What’s in silly putty? – Borax

What does it look like?

What is it?

Sodium tetraborate, more commonly known as Borax, is a substance that has been used for centuries for all sorts of purposes.  First imported from Tibet via the silk road, it has been used as a fire retardant, an anti-fungal, and even a chemical additive in the metal industry.  But today we’ll just tell you about silly putty.

Mix borax with water and polyvinyl acetate (PVA) glue and you can make silly putty!  The borate molecules get cross linked with the polyvinyl within the glue, to make this material strong in short time scales and flowing on a longer timescale!

Where did the structure come from?
This structure of borax (which, in fact is a hydrate as sodium tetraborate usually crystallises with water) was determined by Levy and Lesensky in 1978 at the same time that they determined the structure of Glauber’s salt.

Biominerals #2 – Weddellite (in Antarctica and in you!)

What does it look like?

Two views of the Weddellite structure, image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

Scanning Electron Micrograph of the surface of a kidney stone showing tetragonal crystals of Weddellite, image by Kempf EK

This is another biomaterial that our own bodies form, and it can have some very painful consequences.  Calcium oxalate is one of the materials that forms kidney stones, painful objects that can obstruct some of our internal organs.   There are thought to be many reasons why these form, but the calcium oxalate is one of the minerals that conglomerates to form them.

There a a few ways that calcium atoms and oxalate molecules can crystallise together, and most of these involve water.  The form we’ve featured today is dihydrate, there are two water molecules in the structure for every calcium oxalate.  This material actually has a mineral name, Weddellite, because as well as being found in your body forms in sediment in the bottom of the Weddell sea in Antarctica.

Where did the structure come from?

This structure of Weddellite was determined in a paper that re-examined the crystal structure of weddellite and whewellite (the monohydrate (or one water atoms per calcium oxalate) structure).  It is #9000764 in the Crystallographic Open Database.

A big water cage, sH clathrate hydrate.

What does it look like?

The three host water cages found in sH clathrate hydrates and how they fit together. Image from Loveday and Nelmes 2008 http://pubs.rsc.org/en/content/articlelanding/2008/cp/b704740a#!divAbstract

What is it?

Water is an incredible material for lots of reasons, but a further one is the shear number of materials it will form new compounds with. Known as hydrates, we’ve featured quite a lot of them on the blog so far. Most of the time the water molecules are able to bond with the materials it’s forming with, for instance in copper sulfate hydrate. But sometimes water forms a solid with a material it can’t bond to, often very small organic molecules like methane and propane, so what do those structures look like?

We’ve met these before, a class of materials called clathrate hydrates. These are three dimensional host guest materials where the water molecules form into a giant host cage that trap the small molecular guests. The structure we met before was one of the two cubic (i.e. very symmetrical) clathrates that form when you freeze methane and water. This one is a bit different. It’s named sH clathrate, where the H indicates that it has hexagonal symmetry, and is special because it forms a very big cage able to fit some pretty large molecules in.

It was thought for many years that this material was only synthetic; it was discovered in the laboratory. This structure has been of interest to a range of scientists, especially as it could also be used to stored CO₂. But a few years ago researchers pluming the margins of the Gulf of Mexico discovered that the hexagonal big cage form of clathrate could form in nature too.

Where did the structure come from?

sH clathrate hydrates were first discovered by Ripmeester et al., and were reported in Nature in 1987.

Bonattite – blue copper sulfate, but a lot rarer

What does it look like?

The crystal structure of Bonnattite. The blue atoms are copper, the red oxygen and the yellow are sulfur. Image generated by the VESTA (Visualisation for Electronic and STructual Analysis) software http://jp-minerals.org/vesta/en/

What is it

Discovered in the Toscana region of Italy Bonattite is very similar in composition to copper sulfate pentahydrate, except it has only three water molecules in its structure making it a copper sulfate trihydrate.  There’s not much information out there on this illusive mineral, but is thought to form in hot spring areas like Steamboat Springs in Colorado.

Where did the structure come from?

The structure of Bonattite was determined by Zahrobsky and Baur from an synthetic sample in 1968.  You can find this structure in the American mineralogical database.

