Han purple – colour of the terracotta warriors.

In 1974, local farmers digging a well in China’s Shaanxi province uncovered the tomb of the first Qin Emperor, who died in the 3rd century BC. Buried with the emperor were thousands of life-size terracotta figures, now famously known as the Terracotta Army. The warriors were once brightly painted, and although they have faded over the last 2000 years, the colours can still be seen. One of the prominent pigments used is known as Han purple, which is chemically a barium copper silicate, BaCuSi2O6.

Terracotta warriors, coloured with now-faded Han purple pigment. Source: q-files.com

Terracotta warriors, coloured with now-faded Han purple pigment. Source: q-files.com

Han purple was the first synthetic purple pigment, and was used from at least the 8th century BC until the end of the Han dynasty in the 3rd century AD, when the secrets of its production were lost. It appears to have been made from a mixture of barium and copper minerals, quartz, and a lead salt as a special extra ingredient that acts as a catalyst and flux. The mixture needed to be heated to between 900 and 1000 °C – any hotter and you’ll get Han blue (BaCuSi4O10), which is closely related to Egyptian blue (CaCuSi4O10), the oldest synthetic pigment.

The structure of Han purple, BaCuSi2O6, viewed along two different directions, with barium atoms in green, copper in blue, silicon in yellow, and oxygen in red. It can be found in entry 9001237 of the Crystallography Open Database.

The structure of Han purple, BaCuSi2O6, viewed along two different directions, with barium atoms in green, copper in blue, silicon in yellow, and oxygen in red. It can be found in entry 9001237 of the Crystallography Open Database.

Modern interest in Han purple goes beyond its colour. Crystallography has revealed that it has a layered structure with layers of barium atoms sandwiched between copper silicate layers. In 2006 it was discovered that at temperatures very close to absolute zero, and in the presence of a strong magnetic field, the material effectively loses a dimension, transforming from a 3D material to a 2D Bose-Einstein condensate as it crosses a quantum critical point.

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Mineral in pink – Spherocobaltite

What does it look like?

Two views of the Spherocobaltite structure, blue atoms are cobalt, red oxygen and brown are carbon.

Two views of the Spherocobaltite structure, blue atoms are cobalt, red oxygen and brown are carbon.

What is it?

When you think of the word ‘mineral’ and then imagine the types of colours that associate with that – you’ve probably got ‘grey’ or ‘rock-coloured’ in your head first?  I hope that on this blog so far we’ve managed to show that minerals can really come in all colours – through the ‘magic’ of chemistry.

Spherocobaltite-260478.jpg
Spherocobaltite-260478” by Rob Lavinsky / iRocks.comhttp://www.mindat.org/photo-260478.html. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Today’s is pink!  Spherocobaltite, is colbate carbonate (or CoCO3), and it’s the cobalt that gives this mineral it’s lovely pink hue.  It’s a hydrothermal mineral, forming originally from a hot soup of elements.  It’s usually found in veins, where the hot fluid has flowed through crack in the rock.

It you think you seen this all before, then you’ve obviously been pretty keen on our blog! The structure of spherocobaltite is the same as calcite (CaCO3), only with cobalt atoms instead of calcium ones.

Where did the structure come from?

The structure of Spherocobaltite that we’ve featured was determined by Graf in 1961 and was published in the journal American Mineralogist. It’s #9000101 in the Crystallography Open Database.

The crystal structure rainbow – Glowing in UV, Andersonite

Just to extend on our theme for this week a little – how about a crystal structure that glows in UV light?  It’s also one of the minerals in this post we mentioned yesterday, so it ties up things very nicely!

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Andersonite is a rare Uranium carbonate mineral that was first found in Arizona in the US. It’s fluorescent and will glow a yellowy-green under UV light. As you can see hinted in the picture, it’s crystal structure is quite complex – with units of carbonate (CO3, brown carbon atoms surrounded by red oxygen atoms), sodium (yellow atoms) and calcium (blue) atoms which all surround the uranium (green) atoms. Because of the way uranium are made up, they can bond with up to 6 other atoms at a time. This makes for quite a variety of minerals that it can form.

