Decorative, but a little deadly – Torbernite

 What does it look like?

The crystal structure of Torbernite. Here the atom colours are; blue – uranium, orange – copper, purple - phosphorus, red – oxygen. Image generated by Mercury.

The crystal structure of Torbernite. Here the atom colours are; blue – uranium, orange – copper, purple – phosphorus, red – oxygen. Image generated by Mercury.

What is it?

Torbernite crystals exhibit exceptionally beautiful shades of green, from emerald to grass-green to apple-green, and thus may entice you to collect these crystals as ornaments for your tables – but beware; these crystals are capable of slowly leaking lethal radon gas which can cause lung cancer.

Image of a collection of torbernite crystals. Taken from: http://www.gemstonesadvisor.com/torbernite/

Image of a collection of torbernite crystals. Taken from: http://www.gemstonesadvisor.com/torbernite/

Torbernite crystals, Cu(UO2)2(PO4)2)·12H2O, are formed through a complex reaction of phosphorus, copper, water and uranium and form as secondary uranium deposits in granitic rocks. These materials belong to the autunite group and are found in the alteration zone of hydrothermal veins and pegamites that contain uraninite. Torbernite materials possess a significant environmental interest in that they exert an impact on the mobility of uranium in phosphate bearing systems such as uranium deposits and so can act as a reactive barrier that uses phosphate to limit the transport of uranium in groundwater. As such, the presence of torbernite has been used by prospectors as an indicator of uranium deposits.

Where did the structure come from?

Torbernite occurs in tabular blocks that may be very thin to moderately thick. The crystals have a perfect cleavage parallel to the basal plane and thus can resemble mica. This particular structure of torbernite that we have featured was presented in Locock, A.J. and Burns, B.C. The Canadian Mineralogist, 2003, vol. 41, pp. 489 – 502.

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.

Now you’ve seen everything – superconductive concrete

What does it look like?

AMS_DATA-6What is it?

In 2006 a senior scientist remarked in the journal Nature ‘If that superconductors is made by doping concrete, I’ll know it’s time for me to retire’.  We wonder if, the very next year, he was clearing his desk and getting his gold watch with the discovery of what happen when you dope today’s crystal structure.

Dodecacalcium hepta-aluminate (12CaO.7Al2O3) – often just known as C12A7, is another of those useful crystal phases that crop up in cement, along with tricalcium aluminate.  It even occurs in nature, as the mineral Mayenite.  In the structure the calcium and aliuminium oxides form cages, which hold small amounts of oxygen ions.  A group in Japan, discovered that if you can replace the oxygen ions with electrons you can make the material behave like a metal, which is rather odd for concrete.

Crystal structure of C12A7. The cube is a unit cell. Two of the 12 baskets in the crystal contain oxygen ions. From http://techon.nikkeibp.co.jp/english/NEWS_EN/20070618/134409/

Crystal structure of C12A7. The cube is a unit cell. Two of the 12 baskets in the crystal contain oxygen ions. From http://techon.nikkeibp.co.jp/english/NEWS_EN/20070618/134409/

Then in 2007, the cooled the doped  to 0.4 K and found that C12A7 becomes a superconductor! The key to this was how the electrons can move between the cages in the structure.

Where did the structure come from?

The crystal structure of Mayenite was first discovered by Buessem and Eitel in 1936, and is #1011034 in the crystallography open database.

Stuffing in the hydrogen – Lithium Boro-hydride

What does it look like?

Crystal structure of lithium boro-hydride, the light green atoms are lithium, dark green boron and pink are hydrogen.  Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Crystal structure of lithium boro-hydride, the light green atoms are lithium, dark green boron and pink are hydrogen. Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

One solution to our society’s dependence on carbon dioxide emitting fossil fuels is the suggestion that we move to a ‘hydrogen economy’.  The idea is that we can abundantly form hydrogen, and on releasing it’s energy produce water as a waste product.  But one of the challenges facing this route has been how to store it!  Conventional high-pressure tanks of hydrogen gas are expensive and heavy (and a little dangerous).  So in came crystallography to look for solids with a high-density hydrogen content!

There has been lots of promising candidates for this, principally hydrides formed when hydrogen reacts with metals like magnesium and sodium.  There was an issue with these however, the hydrogen within them were bound too tight – it would cost too much energy to release them.  So one potential solution to this was this material, lithium boro-hydride, where the boron don’t hold onto the hydrogen quite a tight.

Where did the structure come from?

The structure we’ve featured come from work by Soulié et al., published in 2002. They investigated the structure of Lithium boro-hydride and also found a new polymorph that forms at high-temperature.  The crsytal structuere data for this study can be found in the Cambridege structre data base, refcode XIHFAW03.

Beautifully holding buildings together – Tricalcium aluminate

What is it?

Hello, it’s Helen here – as we move into the final furlongs of the crystallography 365 blog we’re scrambling to find structures that we’ve missed and really deserve a post.  I know that you’re not meant to have a favourite in the over a million crystal structures that have been discovered – but between you and me, this is mine. Who else would love a mineral found in cement?