An Earth and Mars mineral – Meridianiite MgSO4.11H2O

What does it look like?

The structure of Meridianiite at 250 K. This image was created using the diamond visualisation software. Mg is pinky-red, S is yellow, O is white and hydrogen (technically deuterium), is grey.

What is it?

MS11, Meridianiite is another lovely (in the opinion of the author), salt hydrates, just like Mirabilite and Epsomite. Like those salts, it is also thought to be a key rock-forming mineral on the icy satellites of Jupiter. But hang on, how come it is named after a place on Mars?, well it is there too…probably…

MS11 is only stable below 2°C. This means that on Earth we mainly find it at the cool extremes of our planet; tundra and cold deserts are MS11’s favourite places to be. On Mars the temperature variations on the planet mean that the conditions that MS11 forms in and is stable at are much more prevalent and the “Mg-sulphates” which have been identified by various instruments on Mars may well be Meridianiite.

As we heard with Bridgmanite, a mineral can only be named once it has been found in nature. This mineral was first synthesised by Fritzsche in 1837, just like Glauber and Mirabilite, this salt was called Fritzsche’s salt to begin with. Fritzsche thought that MS11 was actually MS12 – Magnesium sulphate with 12 water molecules attached rather than 11. He worked out the composition by measuring the mass lost as the compound dehydrated and must have counted 12 waters instead of 11. This was put right in 2006 by Petersen and Wang who solved the structure and realised there were only 11 waters rather than 12. Shortly after this, a natural sample of Meridianiite was found in Canada and so it was officially named and became a fully paid up mineral.

Where did the structure come from?

This structure is actually for deuterated Meridianiite, (the hydrogen atoms are replaced by deuterium) and comes from a study using time-of-flight neutron diffraction to investigate how the structure of MS11 changes as temperature varies from 10 – 300 K.

Fortes A. D., Wood I. G. and Knight K. S. (2008) The crystal structure and thermal expansion tensor of MgSO4-11D2O (meridianiite) determined by neutron powder diffraction. Physics and Chemistry of Minerals 35 207-221 and is available on the American Mineralogist Crystal Structure Database.

Sounds edible, but isn’t Tobermorite

We’re starting to draw a close on the ‘crystal structures inspired by food’ theme on the blog, but room for just a couple more? Tomorrow’s last one is a cracker, but today’s is a little more tenuous – a mineral that sounds like it should be a chocolate bar.

What does it look like?

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

The blue polyhedral are silica units (silicon atoms in the middle with red oxygen atoms in the corners) which make a double chain through the structures. The large light blue atoms are calcium which sit in-between and are calcium atoms with the small grey atoms marking the positions of water molecules.

What is it?

Tobermorite is a mineral named after the Scottish village Tobermory found on the Island of Mull (as well as having a mineral named for it, the village was inspiration for a popular children’s program in the UK). It is a calcium silicate hydrate, and is the natural mineral of this very important ingredient of Portland cement. The hydration and crystallisation of this mineral product is what give the cement its strength.

Where did the structure come from?

There are a couple of variations of Tobermorite, one with four water molecules and the other with five. The one we’ve drawn has five and is #902244 in the open crystallographic database.

Selenite – Crystal Cathedrals

What is it?

It is the dream of all crystallographers to develop a technique which can grow the largest and highest quality crystals possible. Often this is extremely difficult to do due to all of the competing factors which can drive crystal growth, and so it is often considered an art form. The rarity of large single crystals is what makes the Naica crystal caves truly extraordinary.

The great crystal cavern of NAICA mines.

Buried 300 meters below the surface, and discovered at a time when we people believed that they had seen every spectacular sight the earth had to offer, are the selenite crystal caves (Figure 1). No, this is not a carefully constructed Photoshop; those are real people climbing over pure gypsum crystals, otherwise known as selenite. In fact these are the largest of any naturally occurring crystals measuring up to 12 m in length, 4 m in diameter and 55 tons in weight. However, before you rush to pull out your passport and start booking flights, know that these caves are not open to the public. The caves are restricted partially for preservation purposes (instigated by a mining company of all things) but also because conditions within the cave are oppressive. Due to their location above an underground magma chamber and the presence of a large quantity of ground water, temperatures can sit at a constant 58 °C and 90 to 99 % humidity. Without special protection humans can only survive in these conditions for a few minutes. Incidentally, these are ideal conditions for crystal growth. Prior to their discovery, the caves were filled with hot, mineral rich water for 500,000 years. During this time the temperature hardly varied and the crystals were allowed to slowly grow to the size they are today.[1]

Researcher in protective gear among the crystals.

