cryolite); above 881° to the melting temperature there is hexagonal (?) γ-Na3AlF6 (γ-cryolite). When cryolite is heated, birefringence gradually decreases, and movement of twin plates is observed, which disappear above 565° as a result of the transition to cubic β-cryolite. Cooling of β-cryolite is accompanied by a strong cracking sound due to a sharp change in volume during the β → α transformation; a large number of thin polysynthetic twin plates arise. Above the β → γ transformation point (881°), the substance begins to behave as a very plastic solid; a sharp increase in electrical conductivity indicates the dissociation of Na3AlF6. When melting, the volume increases by 25% and the density sharply decreases. Melting occurs congruently, but AlF6, AlF4, Na and F ions are immediately detected in the melt; the unstable AlF6 ion dissociates into AlF4 and F ions (equilibrium constant 0.06). Complete dissociation reaction: Na3AlF6 2NaF + NaAlF4 (equilibrium constant 0.09). In air, especially in the presence of water vapor, part of the NaAlF4 in the melt gives NaF and β-Al2O3, which constitute the final decomposition product of the mineral. There is data on the density of the Na3AlF6 melt, its viscosity and surface tension.

Artificial production of mineral

Cryolite is produced on an industrial scale using various methods. The most common are the following: interaction of Al and NaF sulfates; neutralization of acid fluoride gases (H2SiF6) with aluminum hydroxide and NaOH; treatment with NaF solution of aluminum hydroxide, sodium aluminate, aluminum fluoride. In laboratory conditions, it is easily synthesized by treating metallic Na and Al with hydrofluoric acid, treating NaAlF4 with NaF solution and many other methods. The mineral was synthesized together with cassiterite, topaz, albite and quartz in an autoclave at 500° by heating Al2O3, SnCl2 with Na2SiF6 in the presence of water. Cubic cryolite crystals up to 1 cm in size were formed together with chiolite and ralstonite (?) in steam boilers that received water from the Ivigtut cryolite mine, containing 0.0256% of dissolved mineral. Cryolite was established during the study of the NaF - AlF3 system; in this system the phases NaF (williomite), cryolite, chiolite, NaAlF4 and AlF3 are established; there are no solid solutions between the phases. Eutectic NaF-cryolite crystallizes at 885°.

Diagnostic signs

Similar minerals . Anhydrite, barite.

Similar minerals. Anhydrite, barite. It is characterized by snow-like aggregates and slight hardness; externally similar minerals differ from it: chiolite - medium cleavage, greater hardness (3-4), fluorite - perfect cleavage, hardness, barite - perfect cleavage, greater density, calcite - rhombohedral cleavage, boiling from HCl. Cryolite differs from cryolithionite in its ability to split into cubic fragments. In immersion, it is characterized by very low refractive indices: its transparent fragments are almost invisible in water. It differs from weberite and elpasolite by lower refractive indices, and is easily distinguished from other aluminum fluorides by its very low birefringence.

Related minerals. Quartz, siderite, galena, pyrite, chalcopyrite, sphalerite, cassiterite, topaz, fluorite.

Formula of the stone and its components

Complex stone with varied properties

Cryolite has various properties. The temperature should always be within normal limits. The basis of the stone is crystals: they appear and are created in a monoclinic system; sometimes such specimens can be found in the form of a cube or plate. Everything happens due to the accumulation of many crystals and various plates of white and slightly grayish color with an admixture of yellow tint. The shine from such materials can be glassy, ​​oily and greasy. Cryolite is intertwined with such materials and components as:

• most often, a material such as columbite is used;
• cassiterite mixtures are slightly less common;
• quartz;
• you can also find a mixture with a mineral such as chalcopyrite;
• siderite;
• you can find galena;
• pyrite.

The chemical formula of cryolite shows that this mineral is based on sodium, aluminum and iron. The formula looks like this - Na3AlF6. This strange structure of the entire formula is explained by the fact that such a mineral is of glucose origin, which in its structure has an additive such as Al2O3. They are often found transparent and translucent. But in their structure they have a rather low percentage of strength, hardness on the Mohs scale is 2.5. And its density is 3 g/cm3. From this we can conclude that cryolite has a fairly high percentage of density, but the hardness could be better.