Cement, when you look into it, is a very sophisticated material.  It’s a powder, that when you add water too can then be poured – but hours later its tough enough to build skyscrapers from.  How it does this is due to the interplay of all the mineral phases in that powder – one important one is Ca3Al2O6 or tricalcium aluminate.

Tricalcium aluminate has a bit of a love/hate relationship with cement.  It’s responsible for many of the impurities in the powder, but is needed because of it’s amazing abilty to grab water and help make the right silicate phases that give set cement it’s strength.

So, don’t keep us in suspense… what does it look like?

1000039 You see I love this structure because it’s so complex!  The interplay of forces between the aluminate (the light blue and red atom units) and the calcium atoms (in hot pink) are just enough to shift them apart so they don’t line up.  This means that the repeating ‘unit cell’ structure has to be extra-ordinarily big to describe them all.

1000039_2Though this is tricky to measure and describe, this ‘kincked’ structure is why this material reacts so strongly with water.  A further upside is it does look rather beautiful.

So tricalcium aluminate – builds skyscrapers and is and has an incredible crystal structure – what’s not to love?

Where did the structure come from?

This structure of tricalcium aluminate was found by Mondal and Jeffery in 1975, and is #1000039 in the Crystallography Open Database

A Structure for Summer – Methylammonium Lead Halides

Summer is upon us down in the Southern Hemisphere and as the days heat up we will most likely be seeing a whole lot of sun. Wouldn’t it be great if we could harness all that solar energy? Well, fortunately for us there is literally no single technological issue that can’t be solved with a perovskite.

This is no ordinary perovskite however. In earlier articles I described perovskites with the formula ABX3, where A is a large cation, B is a smaller cation and X is an anion. The large A-site cation does not necessarily have to be an atom, it could be for example, a large positively charge organic compound. The light harvesting perovskites that have been causing quite a stir in the solar cell industry contain a methyammonium cation on the A-site with lead halide octahedra forming the corner linked network (Figure 1) [1].

Figure 1: Crystal structure of methylammonium lead iodide.

Figure 1: Crystal structure of methylammonium lead iodide.

These perovskites have attracted quite a bit of attention due to the unprecedented improvements in energy conversion efficiency (that is how efficiently they convert solar energy to electricity) over the past five years. While they started off at 3.8 % in 2009 [2], the perovskite solar cell efficiency is now 19.3 % [3]. For reference, current commercial crystalline silicon is between 17-23 % efficient [4]. Of course the main concern to the average consumer is cost, and this is where perovskites really begin to shine. Perovskite solar cells are made from cheap starting material and are easier to manufacture compared to the currently employed crystalline silicon solar cells.

Figure 2: Perovskites can be used in two solar cell architectures, (a) a sensitized perovskite solar cell and (b) thin film solar cells ("Perovskite solar cell architectures 1" by Sevhab - Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons [5])

Figure 2: Perovskites can be used in two solar cell architectures, (a) a sensitized perovskite solar cell and (b) thin film solar cells (“Perovskite solar cell architectures 1” by Sevhab – Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons [5])

The other more recently discovered benefit of the methylammonium perovskites is the role they play in the actual solar cell. In the simplest form of a solar cell, there is a material which acts as the light harvester, absorbing visible light and generating a negative charge (electron) and positive charge (“hole”). These charges are transferred into a material which allows the negative charges to be extracted to an external circuit as electricity (Figure 2). Originally titanium dioxide, which acted as the charge transfer material, was coated with the perovskite as a sensitiser to absorb visible light. It was recently discovered that the methylammonium lead halide perovskite can act as both the light harvester and charge transfer material. This discovery paved the way for perovskites to be used in alternate solar cell architectures, such as thin film solar cells.

It isn’t all sunshine and rainbows for perovskites however. The perovskite itself isn’t actually stable under humid conditions as it readily dissolves in water. Further, there have been concerns surrounding the use of toxic lead, although some progress has been made in this area by replacing lead with tin. Also, like any perovskite, the structure is tuneable with different A-site and halide variants possible leading to further improved properties for the tin varieties.

The future is bright, the future is perovskite.

The structure is #4335638 in the Crystallography Open Database

[1] Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Inorg. Chem. 2013, 52, 9019.

[2] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131, 6050.

[3] Service, R. F. Science 2014, 344, 458.

[4] McGehee, M. D. Nature 2013, 501, 323.

[5] Perovskite solar cell architectures 1 <http://commons.wikimedia.org/wiki/File:Perovskite_solar_cell_architectures_1.png#mediaviewer/File:Perovskite_solar_cell_architectures_1.png&gt; (Accessed 12/2014)

Surreal Microscopic Environments – Negative Crystals

A few months ago a story did the rounds about a “negative crystal floating in space” [1] which was accompanied by a rather spectacular image (Image 1).

