# Chalcanthite

- Page: https://taslar.bumba.tr/en/stones/chalcanthite/
- Turkish: Kalkantit
- Also known as: Blue vitriol
- Kind: Mineral
- Classification: Minerals › Sulfates
- Chemical formula: CuSO₄·5H₂O
- Crystal system: Triclinic
- Hardness (Mohs): 2.5
- Specific gravity: 2.28–2.29 g/cm³
- Refractive index: 1.514–1.543
- Lustre: Vitreous (glassy), Resinous
- Transparency: Translucent
- Colours: Berlin blue, sky blue, greenish blue
- Streak: White
- Cleavage: Perfect on {110}
- Fracture: Conchoidal
- Value ladder: 2/9 step, Raw material by the tonne
- Last checked: 2026-10-04

Copper sulfate as nature makes it: a sky-blue, water-soluble and toxic mineral.

Chalcanthite is the natural mineral form of copper sulfate with five water molecules bound into its structure (CuSO₄·5H₂O). It is deep blue or sky blue. It forms in the parts of copper sulfide deposits exposed to air, often after a mine has been opened, building up as crusts and stalactites on tunnel walls. It dissolves in water and turns it blue. The same compound is known as “blue vitriol”; the copper sulfate used in vineyards is made in factories. Its name comes from the old Latin chalcanthum, “flowers of copper”.

## How does it form?

Chalcanthite is a **secondary mineral**: it forms in the parts of copper sulfide deposits near the surface, exposed to air and water. As copper sulfides react with air and break down, the copper and sulfate released dissolve in water; when the water evaporates, chalcanthite crystallises. It therefore builds up mostly in dry regions, and often forms after a mine has been opened, on the tunnel walls. Rarely it also occurs at volcanic vents (fumaroles). It is found with soluble or secondary minerals such as melanterite, epsomite, brochantite and gypsum.

## What is it used for?

- **Copper ore:** As a copper mineral, it can be used as an ore of copper.
- **Farming:** The same compound, mixed with lime, makes Bordeaux mixture, used in vineyards against fungal diseases; the copper sulfate used for this is made in factories.
- **Crystal growing:** Despite its toxicity, copper sulfate is often used to grow blue crystals in schools; these artificial crystals are not natural chalcanthite.

## How to recognise it

- Deep blue, sky blue or greenish blue; usually found as stalactites, kidney-shaped crusts or fibrous veins, and rarely as short crystals.
- It dissolves in water and turns it blue, and has a metallic taste. Never taste it.
- Very soft (Mohs 2.5) and light (specific gravity about 2.3).
- It loses water and breaks down in dry air, and takes up water in humid air.

## Fakes and imitations

- Most of the large, well-formed, deep blue chalcanthite crystals seen for sale are grown in the lab; because such crystals are easy to grow, there are concerns that synthetic specimens are sold as natural.
- Azurite and lazurite are also deep blue, but they do not dissolve in water; chalcanthite does.

## What determines its value?

Chalcanthite can be used as an ore of copper; according to the Handbook of Mineralogy, commercial deposits may occur in arid regions. Its value to collectors is low, because large crystals are easy to grow artificially and natural specimens slowly deteriorate. Specimens proven to be natural, with distinct crystals and in good condition, are more sought after.

This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.

## Myths and misconceptions

- Myth: The large, deep blue chalcanthite crystals sold in mineral shops formed naturally. Fact: Most of these crystals are grown in the lab. Natural chalcanthite usually occurs as crusts, stalactites or fibrous veins; large, well-formed crystals are rare in nature.
- Myth: Once crystallised, chalcanthite is permanent and can simply be left on a shelf. Fact: Chalcanthite loses water in dry air and takes it up in humid air, which slowly breaks down its crystal structure. Specimens must be protected from both humidity and excessive dryness.

## Curious questions

**Is the copper sulfate used in vineyards the same as chalcanthite?**

Chemically it is the same compound: copper(II) sulfate with five bound water molecules. The copper sulfate used to make Bordeaux mixture in vineyards is made in factories; chalcanthite is the mineral form of the same compound that forms naturally in copper deposits. Both are toxic.

## Safety

Chalcanthite (copper sulfate) is toxic: swallowing it can cause dangerous copper poisoning. It irritates the eyes and skin, and copper ions are extremely toxic to fish. Because it dissolves so easily in water, never put it in your mouth, in drinking water or in an aquarium. Wash your hands after handling it and keep specimens out of reach of children.

## Did you know?

- It often forms after mining: water seeping through opened tunnels dissolves copper minerals and leaves them again on the walls as blue crusts and stalactites.
- Because chalcanthite both loses water and takes it up, its crystal structure slowly falls apart; collectors must protect it from both humidity and very dry air.
- Bordeaux mixture, a blend of copper sulfate and lime, is used especially in vineyards against fungal diseases.

## Where is it found?

- Minas de Riotinto, Huelva, Spain (Major deposit): Abundant at the Rio Tinto mines, according to the Handbook of Mineralogy
- Chuquicamata, Antofagasta, Chile (Known locality): The world's largest open-pit copper mine by excavated volume; chalcanthite occurs here too
- Vesuvius, Campania, Italy (Known locality): An example of the volcanic vent (fumarole) settings where chalcanthite rarely forms

## Sources

- [Chalcanthite](https://en.wikipedia.org/wiki/Chalcanthite), Wikipedia
- [Handbook of Mineralogy: Chalcanthite](https://www.handbookofmineralogy.org/pdfs/chalcanthite.pdf), Mineralogical Society of America
- [Bakır(II) sülfat](https://tr.wikipedia.org/wiki/Bak%C4%B1r(II)_s%C3%BClfat), Vikipedi
- [Copper(II) sulfate](https://en.wikipedia.org/wiki/Copper(II)_sulfate), Wikipedia
- [Chuquicamata](https://en.wikipedia.org/wiki/Chuquicamata), Wikipedia


Source: https://taslar.bumba.tr/en/stones/chalcanthite/ · Licence: CC BY-SA 4.0