Alstonite, also known as bromlite,
is a low temperature
hydrothermal mineral
that is a rare
double carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
of
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
and
barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
The ...
with the formula , sometimes with some
strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
.
Barytocalcite
Barytocalcite is an anhydrous barium calcium carbonate mineral with the chemical formula Ba Ca( C O3)2. It is trimorphous with alstonite and paralstonite, that is to say the three minerals have the same formula but different structures. Baryte ...
and
paralstonite have the same formula but different structures, so these three minerals are said to be
trimorphous. Alstonite is
triclinic
180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ )
In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is ...
but
barytocalcite
Barytocalcite is an anhydrous barium calcium carbonate mineral with the chemical formula Ba Ca( C O3)2. It is trimorphous with alstonite and paralstonite, that is to say the three minerals have the same formula but different structures. Baryte ...
is
monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic sys ...
and paralstonite is
trigonal
In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...
. The species was named Bromlite by
Thomas Thomson in 1837 after the Bromley-Hill mine,
[Thomas Thomson, This mine was owned by Thomas Shaw and business partner Jacob Walton between 1816 and 1874, The owners then Subcontracted to groups of independent miner, basing their pay on the amount of drifting accomplished, the amount of ore produced, or a combination of the two, . This system worked well, and the Brownley Hill mine became one of the more productive mines on Alston Moor during the Middle 1800s. by the early 1870s the seems of lead were being exhausted and in 1874 the lease was then taken over by the Brownley Hill Mining which concentrated on Zinc mining, which failed to make a profit. "On the Right Rhombic Baryto-Calcite, with reference to Prof. ''Johnston’s'' Paper in the Phil. Mag. for May 1837", ''The London and Edinburgh Philosophical Magazine and Journal of Science'', vol. XI, july-december 1837]
p. 48
/ref> and alstonite by August Breithaupt
Johann Friedrich August Breithaupt (May 16, 1791 – September 22, 1873) was a German mineralogist and professor at Freiberg Mining Academy in Freiberg, Saxony.
Biography
He was born in Probstzella. He received his doctorate at the Universities ...
of the Freiberg Mining Academy in 1841, after Alston, Cumbria
Alston is a town in Cumbria, England, within the civil parish of Alston Moor on the River Tyne, River South Tyne. It shares the title of the 'highest market town in England', at about above sea level, with Buxton, Derbyshire. Despite its altitud ...
, the base of operations of the mineral dealer from whom the first samples were obtained by Thomson in 1834. Both of these names have been in common use.
Structure
Alstonite is triclinic
180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ )
In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is ...
, but appears pseudo-orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
because of twinning. The space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unch ...
is P1 or P. Alstonite appears to have a superstructure based on paralstonite without long range order of the metal cations
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
or the CO3 groups. The structure of paralstonite is similar to that of other double carbonates.
Unit cell
The number of formula units, Z, in the triclinic unit cell
In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...
is given as 10 or 12, and the unit cell parameters are a = 17.38 Å, b = 14.40 Å, c = 6.123 Å, α = 90.35°, β = 90.12°, γ = 120.08°. The Handbook of Mineralogy, however, describes the mineral in terms of a pseudo-orthorhombic unit cell, with space group C1 or C, Z = 24, and unit cell parameters a = 30.14 Å, b = 17.40 Å, c = 6.12 Å, α = β = γ = 90°.
Appearance
Simple crystals of alstonite are not known. The crystals are invariably complex twins
Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
formed by repeated twinning, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite but more acute. The faces are horizontally striated perpendicular to the pseudohexagonal c crystal axis[Handbook of Mineralogy](_blank)
/ref> and they are divided vertically by a medial, slightly reentrant twinning line parallel to the pseudohexagonal c axis.
Crystals are colourless to snow white, yellow-gray, pale gray, pale cream, pink, or pale rose-red, but the colour may fade on exposure to light. They are transparent to translucent with a white streak and vitreous lustre. The examination in polarized light
Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of th ...
of a transverse section
Section, Sectioning or Sectioned may refer to:
Arts, entertainment and media
* Section (music), a complete, but not independent, musical idea
* Section (typography), a subdivision, especially of a chapter, in books and documents
** Section sig ...
shows that each compound crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...
is built up of six differently oriented individuals arranged in twelve segments.
Optical properties
Alstonite is a biaxial (-) mineral with refractive indices nα =1.526, nβ = 1.671, nγ = 1.672. The maximum birefringence
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefrin ...
