Bararite

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Bararite
Bararite.jpg
Pale pink yellowish crystals of bararite from Shamokin, Northumberland County, Pennsylvania, USA
General
Category Halide mineral
Formula
(repeating unit)		 (NH4)2SiF6
IMA symbol Brr [1]
Strunz classification 3.CH.10
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group P3m1
Unit cell a = 5.77 Å, c = 4.78 Å; Z = 1
Identification
ColorWhite to colorless
Crystal habit Tabular, sometimes elongated on {0001},
also appears in irregularly shaped or mammillary surfaces that comprise mainly cryptohalite
Twinning Interpenetration twins (paddlewheels/darts), axis parallel to {0001}
Cleavage [0001] perfect
Mohs scale hardness2.5
Luster Vitreous
Diaphaneity Transparent
Specific gravity 2.152 (synthetic)
Optical propertiesUniaxial (-)
Refractive index nω = 1.406 ± 0.001,
nε = 1.391 ± 0.003
Birefringence 0.015 ± 0.003
Solubility Dissolves in water
Other characteristicssalty taste
References [2] [3] [4] [5] [6] [7] [8]

Bararite is a natural form of ammonium fluorosilicate (also known as hexafluorosilicate or fluosilicate). It has chemical formula (NH4)2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is named after the place where it was first described, Barari in Jharia Coal Field, Dhanbad, India. [3] It is found at the fumaroles of volcanoes (Vesuvius, Italy), over burning coal seams (Barari, India), and in burning piles of anthracite (Pennsylvania, U.S.). It is a sublimation product that forms with cryptohalite, sal ammoniac, and native sulfur. [4]

Contents

History

A. Scacchi first discovered cryptohalite in 1873. [3] It appeared in a volcanic sublimate from the Vesuvian eruption of 1850. In 1926, W.A.K. Christie reported his own chemical study. A microscope was used to pick out enough material for analysis. Distilling with sodium hydroxide (NaOH) produced ammonia (NH3). The anions of hexafluorosilicic acid (H2SiF6) precipitated as potassium fluorosilicate (K2SiF6). Barium sulfate (BaSO4) was thrown into the filtrate, and then calcium fluoride (CaF2). Christie found 20.43% (NH4)+ and 78.87% (SiF6)2−. [5]

Bararite is named after Barari, a locality in India. This was where the species was first completely described. Earlier, bararite was recognized as part of mixtures with cryptohalite. [3] However, it did not receive its own name until 1951. [3] [9] The East Indian Coal Company provided the sample that Christie used to evaluate bararite. [5]

Bararite has not received a quantitative chemical analysis in its natural form. [4] Christie received far too little for more than qualitative analysis through microchemistry. He utilized F. Emich's methods with capillary tube centrifuges. [5]

Structure

Bararite is the beta, trigonal (scalenohedral) form of ammonium hexafluorosilicate. Its symmetry is 32/m. [4] The space group is P3m1. The a-axes in the unit cell are 5.784 ± 0.005 Å (angstroms), and the c-axis is 4.796 ± 0.006 Å. The unit lattice is primitive. [6] [10] (Note: Data for the space group come from synthetic crystals.) Cryptohalite has the cubic (isometric) crystal structure and corresponds to the alpha form. Both minerals have the chemical formula (NH4)2SiF6. The halides of form AmBX6 fall into two groups: hieratite and malladrite. The hieratite group is isometric whereas the malladrite is hexagonal. [3]

The (SiF6)2− is octahedral—one fluorine atom at each vertex. [11] In bararite, the (NH4)+’s are trigonally coordinated. They all appear at sites of C3v (3m) symmetry. The (NH4)+ has 12 fluorine neighbors, which form four triangles. Three of these triangles are isosceles. These triangles themselves form a triangle—around the 3-fold axis containing the nitrogen atom. One triangle is equilateral. Its symmetry axis is the same axis that goes through the nitrogen atom. [12] (For structural diagrams, see link to unit cell [6] and downloadable articles [12] [13] in “References.”)

The silicon atoms of cryptohalite, α-(NH4)2SiF6 (alpha), have cubic close(st) packing (CCP). A third form (gamma, γ) of (NH4)2SiF6 uses hexagonal close(st) packing (HCP). Bararite, β-(NH4)2SiF6, utilizes hexagonal primitive (HP) packing. Layers with distorted octahedral gaps separate those with the anions. The (NH4)+ ions appear a little below and above the (SiF6)2−. In all three phases, 12 fluorine atoms neighbor the (NH4)+. Distances range from about 3.0 to 3.2 Å. [13] The (NH4)+ has no free rotation. It only librates (oscillates)—at least when vibrationally excited. [12]