Mineral Change

Due to its noticeable solubility in water, its outcrops resemble rock salt outcrops. Long-term exposure to solutions leads to the replacement of cryolite with gearxutite, which occurs even in dumps containing fragments of the mineral. The replacement of cryolite by supergene silica minerals is known. The products of hydrothermal alteration are pachnolite and tomsenolite; the process begins separately along cracks and leads to the formation of aggregates with cubic cells or continuous fine-grained aggregates of the same minerals and other aluminum fluorides Na, K, Mg, Ca. Hollow pseudomorphs of pachnolite and tomsenolite were discovered in cryolite crystals.

Characteristics of the stone

Cryolite is a versatile and precious mineral. Cryolite is used in many areas not only of chemistry, but also of life. But molten cryolite is used in various industrial fields. The gem can be considered sodium hexfluoroaluminate; it is better known as fluoride. Can be used for many industrial works related to metal.

Today it is rare and, if used in its pure form, then the real benefits of the gem are great, but due to the fact that granite is rare, they are trying to dilute it with many different minerals that are similar in chemical formula. The stone received this name from the Greek language and is translated as frosty or snowy cobblestone. And as mentioned earlier, the mineral resembles a large piece of ice, which consists of frozen stones and ice.

Place of Birth

Cryolite is a rare mineral. As an accessory mineral, it is characteristic of albite-ribeckite pyrochlore-containing granites and associated pegmatites. Also known in amazonite pegmatites. Found in the form of inclusions in topaz crystals from granite pegmatites. The large Ivigtut deposit is known, where a stock-shaped body of siderite-cryolite ore rich in sulfides is mined. The genetic connection of this body with pegmatites has not been established. Found in weathering crust opals of williomite-bearing nepheline syenites. Noted among epigenetic minerals of lacustrine continental sediments in nahcolite and trona deposits. In the Erzinsky granite massif (Tuva), cryolite, along with tomsenolite, is a common accessory mineral in albite-riebeckite granites. Its accumulations with a diameter of up to 0.3 m are also found here in quartz-feldspathic nests and veins of similar composition with a sharp predominance of quartz and small amounts of columbite, malacon, galena and sphalerite. Brownish-gray cryolite forms a medium-grained aggregate. In the weathering crust it is replaced along cracks by gearxutite. Accessory dissemination of cryolite was found in albite-ribeckite metasomatites of the Upper Espe (Tarbagatai, Kazakhstan). In Northern Nigeria on the Jos Plateau, in albite-riebeckite granites, cryolite, together with pyrochlore and topaz, are the main accessory minerals. The content of cryolite reaches 3-4%, topaz - 3.9%, pyrochlore - 1%; Astrophyllite and tomsenolite are found in smaller quantities. The rock-forming riebeckite is very rich in fluorine. The Caffo massifs (Learway complex) are the richest in stone material. In St. Peters House (Colorado, USA), in the body of coarse-grained rnbekito - microcline-quartz pegmatite, enclosed in biotite granites, coarse-grained grayish and pink cryolite fills a drusy cavity with large crystals of microcline, riebeckite and quartz; cryolite accumulations reach 0.5–0.9 mm in diameter; for the most part it is replaced by pachnolite, weberite and prosopite. Accumulations of cryolite, also mostly replaced by secondary aluminum fluorides, were also found in a cavity with a diameter of 0.6 m in a white quartz vein. Quartz and cryolite are penetrated with astrophyllite laths. In the Ilmen Mountains in the Southern Urals, giant-grained grayish-white and brownish-gray cryolite, together with cryolithionite, formed a drusy cavity with a diameter of 1 m in the lens-shaped body of amazonite pegmatite. Intergrowths of large grains of the mineral with very large (up to 15-20 cm) grains of cryolithionite are observed (structures resembling graphic ones). Along cracks and at the contact with pegmatite, the mineral is replaced by chiolite, later pachnolite, tomsenolite and prosopite, as well as supergene gearxutite and halloysite. Near the city of Volodarsk-Volynsk (Ukraine), large drusy cavities with giant topaz crystals were discovered in microclino-quartz pegmatites, which contain abundant gas-liquid inclusions containing cryolite, elpasolite, sylvite, halite, quartz and other minerals. The heads of the inclusion crystals contain elpasolite, and the primary inclusions at the base of the topaz crystals contain cuboid crystals of it along with well-formed quartz crystals. It also fills microscopic healed cracks in topaz.