Where did the structure come from?

The crystal structure of Andersonite was determined in 1981, and it #907645 in the open crystallography database.

The crystal structure rainbow – Imperial violet

What does it look like?

—Image generated by the Mercury crystal structure visualisation software http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

—Image generated by the Mercury crystal structure visualisation software http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

What is it?

What are our statuses of wealth? A sporty car? A swimming pool? Or maybe a beach house? In the time of Pliny the Elder a sure sign of a wealthy status was to wear purple.  To colour material purple 2000 years ago required a dye known as Tyrian purple, which at the time cost as much as silver did.  Made from thousands of crushed shells, much of the early production of this material was in the ancient city of Tyre (hence the name).

Pliny the elder wrote about the production of Tyrian purple dye in his ‘Natural History’

The most favourable season for taking these fish [i.e., shellfish] is after the rising of the Dog-star, or else before spring; for when they have once discharged their waxy secretion, their juices have no consistency: this, however, is a fact unknown in the dyers’ workshops, although it is a point of primary importance. After it is taken, the vein is extracted, which we have previously spoken of, to which it is requisite to add salt, a sextarius [just over 560 grams] about to every hundred pounds of juice. It is sufficient to leave them to steep for a period of three days, and no more, for the fresher they are, the greater virtue there is in the liquor. It is then set to boil in vessels of tin [or lead], and every hundred amphoræ ought to be boiled down to five hundred pounds of dye, by the application of a moderate heat; for which purpose the vessel is placed at the end of a long funnel, which communicates with the furnace; while thus boiling, the liquor is skimmed from time to time, and with it the flesh, which necessarily adheres to the veins. About the tenth day, generally, the whole contents of the cauldron are in a liquified state, upon which a fleece, from which the grease has been cleansed, is plunged into it by way of making trial; but until such time as the colour is found to satisfy the wishes of those preparing it, the liquor is still kept on the boil. The tint that inclines to red is looked upon as inferior to that which is of a blackish hue. The wool is left to lie in soak for five hours, and then, after carding it, it is thrown in again, until it has fully imbibed the colour.

Where did the structure come from?

Though it was synthesised in 1903, the crystal structure of Tyrian purple was not determined until 1980 with x-ray crystallography by Larsen and Watjen in 1980. The crystallographic information file for this structure can be found in the Cambridge Structure Database, refcode DBRING01

The crystal structure rainbow – Indigo in your batteries?

What does it look like?

The indigo carmine structure found by Yao et al.  Image take from thier paper.

The indigo carmine structure found by Yao et al. Image take from thier paper.

What is it?

This molecule is colourful, and perhaps an answer to humankind’s energy storage issues? Indigo carmine (or 5,5′-indigodisulfonic acid sodium salt) already has a number of uses because of its colour. This molecule can be used as an indicator of acidity. It also has medical uses, often being used to investigate how a urinary tract is working. It turns your pee indigo blue and can easily be broken down by your kidneys.

But some investigators have looked into the possibility of it being used as a battery material. This is because of the sodium ion in the structure (in the picture above these are purple). Much of the focus on increasing the effectiveness of batteries is investigating lithium ion batteries, but there’s an issue in that there’s only so much lithium the world has to offer. There’s a lot more sodium available, but it’s difficult to use as a battery material because the sodium atoms are quite a bit bigger than lithium ones (and you need them to be able to flow past the rest of the material in the electrode). Electrode materials (those that store up the charge in batteries) are often made out of inorganic materials, which are themselves quite big atoms. The difficulties is finding an electrode that can reproducible give away it’s sodium to generate charge, but then take it back to store up energy again. Could an electrode be made out of a smaller, organic material?       

Where did the structure come from?

These Japanese researchers investigated the potential that indigo carmine has as a battery material, and monitored how the structure changed as it was charged up and as the energy was used. As they were preparing the same they saw that the structure had changed, and this new structure had lots of ‘potential’ as an electrode material.

