Alstonite
Encyclopedia
Alstonite, also known as bromlite, is a member of the aragonite
group of carbonate minerals
. It is a calcium
and barium carbonate
mineral with formula, BaCa(CO3)2. It is triclinic in crystal structure
and differs structurally from barytocalcite
, which has a monoclinic structure. Simple crystals are not known. The crystals are invariably complex twins
, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite
but more acute; the faces are horizontally striated and are divided down their center by a twin suture.
The examination in polarized light of a transverse section shows that each compound crystal
is built up of six differently oriented individuals arranged in twelve segments.
The crystals are translucent and white, sometimes with a shade of pink. Specific gravity
is 3.706, and hardness is 4 - 4.1.
The mineral has been found in a variety of places, two of which are in the north of England
. At the Fallowfield lead
mine, near Hexham in Northumberland
, it is associated with witherite; and at Bromley Hill, near Alston in Cumberland, it occurs in veins with galena
. The species was named bromlite by T. Thomson in 1837, and alstonite by A. Breithaupt in 1841, both of which names, derived from the locality, have been in common use.
It occurs typically in low-temperature hydrothermal lead-zinc ore deposits and has been reported as a rare phase in carbonatite
s. It occurs associated with calcite
, baryte, ankerite
, siderite
, benstonite, galena
, sphalerite
, pyrite
and quartz
.
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
group of carbonate minerals
Carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...
. It is a calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and barium carbonate
Barium carbonate
Barium carbonate , also known as witherite, is a chemical compound used in rat poison, bricks, ceramic glazes and cement.Witherite crystallizes in the orthorhombic system...
mineral with formula, BaCa(CO3)2. It is triclinic in crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
and differs structurally from barytocalcite
Barytocalcite
Barytocalcite is a barium calcium carbonate mineral with the chemical formula BaCa2. It crystallizes in the monoclinic crystal system typically as massive to druzy accumulations of transparent white to yellow to grey aggregates of slender prismatic crystals...
, which has a monoclinic structure. Simple crystals are not known. The crystals are invariably complex 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....
, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite
Witherite
Witherite is a barium carbonate mineral, BaCO3, in the aragonite group. Witherite crystallizes in the orthorhombic system and virtually always is twinned. The mineral is colorless, milky white, grey, pale yellow, green, to pale brown. The specific gravity is 4.3, which is high for a translucent...
but more acute; the faces are horizontally striated and are divided down their center by a twin suture.
The examination in polarized light of a transverse section shows that each compound crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
is built up of six differently oriented individuals arranged in twelve segments.
The crystals are translucent and white, sometimes with a shade of pink. Specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is 3.706, and hardness is 4 - 4.1.
The mineral has been found in a variety of places, two of which are in the north of England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. At the Fallowfield lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
mine, near Hexham in Northumberland
Northumberland
Northumberland is the northernmost ceremonial county and a unitary district in North East England. For Eurostat purposes Northumberland is a NUTS 3 region and is one of three boroughs or unitary districts that comprise the "Northumberland and Tyne and Wear" NUTS 2 region...
, it is associated with witherite; and at Bromley Hill, near Alston in Cumberland, it occurs in veins with galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
. The species was named bromlite by T. Thomson in 1837, and alstonite by A. Breithaupt in 1841, both of which names, derived from the locality, have been in common use.
It occurs typically in low-temperature hydrothermal lead-zinc ore deposits and has been reported as a rare phase in carbonatite
Carbonatite
Carbonatites are intrusive or extrusive igneous rocks defined by mineralogic composition consisting of greater than 50 percent carbonate minerals. Carbonatites may be confused with marble, and may require geochemical verification....
s. It occurs associated with calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, baryte, ankerite
Ankerite
Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with formula: Ca2. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varying amounts of iron and manganese.The crystallographic and...
, siderite
Siderite
Siderite is a mineral composed of iron 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...
, benstonite, galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, sphalerite
Sphalerite
Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...
, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
.