(the difference in refractive index between light travelling through the crystal with different polarizations) is δ = 0.146.
The optic angle 2V is the angle between the two optic axes in a biaxial crystal. The measured values of 2V for this mineral is 6°. It is also possible to calculate a theoretical value of 2V from the measured values of the refractive indices. The calculated value for alstonite is 8°. If the colour of the incident light is changed, then the refractive indices are modified, and the value of 2V changes. This is known as dispersion of the optic axes. For alstonite the effect is weak, with 2V larger for red light than for violet light (r > v).
The optical directions X, Y, and Z are the directions of travel of light with refractive indices nα, nβ, and nγ respectively. In general they are not the same as the directions a, b, and c of the crystallographic axes. For alstonite X, Y, and Z are parallel to the c, a, and b crystal axes respectively.
Alstonite fluoresces weak yellow under shortwave and longwave ultraviolet light.
Physical properties
Twinning in alstonite is ubiquitous, forming pseudohexagonal groups. The mineral has one imperfect cleavage and it breaks with an uneven fracture. It is not very hard, with a Mohs hardness
The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by t ...
of just 4 to , a little harder than fluorite, and its specific gravity is 3.70. It is soluble in dilute HCl and it is not radioactive. The trimorphs alstonite, paralstonite, and barytocalcite all have similar physical properties.
Type locality
There are two type localities, both in the north of England. One is the Bromley Hill Mine (Bloomsberry Horse Level), Nenthead, Alston Moor
Alston Moor, formerly known as Alston with Garrigill, is a civil parish and electoral ward in Cumbria, England, based around the small town of Alston. It is set in the moorlands of the North Pennines, mostly at an altitude of over 1000 feet. T ...
District, North Pennines, Cumbria, and the other is the Fallowfield Mine, Acomb, Hexham
Hexham ( ) is a market town and civil parish in Northumberland, England, on the south bank of the River Tyne, formed by the confluence of the North Tyne and the South Tyne at Warden nearby, and close to Hadrian's Wall. Hexham was the administra ...
, Tyne Valley, Northumberland. The type material is held at the Freiberg Mining Academy, Germany, 15818.
At the type locality at Brownley Hill, alstonite occurs in low-temperature lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...
–zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
hydrothermal deposits associated with witherite, calcite, and baryte. The crystals are white to colourless or faintly pink acute pseudohexagonal pyramids or dipyramids up to 6 mm long. In some specimens the alstonite is intergrown with very thin hexagonal platy crystals of nailhead calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scrat ...
. Alstonite commonly encrusts compact crystalline white to pale pink baryte
Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
.[Mineralogical Record supplement to 41-1 (2010).] Similar crystals have been found at Fallowfield. It occurs typically in low-temperature hydrothermal lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, l ...
–zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
ore deposits, as is the case at the type localities, and it has also been reported as a rare phase in carbonatite
Carbonatite () is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification.
Carbonati ...
s. It occurs associated with calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scrat ...
, baryte
Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
, ankerite
Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with the chemical formula . In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varyi ...
, siderite
Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
, benstonite, galena
Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver.
Galena is one of the most abundant and widely distributed sulfide minerals. It cryst ...
, sphalerite
Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in sedimentary exhalative, Mississippi- ...
, pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic luster and pale brass-yellow hue giv ...
, and quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical fo ...
.
Other localities
*USA: Tiny sharp pseudohexagonal dipyramids of alstonite with benstonite and more commonly with witherite are found at the Minerva Mine, Cave-In-Rock, Illinois.[Rocks & Minerals 85-3:211 (2010)]
*England: Alstonite has been found in veins at New Brancepeth, near Durham.
*England: At Nentsberry Haggs Mine, which, like the Brownley Hill Mine, is on Alston Moor
Alston Moor, formerly known as Alston with Garrigill, is a civil parish and electoral ward in Cumbria, England, based around the small town of Alston. It is set in the moorlands of the North Pennines, mostly at an altitude of over 1000 feet. T ...
, Cumbria, baryte
Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
has been found as pseudomorphs after alstonite up to 2.5 cm long, associated with sphalerite
Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in sedimentary exhalative, Mississippi- ...
, galena
Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver.
Galena is one of the most abundant and widely distributed sulfide minerals. It cryst ...
, and witherite.
References
Bibliography
*Palache, P.; Berman H.; Frondel, C. (1960). "''Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)"'' John Wiley and Sons, Inc., New York, pp. 218-219.
{{Commons
Barium minerals
Calcium minerals
Carbonate minerals
Triclinic minerals