Witherite

Chemical formula: BaCO₃

Witherite is a barium carbonate, prized by collectors for its characteristic pseudohexagonal twin crystals and strong fluorescence under UV light.

## Characteristics Witherite is a mineral from the carbonate group, belonging to the aragonite group. Its chemical composition is barium carbonate (BaCO₃). It forms tabular, columnar, or bipyramidal crystals, which almost always occur as twin aggregates, giving them a characteristic pseudohexagonal (six-sided) outline. It also occurs in radial, fibrous, spherical, and granular aggregates. It is a heavy mineral, which is noticeable even with small specimens. ## Physical Properties This mineral is characterized by a hardness of 3-3.5 on the Mohs scale, making it relatively soft. It has a density of approximately 4.3 g/cm³, which is a high value for a mineral with a non-metallic appearance. The luster is vitreous, and on fracture surfaces, it can be resinous. It is brittle and exhibits imperfect cleavage. Witherite is also known for its strong fluorescence – under ultraviolet light (both short-wave and long-wave), it glows blue or white-blue, often with phosphorescence (afterglow) in a white-green color. ## Colors and Varieties Most often, it is colorless, white, or grayish. It can also take on yellowish, greenish, or light brown hues, usually caused by the presence of impurities. There are no named color or commercial varieties. ## History and Name The mineral's name comes from the English physician, chemist, and naturalist William Withering (1741-1799), who in 1784 was the first to demonstrate that it was a mineral chemically distinct from barite. Withering analyzed "Terra Ponderosa" from a mine in Cumberland and identified a new compound in it – barium carbonate. The mineral was formally named in his honor by the German geologist Abraham Gottlob Werner in 1790. ## Uses Historically, witherite was an important source of barium salts, used in various industries, including the production of glass, ceramics, pigments, and also as rat poison. Due to barium's toxicity, its uses in consumer products have been restricted. Today, it is mainly of collector's interest.

Properties

Mohs hardness: 3-3.5
Color: Colourless, white, greyish, light yellow; colourless in transmitted light.
Luster: Vitreous, Resinous
Streak: White
Density: 4.289
Cleavage: On {010} distinct; on {110} imperfect. Possibly also on {110} and {112}.
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features of witherite are its high density (it feels unnaturally heavy for its appearance), the pseudohexagonal shape of its twin crystals, and its violent reaction with acids (it effervesces). Strong blue fluorescence under UV light is also very characteristic. Its relatively low hardness (it can be scratched with a knife) aids in identification. ## Distinguishing from Similar Minerals Witherite is sometimes confused with aragonite, which also forms pseudohexagonal twins but is much lighter. It can be distinguished from barite, with which it often co-occurs, based on its reaction with acid (barite does not react) and crystal form. Cerussite also forms similar, heavy crystals, but has an adamantine luster and is even denser. Calcite and dolomite are much lighter and have different cleavage. ## Crystal Forms The most typical are pseudohexagonal crystals, resulting from cyclic twinning of three orthorhombic crystals. Single, untwinned crystals are rare and have a bipyramidal form. Witherite also forms fibrous, radial, spherical (spherulitic), botryoidal, and massive, granular aggregates.

Geological environment

## Genesis Witherite is a low-temperature hydrothermal mineral. It forms in ore veins, often associated with lead and zinc deposits, where it crystallizes from barium-rich solutions. It can also form in coal deposits and as a product of alteration of other barium minerals, mainly barite. ## Mineral Associations It most commonly co-occurs with barite, fluorite, calcite, galena, and sphalerite. In some localities, it is also accompanied by strontianite, celestine, and native sulfur. ## Localities The classic and most historically important locality, from which the best specimens originate, is the Alston Moor mine in Cumbria (England). Other significant localities include Rosiclare in Illinois (USA), where it occurred in fluorite deposits, as well as Thunder Bay in Ontario (Canada). It has also been found in Germany (North Rhine-Westphalia), Austria, France, and Siberia in Russia.

Rarity

Not very common

For collectors

## Quality Criteria The most prized by collectors are well-formed, sharp, and large pseudohexagonal twin crystals. Transparent or translucent specimens, with a clear, white, or colorless hue, are highly valued. The attractiveness is enhanced by the presence of the mineral on a contrasting rock matrix and aesthetic associations with other minerals, such as fluorite or galena. Specimens exhibiting strong fluorescence are particularly sought after. ## Popular Localities The specimens from the historic mines of the Alston Moor area in Cumbria, England, are considered absolutely the best in the world. Crystals from there, often in the form of large, complex twins, set the standard for this mineral. Specimens from Rosiclare, Illinois (USA) are also valued, although they often have a less distinct crystalline form.

Care and storage

## Cleaning It should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. The specimen should be thoroughly dried after contact with water. ## What to Avoid Witherite is sensitive to acids – even weak acids, such as vinegar, cause it to effervesce violently and decompose. Contact with all household and laboratory chemicals should be avoided. It is a soft and brittle mineral, susceptible to scratches and mechanical damage. It should not be heated or exposed to sudden temperature changes. ## Storage Witherite specimens should be stored in separate, padded boxes to prevent scratching by harder minerals. Due to its toxicity (barium content), it is recommended to store it out of reach of children and pets, and hands should be washed after contact with the mineral. It does not require special lighting conditions, but areas with high humidity should be avoided.