As a salt, bararite is an ionic compound. The ions, of course, have ionic bonding. The atoms of polyatomic ions are held together covalently. The orientation of (NH4)+ is sustained by four trifurcated (three-branch) hydrogen bonds. These bonds point toward the triangles containing the 12 fluorine neighbors. Three H bonds are equivalent. The fourth bond, pointing toward the equilateral triangle, has a shorter distance. [12]

The intermolecular distances between fluorine atoms are smaller in bararite (3.19 and 3.37 Å) than cryptohalite. In cryptohalite, each anion is coordinated to 12 others. Bararite has (2+6)-fold coordination. The two Si-Si distances between layers (4.796 ± 0.006 Å) do not equal the six within a layer (5.784 ± 0.005 Å). Bararite is more compressible along the c-axis than the a-axis. [13]

Bararite has no known solution or exsolution, but it is always mixed with other substances (cryptohalite, sal ammoniac, and sulfur). [4] Due to thermal motion, atomic behavior of ammonium salts can be very hard to evaluate. [11] The anions, however, are ordered and have no unusual motion from heat. [6]

A third form of (NH4)2SiF6 was discovered in 2001 and identified with the 6mm symmetry (hexagonal). [13] In all three arrangements, the (SiF6)2− octahedra come in layers. In the cubic form (cryptohalite), these layers are perpendicular to [111]. [13] In the trigonal (bararite) and hexagonal (gamma, γ) forms, the layers are perpendicular to the c-axis. [13] (Note: Trigonal crystals are part of the hexagonal group. But not all hexagonal crystals are trigonal. [14] )

Although bararite was claimed to be metastable at room temperature, [11] it does not appear one polymorph has ever turned into another. [13] Still, bararite is fragile enough that grinding it for spectroscopy will produce a little cryptohalite. [12] Even so, ammonium fluorosilicate assumes a trigonal form at pressures of 0.2 to 0.3 giga-pascals (GPa). The reaction is irreversible. If this phase is not bararite, it is at least very closely related. [13]

The hydrogen bonding in (NH4)2SiF6 allows this salt to change phases in ways that normal salts cannot. Interactions between cations and anions are especially important in how ammonium salts change phase. [13]

Physical properties

Bararite forms tabular crystals. They are flattened, sometimes elongated, on {0001} (perpendicular to c). [3] Christie reported tiny, transparent crystals of bararite that looked like paddlewheels and darts. Each had four barbs at 90°. The crystals reached up to 1 mm long, the barbs up to 0.2 mm wide. They were interpenetration twins, the twin axis perpendicular to the c-axis. [5] Visually, cryptohalite crystals are almost impossible to discern from sal ammoniac (NH4Cl). [15] Inclusions of bararite in cryptohalite can be seen only with plane-polarized light. [16]

Bararite has perfect cleavage on the {0001} plane. The hardness is probably 2+1⁄2. [3] The anions (as already shown) are bonded much more strongly within layers than between layers. Also, ionic bonds are not the strongest bonds, and halides cannot normally scratch glass plates. [14]

Bararite has a measured density of 2.152 g/mL (synthetic)—but a calculated density of 2.144 g/mL. It tastes salty, and it dissolves in water. Its luster is vitreous (like glass). Bararite is white to colorless. [3] These properties are similar to halite (NaCl) [14] —which gave the halide group its name.

Whereas cryptohalite belongs to the isotropic optical class, bararite is uniaxial negative. [17] [3] At 1.391 ± 0.003, the refractive index through c is smaller than through a (1.406 ± 0.001). [5] The c-axis in bararite is shorter than the a-axes (see “Structure”). Furthermore, only this path lets light hit nothing but the same ion in the same orientation (all the layers have the same structure and orientation [13] ).

Bararite has about a 6% greater density than cryptohalite. [13] As discussed before, its structure is more packed. This substance can be produced easily from aqueous solution, [3] but only below 5 °C (41 °F) will pure bararite form. [3] [18] Above 13 °C (55 °F), almost pure cryptohalite emerges. [3] [5] Bararite sublimes without leaving residue. [3]

Geologic occurrence

In nature, bararite appears with cryptohalite, sal ammoniac, and native sulfur. [5] [15] It is found over a burning coal seam in Barari, India, [5] and as a sublimation product in Vesuvius, Italy, at fumaroles (opening in or near a volcano where hot sulfurous gases come out). [4] [19] It also is found in the United States, in Pennsylvania. It appears in burning piles of anthracite (highest grade of coal)—again as a sublimation product. [15]

Christie found translucent arborescent (treelike) crystals, with vitreous luster. He found white, opaque lumps that were a mixture of (NH4)2SiF6 with SiO2. They were irregularly shaped but usually had a mammillary surface (several convex surfaces smoothly rounded). These hold primarily cryptohalite but also some bararite. [5] In Pennsylvania, bararite normally comes as tiny inclusions in cryptohalite crystals. [15] [16] It appears that first, bararite forms through direct sublimation. Afterward, it quickly changes to cryptohalite. [16]

