Blue skies and copper demons: a story of mysterious purple crystals

Mystery purple crystals (posted with permission of Caroline Hedge, @CM_Hedge)

Today, a little story about some mysterious, purple crystals. On Tuesday, Twitter user Caroline Hedge posted this photo with the question: “What the %#&$ is lab putting down the drain to cause this?”

The post spawned lots of responses, some more serious than others. One of the sensible ones came from Roland Roesler, who thought that the pipe had corroded from the outside, suggesting that a leaky connection at the top right had allowed sewage to drip down the right-hand side of the copper pipe and drip from the bottom, which explained why the left-hand half of the pipe appeared unscathed.

I agreed. The pipe is clearly made of copper, and blue colours are characteristic of hydrated copper salts. Inside the pipe, the flow of water would wash any solution anyway before corrosion could occur, but on the outside, drips could sit on the surface for long periods of time. There’d be plenty of time for even a slow reaction to occur, and then for water to slowly evaporate, allowing the growth of spectacular crystals.

Hydrated copper(II) sulfate crystals are bright blue. (Image from Wikimedia Commons)

But what exactly where they? There were several theories, but for me the interesting thing was the colour. Hydrated copper(II) sulfate crystals are bright blue. The colour arises due to an effect called d orbital splitting, which is a tad complicated but, in short, means that complex absorbs light from the red end of the visible light spectrum, allowing all the other colours of light to pass through. As a result, our eyes “see” blue.

But these crystals, assuming it’s not a photographic effect, had a purplish hue. At least, some of them do. So… not copper sulfate, or not entirely copper sulfate (given the situation, a mixture seemed entirely likely). Which begs the question, which copper complex produces a purple colour?

A little bit of Googling and I was pretty sure I’d identified it: copper azurite, Cu₃(CO₃)₂(OH)₂. This fit for two reasons: firstly, it’s a mineral that could (does) readily form in the presence of water and air (which, of course, contains carbon dioxide), and secondly it’s exactly the right colour.

Many will recognise the word “azure” as being associated with the deep, rich blue of a summer sky, and in fact the English name of this mineral comes from the same word-root: the Persian lazhward, a place known for its deposits of another deep-blue stone, lapis lazuli (meaning “stone of azure”).

Blue-purple copper azurite and green malachite (image from Wikipedia)

Azurite is often found with malachite, the better-known green copper mineral that we recognise from copper roofs and statues. Malachite is sometimes simplistically described as copper carbonate, implying CuCO₃, but in truth it’s Cu₂CO₃(OH)₂ pure copper(II) carbonate doesn’t form in nature.

You can see malachite co-existing with azurite in the photo on the right. The azurite will, over time, tend to morph into malachite when the level of carbon dioxide in the air is relatively low, as in ‘normal’ air—which explains why we don’t usually see purple ‘copper’ roofs—but the carbon dioxide levels were probably higher in that cupboard. There was almost certainly acidic sewage reacting with carbonate, combined with a lack of ventilation, so it makes sense that we might see more azurite.

Azurite has an interesting history as a pigment. Historically blue colours were rare and expensive—associated with royalty and divinity—which is one reason why the Virgin Mary was often depicted wearing blue in paintings. Azurite was used to make blue pigments, but (as I mentioned above) it’s unstable, tending to turn greenish over time, or black if heated. Ultramarine blue (made from lapis lazuli) is more stable, particularly when heated, but it was even more expensive. A lot of blue pigments in medieval paintings have been misidentified as coming from lapis lazuli, when in fact they were azurite—a more common mineral in Europe at the time.

There’s a fun piece of etymology here, too. Copper, of course, has been valuable metal since, well, the Bronze Age. The presence of purple azurite and green malachite are surface indicators of copper sulfide ores, useful for smelting. This lead to the name of the element nickel, because an ore of nickel weathers to produce a green mineral that looks a little like malachite. And this, in turn, lead to attempts to smelt it in the belief that it was copper ore. But, since it wasn’t, the attempts to produce copper failed (a much higher smelting temperature is needed to produce nickel).

The mineral nickeline can resemble malachite, and was dubbed kupfernickel in Germany, literally “copper demon”

As a result, the mineral, nickeline, was dubbed kupfernickel in Germany, literally “copper demon”. When the Swedish alchemist Baron Axel Fredrik Cronstedt succeeded, in 1751, in smelting kupfernickel to produce a previously unknown silvery-white, iron-like metal he named it after the nickel part of kupfernickel.

And this is how we go from a corroded pipe to sky-blue colours to medieval paintings to copper demons to nickel. But what happened to the pipe in the original tweet? Well, in an update, Caroline Hedge told us that it had been removed and disposed of, and so we’ll never be completely sure what the pretty crystals were, but they certainly lead to an interestingly twisty-turny chemistry story.


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