Bararite
Encyclopedia
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, India
. It is found at the fumarole
s of volcano
es (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
.
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-.
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. However, it did not receive its own name until 1951. The East Indian Coal Company
provided the sample that Christie used to evaluate bararite.
Bararite has not received a quantitative chemical analysis in its natural form. Christie received far too little for more than qualitative analysis through microchemistry. He utilized F. Emich’s methods with capillary tube
centrifuge
s.
. Its symmetry is 2/m. The space group
is Pm1. The a-axes in the unit cell are 5.784 ± 0.005 Å (angstrom
s), and the c-axis is 4.796 ± 0.006 Å. The unit lattice is primitive. (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 halide
s of form AmBX6 fall into two groups: hieratite and malladrite. The hieratite group is isometric whereas the malladrite is hexagonal.
The (SiF6)2- is octahedral
—one fluorine
atom at each vertex
. 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. (For structural diagrams, see link to unit cell and downloadable articles 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. The (NH4)+ has no free rotation. It only librates
(oscillates)—at least when vibrationally excited.
As a salt, bararite is an ionic compound
. The ions, of course, have ionic bond
ing. The atoms of polyatomic ion
s are held together covalently
. The orientation of (NH4)+ is sustained by four trifurcated (three-branch) hydrogen bond
s. 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.
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.
Bararite has no known solution
or exsolution, but it is always mixed with other substances (cryptohalite, sal ammoniac
, and sulfur
). Due to thermal motion, atomic behavior of ammonium salts can be very hard to evaluate. The anions, however, are ordered and have no unusual motion from heat.
A third form of (NH4)2SiF6 was discovered in 2001 and identified with the 6mm symmetry (hexagonal
). In all three arrangements, the (SiF6)2- octahedra
come in layers. In the cubic form (cryptohalite), these layers are perpendicular to [111
]. In the trigonal (bararite) and hexagonal (gamma, γ) forms, the layers are perpendicular to the c-axis. (Note: Trigonal crystals are part of the hexagonal group. But not all hexagonal crystals are trigonal.)
Although bararite was claimed to be metastable
at room temperature
, it does not appear one polymorph
has ever turned into another. Still, bararite is fragile enough that grinding it for spectroscopy
will produce a little cryptohalite. 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.
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.
. They are flattened, sometimes elongated, on {0001}
(perpendicular to c). 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. Visually, cryptohalite crystals are almost impossible to discern from sal ammoniac
(NH4Cl). Inclusions of bararite in cryptohalite can be seen only with plane-polarized light.
Bararite has perfect cleavage
on the {0001} plane. The hardness
is probably 2½. The anions (as already shown) are bonded much more strongly within layers than between layers. Also, ionic bond
s are not the strongest bonds, and halide
s cannot normally scratch glass plates.
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. These properties are similar to halite
(NaCl)—which gave the halide group its name.
Whereas cryptohalite belongs to the isotropic
optical class, bararite is uniaxial negative. At 1.391 ± 0.003, the refractive index through c is smaller than through a (1.406 ± 0.001). 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).
Bararite has about a 6% greater density than cryptohalite. As discussed before, its structure is more packed. This substance can be produced easily from aqueous solution
, but only below 5 °C (41 °F) will pure bararite form. Above 13 °C (55 °F), almost pure cryptohalite emerges. Bararite sublimes without leaving residue.
, and native sulfur
. It is found over a burning coal seam in Barari, India
, and as a sublimation product in Vesuvius, Italy
, at fumarole
s (opening in or near a volcano
where hot sulfur
ous gases come out). 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.
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. In Pennsylvania, bararite normally comes as tiny inclusions in cryptohalite crystals. It appears that first, bararite forms through direct sublimation. Afterward, it quickly changes to cryptohalite.
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 silicate
s. 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.
Fluorosilicate minerals are thermodynamically unstable
in soil. 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. Both burning coal and volcano
es are important sources of SO2
and SiF4
.
, however, is very rare in nature and apparently much easier to synthesize
.
Ammonium fluorosilicate
Ammonium fluorosilicate has the formula 2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid...
(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, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. It is found at the fumarole
Fumarole
A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...
s of volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es (Vesuvius, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
), over burning coal seams (Barari, India), and in burning piles of anthracite (Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
, U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
). It is a sublimation product that forms with cryptohalite, sal ammoniac
Sal ammoniac
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and has a low specific...
, and native sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
.
History
A. Scacchi first discovered cryptohalite in 1873. It appeared in a volcanic sublimate from the Vesuvian eruptionMount Vesuvius
Mount Vesuvius is a stratovolcano in the Gulf of Naples, Italy, about east of Naples and a short distance from the shore. It is the only volcano on the European mainland to have erupted within the last hundred years, although it is not currently erupting...
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
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3). The anions of hexafluorosilicic acid (H2SiF6) precipitated as potassium fluorosilicate (K2SiF6). Barium sulfate
Barium sulfate
Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...
