Quick answer: Blue aragonite is calcium carbonate, the same chemistry as calcite but with a different crystal structure, tinted sky-blue by traces of copper. Fine specimens come from the Rio Tinto district of Andalusia, Spain, one of the oldest mining areas on Earth. It is soft, about 3.5 to 4 on the Mohs scale, and should be kept away from heat and acids.
The Río Tinto in southwestern Spain is one of the strangest rivers on Earth. Its water runs deep red-orange and is about as acidic as lemon juice. From the mining district at its headwaters comes a mineral in almost the opposite color: soft, sky-blue aragonite.
Want the full picture? See Calcite: The Complete Guide.
Five thousand years of mining
The Rio Tinto district in Huelva province sits on the Iberian Pyrite Belt, one of the largest concentrations of sulfide ore in the world. People have mined copper, silver, and gold here for roughly five thousand years. The Tartessians and Phoenicians worked it, and the Romans ran huge underground operations whose tunnels and water wheels have been found in modern workings. In 1873 a British-led consortium bought the mines from the Spanish government and founded the Rio Tinto Company, which grew into one of the largest mining corporations in the world and still carries the name.
Why the river is red
The ore bodies are packed with pyrite and other iron sulfides. Where water and air reach them, the sulfides oxidize and produce sulfuric acid and dissolved iron, which gives the river its color. Whether the acidity is purely a product of mining or partly natural is still debated, since the ore bodies were weathering long before people arrived. The river’s microbes, which thrive in acid and iron, have made it a study site for scientists thinking about possible life on Mars. The same weathering process is described in our post on gossan, the rusty cap that forms over sulfide ore.
What makes it aragonite, not calcite
Aragonite and calcite are both calcium carbonate, CaCO3, but the atoms are packed differently. Aragonite is orthorhombic, slightly harder (about 3.5 to 4) and denser, and it tends to form needles, columns, and radiating sprays. At the Earth’s surface it is the less stable form and, given enough time, it slowly converts to calcite. We cover this in depth in aragonite and calcite: polymorphs.
Why it is blue
Pure aragonite is colorless or white. Blue aragonite is generally attributed to copper, which is abundant in the ore-rich waters of a district like this. It is the same element behind the blues and greens of turquoise and chrysocolla, and another example of trace elements controlling color. Blue aragonite is often dyed on the wider market, but natural material shows a soft, slightly uneven color that sits within the stone, not concentrated in cracks.
Caring for blue aragonite
- Keep it away from acid. Vinegar or acidic cleaners will etch any carbonate.
- Avoid long soaks. A quick rinse is fine, but dry it promptly.
- Limit strong sunlight. Some colored carbonates can fade over long exposure.
- Store it apart from harder stones. At under 4 on the Mohs scale, quartz will scratch it.
Our full cleaning and storage guide covers the rest.