In Barari, burning-coal gases go through a dike (igneous intrusion) of mica and peridotite. The sulfur dioxide must attack apatite in the dike, which produces hydrofluoric acid that attacks the abundant silicates. Silicon fluoride is formed. Ammonia also comes from burning coal. From there, ammonium fluorosilicate can form. A slight excess of ammonia could lead to the white lumps of silica and cryptohalite. Bararite and cryptohalite in their pure forms, for the most part, grow out of these nodules. Recrystallization from the rain is probably responsible. [5]

Fluorosilicate minerals are thermodynamically unstable in soil. [20] Still, intense heat promotes the formation of (NH4)2SiF6 to some degree—as seen in some experiments by Rehim. But this compound will break up at 320 to 335 °C. [21] Both burning coal [5] [15] and volcanoes are important sources of SO2 and SiF4. [22]

Chemical properties and uses

Fluorosilicic acid and its salts are poisonous. [23] Ammonium fluorosilicate, however, is very rare in nature [15] and apparently much easier to synthesize. [3]

Related Research Articles

<span class="mw-page-title-main">Silicate</span> Any polyatomic anion containing silicon and oxygen

A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula [SiO(4-2x)−
4−x]
n, where 0 ≤ x < 2. The family includes orthosilicate SiO4−4, metasilicate SiO2−3, and pyrosilicate Si2O6−7. The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate. The name "silicate" is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as hexafluorosilicate [SiF6]2−. Most commonly, silicates are encountered as silicate minerals.

A borate is any of a range of boron oxyanions, anions containing boron and oxygen, such as orthoborate BO3−3, metaborate BO−2, or tetraborate B4O2−7; or any salt of such anions, such as sodium metaborate, Na+[BO2]− and borax (Na+)2[B4O7]2−. The name also refers to esters of such anions, such as trimethyl borate B(OCH3)3 but they are alkoxides.

<span class="mw-page-title-main">Salammoniac</span> Halide mineral

Salammoniac, also sal ammoniac or salmiac, is a rare naturally occurring mineral composed of ammonium chloride, NH4Cl. It forms colorless, white, or yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and is brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and it has a low specific gravity of 1.5. It is water-soluble. Salammoniac is also the archaic name for the chemical compound ammonium chloride.

<span class="mw-page-title-main">Ammonium fluoride</span> Chemical compound

Ammonium fluoride is the inorganic compound with the formula NH4F. It crystallizes as small colourless prisms, having a sharp saline taste, and is highly soluble in water. Like all fluoride salts, it is moderately toxic in both acute and chronic overdose.

<span class="mw-page-title-main">Tetrafluoroborate</span> Anion

Tetrafluoroborate is the anion BF−
4. This tetrahedral species is isoelectronic with tetrafluoroberyllate (BeF2−
4), tetrafluoromethane (CF4), and tetrafluoroammonium (NF+
4) and is valence isoelectronic with many stable and important species including the perchlorate anion, ClO−
4, which is used in similar ways in the laboratory. It arises by the reaction of fluoride salts with the Lewis acid BF3, treatment of tetrafluoroboric acid with base, or by treatment of boric acid with hydrofluoric acid.

<span class="mw-page-title-main">Hexafluorosilicic acid</span> Octahedric silicon compound

Hexafluorosilicic acid is an inorganic compound with the chemical formula H
2SiF
6. Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless.

<span class="mw-page-title-main">Ammonium bifluoride</span> Chemical compound

Ammonium bifluoride is an inorganic compound with the formula [NH4][HF2] or [NH4]F·HF. It is produced from ammonia and hydrogen fluoride. This colourless salt is a glass-etchant and an intermediate in a once-contemplated route to hydrofluoric acid.

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<span class="mw-page-title-main">Tin(IV) fluoride</span> Chemical compound

Tin(IV) fluoride is a chemical compound of tin and fluorine with the chemical formula SnF4 and is a white solid with a melting point above 700 °C.

<span class="mw-page-title-main">Halide mineral</span> Minerals with a dominant fluoride, chloride, bromide, or iodide anion

Halide minerals are those minerals with a dominant halide anion. Complex halide minerals may also have polyatomic anions.

<span class="mw-page-title-main">Hexagonal crystal family</span> Union of crystal groups with related structures and lattices

In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems and two lattice systems. While commonly confused, the trigonal crystal system and the rhombohedral lattice system are not equivalent. In particular, there are crystals that have trigonal symmetry but belong to the hexagonal lattice.

Ammonium fluorosilicate (also known as ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (NH4)2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid. It is made of white crystals, which have at least three polymorphs and appears in nature as rare minerals cryptohalite or bararite.