Image 1: Negative crystal in spinel. Image by Danny Sanchez

Image 1: Negative crystal in spinel. Image by Danny Sanchez

This picture was described as a negative spinel and was one of the most perfectly formed crystal inclusions found and photographed by Danny Sanchez using photomicrography techniques [2]. This unique photography technique allows small regions within crystals to be photographed with exceptional clarity, creating extraordinarily surreal images of crystal inclusions. Negative crystals aren’t the only type of crystal inclusion however, and they certainly aren’t the only ones to produce stunning images.

Crystal Inclusions

Generally speaking, negative crystals are a specific kind of crystal inclusion. Inclusions are when a foreign material is trapped within a host crystal. The foreign substance can be solid, liquid or gaseous and can become trapped within the crystal either during its growth or once the host crystal has formed. The exact method of inclusion formation depends on the type of inclusion and the conditions under which the crystal was grown [3].

Solid inclusions

The most well known solid inclusions are ancient insects or plant life suspended in amber. However, solid inclusions can also include different types of gemstones embedded inside another crystal, typically quartz (Image 2).

Image 2: Rutile on hematite in quartz. Image by Danny Sanchez

Image 2: Rutile on hematite in quartz. Image by Danny Sanchez

These mineral inclusions either form simultaneously with the host crystal or were pre-existing and the host crystal has grown around it, encasing the foreign material. In the case of quartz which usually forms in hydrothermal conditions (basically a hot water soup of dissolved minerals) the included gem existed in the solution prior to the growth of the quartz and over time the quartz grows around it (Image 3).

Image 3: Purple and blue fluorite in quartz. Image by Danny Sanchez

Image 3: Purple and blue fluorite in quartz. Image by Danny Sanchez

Alternatively, as many crystal growth conditions are under high temperature and pressure conditions and in aqueous media, liquid inclusions highly saturated with dissolved minerals can form. As the crystal cools these minerals begin to precipitate forming a second crystal inside the host.

Liquid inclusions

As mentioned, many famous crystals (such as the Naica crystal caverns) were formed in hot, mineral-rich water solutions. As a result, sometimes this solution can become trapped inside the crystal during growth. A rather unique liquid inclusion which occurs under pressurised conditions is liquid CO2, which remains trapped in liquid form due to the pressure maintained inside the inclusion.

Quite often liquid inclusions come paired with a bubble of gas, such as in Image 4 in which liquid petroleum and a bubble of methane are trapped in quartz.

Image 4: Petroleum and methane bubble in quartz. Image by Danny Sanchez

Image 4: Petroleum and methane bubble in quartz. Image by Danny Sanchez

Gaseous inclusions

Finally, “negative crystals” form when a pocket of air is trapped within a crystal. These gaseous inclusions can be a specific gas, depending on the conditions the crystal grew in, or just plain old air. These inclusions can form either due to crystal growth which occurs in multiple directions that then intersect or because crystal growth in a particular direction is inhibited temporarily. Since gaseous inclusions are enclosed by crystal faces the shape will reflect the crystal habit (or defined external shape of the crystal) of the host and they are always oriented parallel to the host crystal (Image 5). While they may look like a typical gemstone, it is actually an inversion with air “inside” the crystal boundaries and crystal material on the “outside”. Hence these inclusions are named “negative crystals”.

Image 5: Negative crystals in amethyst. Image by Danny Sanchez

Image 5: Negative crystals in amethyst. Image by Danny Sanchez

A special aspect of any type of inclusion is that the foreign material is suspended in time. In this way, the inclusion contains a “piece of the past” which provides insight into what the earth and the environment were like when the crystal grew, often millions of years ago. For example, the discovery from air inclusions in amber that the oxygen content in air reached 35% during the Cretaceous period, before suddenly dropping to near the current level of ~21% [4].

There are many other forms of inclusions in addition to those shown. Quite often they can produce stunning visual effects and increase the value of the gemstone. For example, star sapphires which produce a unique six-rayed star effect under certain light sources contain tiny inclusions of aligned needle shaped rutile (Image 6).

Image 6: Star sapphire under a direct light source. Image from http://www.gemselect.com/.

Image 6: Star sapphire under a direct light source. Image from http://www.gemselect.com/.

Ordinarily, most inclusions are microscopic in size and can only be properly visualised under a microscope. For the majority of images featured here highly specialised and expensive equipment is required.

More images of crystal inclusions can be found in Gübelin and Koivula’s Photoatlas of inclusions in gemstones [5] or at http://www.dannyjsanchez.com/.

[1] Hooper, R.; New Scientist 2014, 2975, 24.
[2] The Art of Photomicrography: Gemstone Inclusions by Danny Sanchez <http://www.dannyjsanchez.com/&gt; (Accessed 11/14)
[3] Benz, K. W.; Neumann, W. Introduction to Crystal Growth and Characterization; Wiley, 2014
[4] Kump, L.R.; Kasting, J.F.; Robinson, J.M. Global and Planetary Change 1991, 5, 1.
[5] Gübelin, E.J.; Koivula, J.I. Photoatlas of Inclusions in Gemstones; Vol 1-3, Opinio Publishers