The crystal structure rainbow – Yellow sulfur

What does it look like?

Cyclodecasulfur, Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Cyclodecasulfur, Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Io, a picture taken by NASA's Galileo spacecraft.  NASA / JPL / University of Arizona

Io, a picture taken by NASA’s Galileo spacecraft. NASA / JPL / University of Arizona

Another element of colour and perhaps not such a surprising inclusion in our Rainbow series is sulfur.  Sulfur really stands out as the element with the most currently known allotropes, standing at 30 currently.  Like carbon can come in graphite, diamond and buckyballs, the different structures of sulfur are contrasted by the rings that the sulfur atoms can from.  There has been observations of 6, 7 and even 16 member rings making up the basis of the large number of allotropes.  A ring ten of sulfur atoms (cyclodecasulfur) which we’ve chose to feature was discovered in 1968.  This large range of sulfur compound could be the explanation for the large range of yellows we see on the surface of Jupiter’s moon Io.     

Where did the structure come from?

We’ve taken the structure of cyclodecasulfur from the work by Steudel et al. and it is number # 9012362 in the open crystallography database

Colour me beautiful – Carotenoids

What are they?

Carotenoids are natural pigments that are found in plants and some bacteria – generally we animals don’t produce them so have to get our quota of carotenoids from eating the producers! There are over 600 types of carotenoids, in humans, beta caratone is useful for the production of retinal.

Some examples of the colours of carotenoids, from left to right these are astaxanthin, canthaxanthin, zeaxanthin and beta-carotene

Some examples of the colours of carotenoids, from left to right these are astaxanthin, canthaxanthin, zeaxanthin and beta-carotene

To gain further insight into the conformation colour relationships in marine crustacea [1], research led by Dr Madeleine Helliwell set about crystallising and determining the crystal structures of free carotenoids [2,3]. From these X-ray crystal structures variations were seen in the free carotenoid C5-C6-C7-C8 torsion angle for zeaxanthin (-75º) and the trans-astaxanthin ester (178º), and differences in colours of the crystalline solids arose from the variation of the degree of conjugation into the end rings and thereby their colours.

The astaxanthin molecule, a type of carotenoid found in marine crustacea

The astaxanthin molecule, a type of carotenoid found in marine crustacea

What do they look like?

Giuditta Bartalucci [3] synthesised the diester of astaxanthin and then grew crystals where she found both s-cis and s-trans configurations! There were colour differences between them.

The different colours of astaxanthin depending on the conformation of the molecule.

The different colours of astaxanthin depending on the conformation of the molecule.

Where did the structure come from?

In summary the ensemble of crystal structures in refs [2,3] allowed tests of the effect of carotenoid conformation, as well as their crystal-packing arrangements, and to some extent the effect of solvation, on the colours of the crystals. However all the samples studied in [2,3] were red in hue and not the blue colour of beta crustacyanin [1].

[1] M. Cianci, P.J. Rizkallah, A. Olczak, J. Raftery, N.E. Chayen, P.F. Zagalsky and J.R. Helliwell “The molecular basis of the coloration mechanism in lobster shell: β-crustacyanin at 3.2 A resolution” (2002) PNAS USA 99, 9795-9800.

[2] G Bartalucci, J Coppin, S Fisher, G Hall, J R Helliwell, M Helliwell and S Liaaen-Jensen “Unravelling the Chemical Basis of The Bathochromic Shift in the Lobster Carapace; New Crystal Structures of Unbound Astaxanthin, Canthaxanthin and Zeaxanthin” Acta Cryst B 2007 B63, 328-337.

[3] G. Bartalucci, S. Fisher, J.R. Helliwell, M. Helliwell1, S. Liaaen-Jensen, J.E. Warren and J. Wilkinson “X-ray crystal structures of diacetates of 6-s-cis and 6-s-trans astaxanthin and of 7,8-didehydroastaxanthin and 7,8,7’,8’-tetradehydroastaxanthin: comparison with free and protein bound astaxanthins’ Acta Cryst (2009) B65, 238-247.