In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula AOmXn, where X is a halogen. Known oxohalides have fluorine (F), chlorine (Cl), bromine (Br), and/or iodine (I) in their molecules. The element A may be a main group element, a transition element, a rare earth element or an actinide. The term oxohalide, or oxyhalide, may also refer to minerals and other crystalline substances with the same overall chemical formula, but having an ionic structure.

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References

  1. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi: 10.1180/mgm.2021.43 . S2CID   235729616.
  2. ↑ Mineralienatlas
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Palache, C., Berman, H., and Frondel, C. (1951) Dana’s System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, etc. John Wiley and Sons, Inc., New York, 7th edition.
  4. 1 2 3 4 5 6 Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 Christie, W.A.K. (1926) An Occurrence of Cryptohalite (Ammonium Fluosilicate). Records of the Geological Survey of India, 59, 233.
  6. 1 2 3 4 Schlemper, E.O. and Hamilton, W.C. (1966) On the Structure of Trigonal Ammonium Fluorosilicate. Journal of Chemical Physics, 45, 408–409.
  7. ↑ Bararite on Mindat.org
  8. ↑ Bararite data on Webmineral
  9. ↑ Fleischer, M. (1952) "New Mineral Names". American Mineralogist, 37, 359–362.
  10. ↑ Anthony et al. (1997) and Palache et al. (1951) use outdated information for the crystal axes. The information in these handbooks is linked ultimately to two articles by Gossner and Krauss from 1934, in Zeitschrift für Kristallographie. The replacement source, Schlemper and Hamilton (1966), is cited not just by this article but also Boldyreva et al. (2007).
  11. 1 2 3 Schlemper, Elmer O. (1966). "Structure of Cubic Ammonium Fluosilicate: Neutron-Diffraction and Neutron-Inelastic-Scattering Studies". The Journal of Chemical Physics. 44 (6): 2499–2505. Bibcode:1966JChPh..44.2499S. doi:10.1063/1.1727071.
  12. 1 2 3 4 5 Oxton, I.A., Knop, O., and Falk, M. (1975) "Infrared Spectra of the Ammonium Ion in Crystals". II. The Ammonium Ion in Trigonal Environments, with a Consideration of Hydrogen Bonding. Canadian Journal of Chemistry, 53, 3394–3400.
  13. 1 2 3 4 5 6 7 8 9 10 11 Boldyreva, E.V.; Shakhtshneider, T.P; Sowa, H. & Ahsbas, H. (2007). "Effect of hydrostatic pressure up to 6 GPa on the crystal structures of ammonium and sodium hexafluorosilicates, (NH4)2SiF6 and Na2SiF6; a phase transition in (NH4)2SiF6 at 0.2–0.3 GPa". Zeitschrift für Kristallographie. 222 (1): 23–29. Bibcode:2007ZK....222...23B. doi:10.1524/zkri.2007.222.1.23. S2CID   97174719.
  14. 1 2 3 Klein, C. and Dutrow, B. (2008) The 23rd Edition of the Manual of Mineral Science. John Wiley & Sons, Hoboken, NJ.
  15. 1 2 3 4 5 6 Barnes, J. and Lapham, D. (1971) Rare Minerals Found in Pennsylvania. Pennsylvania Geology, 2, 5, 6–8.
  16. 1 2 3 Lapham, D.M., Barnes, J.H., Downey, W.F., Jr., and Finkelman, R.B. (1980) Bararite. Mineralogy associated with burning anthracite deposits of eastern Pennsylvania. Pennsylvania Geological Survey: Mineral Resource Report, 78, 45–47.
  17. ↑ To learn what makes a uniaxial crystal, visit Introduction to Uniaxial Minerals.
  18. ↑ Gossner, B. (1903) Ammoniumsiliciumfluorid. Zeitschrift für Kristallographie, 38, 147–148.
  19. ↑ Scacchi, A. (1874) "Appendice alle contribuzioni mineralogiche sull’ incendio vesuviano del 1872". Rendiconto dell’Accademia delle scienze fisiche e matematiche (sezione della Società reale di Napoli), 8, 179–180.
  20. ↑ Elrashidi, M.A. and Lindsay, W.L. (1986) Chemical Equilibria of Fluorine in Soils: A Theoretical Development. Soil Science: An Interdisciplinary Approach to Soil Research, 141, 274–280.
  21. ↑ Rehim, A. M. Abdel (1992). "Thermal study of synthesis of cryptohalite". Journal of Thermal Analysis. 38 (3): 475–486. doi:10.1007/BF01915512. S2CID   95930085.
  22. ↑ Mori, T., Sato, M., Shimoike, Y., and Notsu, K. (2002) "High SiF4 ratio detected in Satsuma-Iwojima volcano’s plume by remote FT-IR observation". Earth Planets Space, 54, 249–256.
  23. ↑ Wiberg, E., Wiberg, N., and Holleman, A.F. (2001) Inorganic chemistry. Academic Press, San Diego.
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