(BaSO4) was thrown into the filtrate, and then calcium fluoride
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
(CaF2). Christie found 20.43% (NH4)+ and 78.87% (SiF6)2-.
Bararite is named after Barari, a locality
Locality
Locality may refer to:* Locality, a.k.a. human settlement* Locality of reference in computer-science data-access issue* Locality * Locality * Locality, physics concept re principle of locality* Locality in astronomy...
in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. This was where the species was first completely described. Earlier, bararite was recognized as part of mixtures with cryptohalite. However, it did not receive its own name until 1951. The East Indian Coal Company
Coal mining in India
Coal mining in India has a long history of commercial exploitation covering nearly 220 years starting in 1774 with John Sumner and Suetonius Grant Heatly of the East India Company in the Raniganj Coalfield along the Western bank of river Damodar...
provided the sample that Christie used to evaluate bararite.
Bararite has not received a quantitative chemical analysis in its natural form. Christie received far too little for more than qualitative analysis through microchemistry. He utilized F. Emich’s methods with capillary tube
Capillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...
centrifuge
Centrifuge
A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...
s.
Structure
Bararite is the beta, trigonal (scalenohedral) form of ammonium hexafluorosilicateAmmonium fluorosilicate
Ammonium fluorosilicate has the formula 2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid...
. Its symmetry is 2/m. The space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
is Pm1. The a-axes in the unit cell are 5.784 ± 0.005 Å (angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
s), and the c-axis is 4.796 ± 0.006 Å. The unit lattice is primitive. (Note: Data for the space group come from synthetic crystals.) Cryptohalite has the cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
(isometric) crystal structure and corresponds to the alpha form. Both minerals have the chemical formula (NH4)2SiF6
Ammonium fluorosilicate
Ammonium fluorosilicate has the formula 2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid...
. The halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s of form AmBX6 fall into two groups: hieratite and malladrite. The hieratite group is isometric whereas the malladrite is hexagonal.
The (SiF6)2- is octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
—one fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
atom at each vertex
Vertex (geometry)
In geometry, a vertex is a special kind of point that describes the corners or intersections of geometric shapes.-Of an angle:...
. 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
Triangle
A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....
. These triangles themselves form a triangle—around the 3-fold axis
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...
containing the nitrogen atom. One triangle is equilateral
Triangle
A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....
. Its symmetry axis
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...
is the same axis that goes through the nitrogen atom. (For structural diagrams, see link to unit cell and downloadable articles in “References.”)
The silicon atoms of cryptohalite, α-(NH4)2SiF6 (alpha), have cubic close(st) packing
Sphere packing
In geometry, a sphere packing is an arrangement of non-overlapping spheres within a containing space. The spheres considered are usually all of identical size, and the space is usually three-dimensional Euclidean space...
(CCP). A third form (gamma, γ) of (NH4)2SiF6 uses hexagonal close(st) packing
Sphere packing
In geometry, a sphere packing is an arrangement of non-overlapping spheres within a containing space. The spheres considered are usually all of identical size, and the space is usually three-dimensional Euclidean space...
(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. The (NH4)+ has no free rotation. It only librates
Libration (molecule)
Libration is a type of reciprocating motion in which an object with a nearly fixed orientation repeatedly rotates slightly back and forth...
(oscillates)—at least when vibrationally excited.
As a salt, bararite is an ionic compound
Ionic compound
In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together...
. The ions, of course, have ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
ing. The atoms of polyatomic ion
Polyatomic ion
A polyatomic ion, also known as a molecular ion, is a charged species composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in...
s are held together covalently
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
. The orientation of (NH4)+ is sustained by four trifurcated (three-branch) hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s. 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.
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.
Bararite has no known solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
or exsolution, but it is always mixed with other substances (cryptohalite, sal ammoniac
Sal ammoniac
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and has a low specific...
, and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
). Due to thermal motion, atomic behavior of ammonium salts can be very hard to evaluate. The anions, however, are ordered and have no unusual motion from heat.
A third form of (NH4)2SiF6 was discovered in 2001 and identified with the 6mm symmetry (hexagonal
Hexagonal crystal system
In crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families...
). In all three arrangements, the (SiF6)2- octahedra
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
come in layers. In the cubic form (cryptohalite), these layers are perpendicular to [111
Miller index
Miller indices form a notation system in crystallography for planes and directions in crystal lattices.In particular, a family of lattice planes is determined by three integers h, k, and ℓ, the Miller indices. They are written , and each index denotes a plane orthogonal to a direction in the...
]. In the trigonal (bararite) and hexagonal (gamma, γ) forms, the layers are perpendicular to the c-axis. (Note: Trigonal crystals are part of the hexagonal group. But not all hexagonal crystals are trigonal.)