So if the caves are closed to public access, what are people doing there? A huge research project is being undertaken covering fields from medical research to regional geology. Some examples of the research being performed include, investigating how the crystals became so large and how old they are, studying pollen and microorganisms trapped inside the crystals, developing technology which enables humans to work in extreme conditions for long periods of time and building an understanding of the physiological effects of exposure to these extreme conditions over a long period of time. However, the longer the crystals are exposed to the air the more faded and brittle they become, eventually cracking under their own weight. As a result there are constant discussions on whether research should continue in order to understand this unique natural resource or whether the caves should be re-flooded with water in order to preserve their current form.

What does it look like?

Structure was generated in VESTA

As mentioned, the crystals are a pure form of gypsum known as selenite. The crystal structure of selenite/gypsum (CaSO₄•2(H₂O)) has been featured before on this blog in its “desert rose form”. The main difference between the desert rose crystals and selenite is the presence of sand impurities in desert rose. Another view of the structure is provided below where calcium is shown in blue, SO₄ tetrahedra are yellow, oxygen is red and hydrogen is white.

Where did the structure come from?

The image was generated from a structure in the paper Schofield P. F., Knight K. S., Stretton I. C. American Mineralogist 81 (1996) 847-851 and can be found on the American Mineralogical database.

Images from:

Cave of Crystal Giants – National Geographic Magazine, Available from: <http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text&gt; [November 2008]

[1]          NAICA PROJECT/CRYSTALS’ CAVE, Available from: <http://www.naica.com.mx/english/internas/interna4_1.htm&gt; [Accessed May 2014]

Confining water – the structure of HHTP tetrahydrate

What does it look like?

The picture was generated by CrystalMaker and MacPyMOL software.

The structure of 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) tetrahydrate involves π-stacking of the disc-shaped HHTP molecules into columns, which are arranged in a pseudo-square lattice. The structure is porous and the pores between the HHTP stacks are filled with water molecules, where hydrophilic hydroxyl groups on the edge of the HHTP molecules act as chemical anchors for the water chains.

What is it?

Water is perhaps one of the most well studied substances due, in part, to its pervasiveness in the environment, the essential role it plays in biological systems and its remarkable physical properties. More solid forms of water have been documented than any other material yet, despite the best efforts of numerous researchers, the dipolar nature of the water molecule has not yet been observed unambiguously to induce ordering – creating a polar ice – when originating from a proton-disordered solid.

One approach to achieving a ferroelectric response in solid water is to place it in a confined geometry where spatial limitations mean the molecules are restricted to have a correlated, polar arrangement. The use of confined water chains has near-accomplished this where ferroelectric ordering has indeed been observed but only on cooling from a liquid state. Clearly, if ice is to be exploited in switchable devices, solid-solid transformations will be far more desirable.

The present structure undergoes a polar to non-polar phase transition with increasing temperature. At low temperatures, the HHTP molecules mediate weak interactions (ca. 0.9 kJmol^-1) between the water chains and lead to the generation of an overall dipole moment with the dipole moments of each water channel pointing in the same direction. On raising the temperature above a modest 240 K, sufficient energy is supplied to disrupt the inter-chain communication, causing the system to adopt a paraelectric state. It is noteworthy that the phase transition from a polar to a non-polar structure solely originates from alignment of the water molecules. What remains now is to measure the strength of the ferroelectric response in the ordered, low-temperature phase.

Where did this structure come from?

The structures are deposited in the Cambridge Crystallographic Data Centre; CCDC 962532 for the high temperature structure and CCDC 962533 – low temperature structure.

The variable temperature study is published in Physical Chemistry Chemical Physics in 2014.