In Ivigtut (Southwestern Greenland), an elongated stock-shaped body of siderite-cryolite composition is enclosed in the apical part of a dome-shaped stock of leucocratic fine-grained porphyritic granites occurring in gneiss-like rocks of brecciated composition. On the surface, the siderite-cryolite body is 115 m long and 30 m wide; at depth the stock expands. In the near-contact part of the cryolite body, a complex network of veins of quartz-microcline pegmatite is observed in granite. The bulk of pegmatite veins is concentrated at the southern contact of the cryolite body. In the west and north, cryolite is largely in direct contact with granite. The marginal part of the cryolite stock forms the so-called “near-contact shell”, composed of large fragments of host pegmatites and granites, cemented by a quartz-cryolite aggregate. The apophyses of the cryolite body intersect the pegmatites, and cassiterite-quartz and cryolite-quartz veins are observed in the granites. The siderite-cryolite body of Ivigtut contains on average 70-80% cryolite, 15-20% siderite, 1-2% quartz, and 1-2% sulfides. Its intergrowths with siderite are very characteristic: smaller siderite grains are located along the boundaries between large cryolite grains. As the size of cryolite grains increases, the size of siderite grains increases proportionally. In the pegmatite, cryolite pseudomorphs are noted along with graphic quartz ingrowths. The Ivigtut cryolite body contains areas enriched in fine-flaky paragonite and muscovite, porcelain-like microspherulite topaz, fluorite, pyrite, chiolite, weberite, stenonite and jarlite. Dark cryolite is often observed in these areas. The most recent formations are druses of stepped pseudocubic crystals of cryolite on the walls of open cracks, as well as products of its replacement - tomsenolite, pachnolite and ralstonite, which are confined to cracks in cryolite. White porcelain-like crusts representing a cryptocrystalline mixture of supergene cryolite and opal were discovered in cracks of weakly weathered villiomit-bearing nepheline syenites of the Lovozero massif (Kola Peninsula). It is established in bituminous shales of lacustrine continental sediments of the Green River Formation in the state. Colorado (USA) along with pachnolite and dawsonite; small grains and crystals of these minerals form disseminations in obviously unmetamorphosed rocks of Eocene age. The indication by Tene and Calderon of the discovery of a small amount of cryolite in fluorite-quartz low-temperature veins near Sallent (Pyrenees, Spain) is unreliable.

Production

Deposits of natural cryolite are very rare. It is interesting that one of the largest stone mining sites is located far from the continents - in New Zealand. There are not only large deposits here, but the stone itself is of extremely high quality. Local residents use it as an amulet, a talisman, they believe that it has protective properties, protects against danger, grief, bad people, and misfortune.

Since the late 90s, this field has been developed more and more, and now the scope is truly enormous. Of course, this brings considerable income to the budget of such a small country. Cryolite is also mined in the Urals and North America. The US has a fairly promising deposit, but it is currently poorly developed. The fact is that it is not so easy to extract cryolite; in some cases it is easier to make artificial one.

White smoke swirled in the sky again, Everyone knows that it is very poisonous, It will cause harm faster than it will benefit the living. This is the production of cryolite.

Practical use

The practical significance of the mineral is very great. Initially, cryolite was used to produce caustic soda, then it served as an ore to produce metallic Al. Since 1886, it has been used as an electrolyte for the production of Al by electrolysis according to the method of Guerou and Hall. Cryolite is also used as a flux in aluminum welding, for degassing Al alloys, in the production of special types of steel castings, in the ceramic industry in the production of milk glass and enamels, as an abrasive and catalyst, and as a filler for rubber and paper. It is a very effective insecticide.

Until 1922, about 500 thousand tons of Greenland stone were mined. Most of the mineral used in industry is currently obtained synthetically.