Weddellite

Chemical formula: Ca(C<sub>2</sub>)<sup>6+</sup>O<sub>4</sub>·2H<sub>2</sub>O

Weddellite is a hydrated calcium oxalate, forming characteristic bipyramidal crystals, known primarily for its occurrence in bottom sediments and kidney stones.

## Characteristics Weddellite is a mineral from the oxalate group, chemically hydrated calcium oxalate. Its most typical and recognizable form consists of well-formed, sharp crystals with a tetragonal bipyramidal habit, resembling two pyramids joined at their bases. Crystals are most often colorless, whitish, yellowish, or light brown. It also occurs as granular aggregates or as a component of concretions and sediments. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 4. It has a vitreous luster and is transparent to translucent. The density of weddellite is low, approximately 2.02 g/cm³. It is brittle and exhibits good cleavage. ## Colors and Varieties Weddellite is usually colorless, white, gray, yellowish to light brown. The coloration often depends on impurities and contaminants present during crystallization. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name comes from its first discovery location – the Weddell Sea in Antarctica. It was identified in bottom sediment samples collected during the Scottish National Antarctic Expedition in 1902-1904. F. A. Bannister and M. H. Hey described it as a new mineral in 1936. ## Uses Weddellite has no industrial applications. Its significance is primarily scientific, as an indicator of biomineralization processes. It is one of the main components of some types of kidney stones in humans and animals, making it a subject of interest in medicine and urology. Collector specimens are rare and sought after mainly by specialized collectors.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
White
Density
2.016
Cleavage
Good on {101}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Tetragonal

Diagnostic features

## Identification The most characteristic feature of weddellite is the form of its crystals – sharp, tetragonal bipyramids. It is a soft mineral (hardness 4) with a relatively low density. It dissolves in acids. Under low humidity conditions, it can dehydrate, transforming into whewellite, which sometimes manifests as dulling or the appearance of a white coating. ## Distinguishing from Similar Minerals Whewellite, which is calcium oxalate monohydrate, is the most similar mineral. However, whewellite crystallizes in the monoclinic system and forms different crystal habits (often tabular or prismatic). Under laboratory conditions, they can be distinguished by analytical methods, e.g., by thermal analysis (TGA) or X-ray diffraction (XRD). ## Crystal Forms Weddellite crystals almost always take the form of simple, sharp tetragonal bipyramids {101}. Less commonly, they form granular aggregates or radial aggregates. They occur as single, well-formed crystals in sediments or as overgrown crystals in concretions.

Geological environment

## Genesis Weddellite is a mineral of sedimentary and biogenic origin. It forms by precipitation from solutions rich in calcium and oxalate ions under low-temperature conditions. Its crystallization occurs in an aqueous environment, often with the participation of organic matter. It forms in marine and lake bottom sediments, in soils, and also within living organisms as a product of metabolic processes (e.g., in kidney stones). ## Mineral Associations This mineral often co-occurs with other evaporitic and sedimentary minerals. In bottom sediments, it is accompanied by calcite, gypsum, halite, and whewellite, into which it can transform. It also occurs with organic matter and various clay minerals. ## Localities The most important weddellite localities are the bottom sediments of the Weddell Sea in Antarctica (type locality). It is also found in Barents Sea sediments. Terrestrial occurrences are rare; it has been reported in lake sediments and soils in various parts of the world. As a component of kidney stones, it is found globally.

Rarity

Very rare

For collectors

## Quality Criteria The most prized by collectors are well-formed, sharp, transparent, and colorless crystals with an ideal bipyramidal form. Specimens where individual crystals are overgrown on a contrasting matrix or form aesthetic groups are highly valued. Crystal size is crucial, as large ones, exceeding a few millimeters, are exceptionally rare. Purity (lack of inclusions and dulling) also significantly increases the value of a specimen. ## Popular Localities The vast majority of collector specimens do not come from typical geological localities but are a result of crystallization under laboratory conditions or, interestingly, originate from industry. Finds from pipes and tanks in sugar beet processing plants (e.g., in Belgium) are known, where weddellite crystallized in large, attractive forms. Specimens from natural localities, such as bottom sediments, are practically unavailable on the market.

Care and storage

## Cleaning Weddellite specimens are very delicate and sensitive to chemical and physical changes. For cleaning, use only distilled water and a very soft brush, removing dust with the utmost care. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral is sensitive to heat – heating causes water loss and its transformation into whewellite. Contact with acids, which dissolve it, must be strictly avoided. Prolonged exposure to dry air can also lead to dehydration and dulling of crystal surfaces. It should be protected from direct sunlight. ## Storage It is recommended to store specimens in closed, padded boxes or display cases, at a stable temperature and humidity, to prevent dehydration. Due to its softness and brittleness, it should be kept away from harder minerals to avoid scratches.

External references

Sources

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