Although bararite was claimed to be metastable
Metastability
Metastability describes the extended duration of certain equilibria acquired by complex systems when leaving their most stable state after an external action....
at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, it does not appear one polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
has ever turned into another. Still, bararite is fragile enough that grinding it for spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
will produce a little cryptohalite. 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.
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.
Physical properties
Bararite forms tabular crystalsCrystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
. They are flattened, sometimes elongated, on {0001}
Miller index
Miller indices form a notation system in crystallography for planes and directions in crystal lattices.In particular, a family of lattice planes is determined by three integers h, k, and ℓ, the Miller indices. They are written , and each index denotes a plane orthogonal to a direction in the...
(perpendicular to c). 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
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....
, the twin axis perpendicular to the c-axis. Visually, cryptohalite crystals are almost impossible to discern from sal ammoniac
Sal ammoniac
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and has a low specific...
(NH4Cl). Inclusions of bararite in cryptohalite can be seen only with plane-polarized light.
Bararite has perfect cleavage
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
on the {0001} plane. The hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
is probably 2½. The anions (as already shown) are bonded much more strongly within layers than between layers. Also, ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
s are not the strongest bonds, and halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s cannot normally scratch glass plates.
Bararite has a measured density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of 2.152 g/mL (synthetic)—but a calculated density of 2.144 g/mL. It tastes salty, and it dissolves
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
in water. Its luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
is vitreous (like glass). Bararite is white to colorless. These properties are similar to halite
Halite
Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...
(NaCl)—which gave the halide group its name.
Whereas cryptohalite belongs to the isotropic
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...
optical class, bararite is uniaxial negative. At 1.391 ± 0.003, the refractive index through c is smaller than through a (1.406 ± 0.001). 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).
Bararite has about a 6% greater density than cryptohalite. As discussed before, its structure is more packed. This substance can be produced easily from aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
, but only below 5 °C (41 °F) will pure bararite form. Above 13 °C (55 °F), almost pure cryptohalite emerges. Bararite sublimes without leaving residue.
Geologic occurrence
In nature, bararite appears with cryptohalite, sal ammoniacSal ammoniac
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and has a low specific...
, and native sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
. It is found over a burning coal seam in Barari, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, and as a sublimation product in Vesuvius, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, at fumarole
Fumarole
A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...
s (opening in or near a volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
where hot sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
ous gases come out). It also is found in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, in Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
. It appears in burning piles of anthracite (highest grade of coal)—again as a sublimation product.
Christie found translucent arborescent
Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
(treelike) crystals, with vitreous luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
. He found white, opaque lumps that were a mixture of (NH4)2SiF6
Ammonium fluorosilicate
Ammonium fluorosilicate has the formula 2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid...
with SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
. They were irregularly shaped but usually had a mammillary
Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
surface (several convex surfaces smoothly rounded). These hold primarily cryptohalite but also some bararite. In Pennsylvania, bararite normally comes as tiny inclusions in cryptohalite crystals. It appears that first, bararite forms through direct sublimation. Afterward, it quickly changes to cryptohalite.
In Barari, burning-coal gases go through a dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
(igneous intrusion) of mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
and peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
. The sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
must attack apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
in the dike, which produces hydrofluoric acid
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
that attacks the abundant silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s. Silicon fluoride
Silicon tetrafluoride
Silicon tetrafluoride or Tetrafluorosilane is the chemical compound with the formula SiF4. This tetrahedral molecule is notable for having a remarkably narrow liquid range...
is formed. Ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
also comes from burning coal. From there, ammonium fluorosilicate
Ammonium fluorosilicate
Ammonium fluorosilicate has the formula 2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid...
can form. A slight excess of ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
could lead to the white lumps of silica
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
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.
Fluorosilicate minerals are thermodynamically unstable
Chemical stability
Chemical stability when used in the technical sense in chemistry, means thermodynamic stability of a chemical system.Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment. This may be a dynamic equilibrium, where individual atoms...
in soil. Still, intense heat promotes the formation of (NH4)2SiF6 to some degree—as seen in some experiments by Rehim. But this compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
will break up
Thermal decomposition
Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes....
at 320 to 335 °C. Both burning coal and volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es are important sources of SO2
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
and SiF4
Silicon tetrafluoride
Silicon tetrafluoride or Tetrafluorosilane is the chemical compound with the formula SiF4. This tetrahedral molecule is notable for having a remarkably narrow liquid range...
.
Chemical properties and uses
Fluorosilicic acid and its salts are poisonous. Ammonium fluorosilicateAmmonium fluorosilicate
Ammonium fluorosilicate has the formula 2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid...
, however, is very rare in nature and apparently much easier to synthesize
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
.
External links
- Entry on mindat.org
- Entry on webmineral.com
- AMCSD search engine (cannot link directly to result)
- Introduction to Uniaxial Minerals
- Handbook of Mineralogy