Mirabilite: “The Miracle Salt”

Today Dr Helen Brand gives us a perspective on her favourite mineral:

What does it look like?

Figure 1. The structure of mirabilite. This picture was made using the Diamond Visualisation software.

What is it?

Mirabilite: Na₂SO₄.10H₂O

Throw some sodium sulphate into a beaker, leave it on a windowsill for a few days and you will come back to a mass of lumpy crystals (figure 2.).

Figure 2. Synthetic mirabilite

Possibly my favourite mineral, this is mirabilite. Whilst I love a bit of mirabilite, a lot of people, especially those in the building trade, hate it. Mirabilite is stable to just above room temperature. Imagine it is a sunny day and you have a building stone, now add some rain. Some of the sodium sulphate in the rock is dissolved into the rain water which sits in pores within the rock. As night falls, so does the temperature, and solid mirabilite becomes stable and crystallises in the pore spaces in the rock. Just like ice, the volume of the solid is greater than that of the liquid. For mirabilite, the volume change upon crystallisation is even bigger than for ice. This large volume change puts stress on the building stone and it cracks, weakening any buildings constructed from the stone.

Mirabilite was first synthesised and described by Johann Glauber in 1648, while he was trying to make nitric acid. He called it “sal mirabilis”, the miracle salt, for the health benefits it provided as a laxative and because he wasn’t expecting to form it as a by-product of his acid-making process. It became known as Glauber’s salt and was finally found in nature and the mineral mirabilite named around 1842.

So why do I love mirabilite? Mirabilite is an abundant but elusive mineral, largely thanks to the dehydration temperature being not very far above room temperature. It is found in evaporite deposits here on Earth and is also thought to be a rock forming mineral on the icy satellites of Jupiter. Mirabilite has a fascinating crystal structure; it is a layered structure with chains of sodium octahedra along the c-axis which are separated by sulphate tetrahedra which are connected together through hydrogen bonds. It also contains disorder of the sulphate tetrahedra and the hydrogen bonds attached to the sodium sulphate tetrahedra. These involve atoms “hopping” between possible positions that they can’t decide between. The behaviour of this mineral with changes in temperature is controlled by the interaction of 2 of the hydrogen bonds with the sulphate tetrahedra (figure 3).

Figure 3. The hydrogen bonds and sulphate tetrahedra which control the behaviour of mirabilite with temperature.

Where did the structure come from?

This structure (figure 1), was produced by the author’s own fair hand from neutron diffraction data obtained at 4.2K at the High Resolution Powder diffraction beamline at the ISIS neutron spallation source, UK. We started with the coordinates determined by Levy and Lisensky’s 1967 single crystal study and refined the structure from there (Brand et al. 2009). The original structure was first determined by Ruben et al. 1961. All these structures are available in the American mineralogist crystal structure database.

References

Brand H.E.A., Fortes A.D., Wood I.G., Knight K.S., Vocadlo L. (2009) The thermal expansion and crystal structure of mirabilite (Na2SO4.10D2O) from 4.2–300 K, determined by time-of-flight neutron powder diffraction. Phys. Chem. Min. 36:29–46.

Levy H.A., Lisensky G.C. (1978) Crystal structures of sodium sulphate decahydrate (Glauber’s salt) and sodium tetraborate decahydrate (borax). Redetermination by neutron diffraction. Acta Cryst Sect. B Struct. Sci. 34:3502–3510.

Ruben H.W., Templeton D.H., Rosenstein R.D., Olovsson I. (1961) Crystal structure and entropy of sodium sulfate decahydrate. J. Am. Chem. Soc. 83:820–824.

Bath salts on the moons of Jupiter

What does it look like?

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

What it is?

Epsomite is a magnesium sulfate hydrate, it forms with seven water molecules for every magnesium sulfate molecule. It is a common evaporate mineral that was first identified in the caves of Epsom in Surrey, UK in 1816.   Then it was first called Epsom salts and became a nice thing to put in your bath to ease aches and pains. But it’s also thought to be found on the moons of Jupiter!

Where did the structure come from?

Epsomite is #1100091 in the open crystallographic database.