Pasteurized Crystals – Tartaric acid.

What is it?

December 27 marks the 192nd birthday of Louis Pasteur, which means that (a) he’d be really old if he hadn’t died in 1895, and (b) today is the perfect day to talk about tartaric acid.

Tartaric acid occurs naturally in many plants, particularly grapes. You’ve already read about ‘wine diamonds’ (potassium bitartrate), but you may not be aware of the contribution tartaric acid has made to scientific language.

Naturally occurring tartaric acid, first isolated in 1769, was found to rotate plane polarized light to the right. When it was prepared synthetically, tartaric acid had identical properties, except that it didn’t rotate plane polarized light. The synthetic material was thought to be a different compound, and was named racemic acid (from racemus, Latin for ‘a bunch of grapes’). It was subsequently determined that tartaric acid can exist in two different forms; and that naturally occurring tartaric acid was L-tartaric acid, while ‘racemic acid’ was actually an equal mixture of D and L-tartaric acid, mirror image isomers (enantiomers). These enantiomers were optically active in opposing directions, appearing optically inactive; this explained the otherwise identical properties of tartaric acid and racemic acid.

For this reason, ‘racemic’ came to mean ‘an equal mixture of enantiomers’, and this term continues to be ubiquitous in organic chemistry today.

So where does Pasteur fit into this story? Early in his career, before he discovered vaccination, microbial fermentation and invented the process which still bears his name (pasteurization), Pasteur studied crystals of tartaric acid and ‘paratartaric acid’ obtained from wine sediments. In particular, he wondered why (as described above) tartaric acid rotated light, while paratartaric acid did not, even though the chemistry and elemental composition of the two were identical. In one of the most beautiful and famous experiments in the history of science, Pasteur noticed, while squinting down a microscope, that there were two subtlety different types of crystals in the samples of paratartaric acid, each the mirror image of the other (see diagram below). He very carefully (and tediously) separated the two types of crystals into separate piles, redissolved each pile, and found that each did indeed rotate light, but in opposite directions. He had, in effect, separated the two enantiomers from the paratartaric acid (a.k.a. racemic acid) and discovered molecular chirality.

The two types of crystals found in paratartaric acid, which are mirror images of each other.

The two types of crystals found in paratartaric acid, which are mirror images of each other.

What does it look like?

The structure of D-tartaric acid (left) and its mirror image, L-tartaric acid (right).

The structure of D-tartaric acid (left) and its mirror image, L-tartaric acid (right).

 

Where did the structure come from?

D-tartaric acid can be found under CCDC refcode TARTAC, while L-tartaric acid is at CCDC refcode TARTAL.

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

Crystals you can grow at home – Nickel Sulphate hexahydrate

What does it look like?

niso46h2o_62051

Spot the helix! Drawn using Jmol.

What is it?

Nickel sulphate hexahydrate (NiSO4.6H2O) forms large, perfect crystals
that can easily be grown at home – just dissolve plenty of NiSO4 in
water and let the water evaporate. You can see a beautiful example here . As an extra bonus, by choosing the appropriate
temperature you can get slightly different coloured crystals – up to
53C the crystals will be dark blue-green, and above 53C they will be
clear green. These colours correspond to slightly different
structures, and this is an apparently unique situation, as similar
sulphate compounds are only stable as a single structure type. As
might be expected for something so easily grown, it is found in nature
as the mineral retgersite.

 

NiSO4.6H20 is also interesting because of its ability to rotate the
direction of polarisation of light – and the strength of this rotation
depends on the wavelength of the light, even switching rotation
direction at 0.5um. In a fascinating paper, Stadnicka, Glazer and
Koralewski [1] were able to relate the direction and strength of the
optical rotation to the properties of the structural helix formed by
the O atoms. The helix goes from top to bottom on the structural
diagram above.

Where did the structure come from?

The structure of the lower-temperature phase was first determined in
1932 by Beevers and Lipson [2]. It was sufficient to classify X-ray
reflections into 8 intensity classes ranging from “very strong” to
“absent” in order to derive and verify their model. Both forms have
since been extensively studied by X-ray and neutron diffraction.

[1] Stadnicka, K., Glazer, A.M. and Koralewski, M. (1987), Acta Cryst.B43, 319-325
http://dx.doi.org/10.1107/S0108768187097787
[2] Beevers, C. A. and Lipson, H. (1932) Z. Kristallogr. 83, 123-135
(http://rruff.info/doclib/zk/vol83/ZK83_123.pdf)

Further reading

Angel, R. J. and Finger, L. W. (1988) “Polymorphism of Nickel Sulfate Hexahydrate”,
Acta Cryst. C44, 1869-1873 http://dx.doi.org/10.1107/S0108270188006717

Dusty crystals

What does it look like?

The image to the right shows the sideview of a plasma crystal in the laboratory. Dust particles are suspended in an argon plasma above a high-frequency electrode (bottom). The horizontal field of view is 2 cm. From the Max Plank Plasma Crystal Experiment http://www2011.mpe.mpg.de/pke/index_e.html

The image to the right shows the sideview of a plasma crystal in the laboratory. Dust particles are suspended in an argon plasma above a high-frequency electrode (bottom). The horizontal field of view is 2 cm. From the Max Plank Plasma Crystal Experiment http://www2011.mpe.mpg.de/pke/index_e.html

What is it?

It’s a crystal you can see! This is a photograph of a plasma crystal (sometimes called a Coulomb crystal). The width of the photo corresponds to about 4cm. The dots are micrometer size particles suspended in an argon gas discharge. The inset at top right shows a view of the crystal looking from above.

The gas discharge produces the diffuse purple glow, but the particles don’t emit light themselves. They’re being illuminated by a laser, which is why there’s a yellow line along the disc at the bottom of the image. That disc, and the other one at the top, are the two electrodes for the high voltage that drives the discharge.

Gas discharges are commonly used in industry, for example in silicon chip manufacture and materials processing. Closer to home, sodium lamps and fluorescent tubes are also gas discharges.

Gas discharges are a type of plasma- the “fourth state of matter”, which consists of free electrons and ions rather than the neutral atoms of a gas. It’s probably no surprise to discover that a plasma with larger particles floating around in it (usually between a nanometer and a micrometer in size) is referred to as a “dusty plasma”.

In industrial use, the dust is often an inconvenience (that’s why silicon chips are made in clean rooms), but researcher H. Izeki realized in 1986 that it might be possible to engineer the dust to form crystals. The dust picks up a charge as a result of being bombarded by electrons, and the individual particles form a lattice to minimize the electrostatic repulsion. Dusty plasmas were “crystallised” by a number of groups (for example in Japan and Germany) in 1994 and dusty plasma research has exploded since then.

Where does it come from?

This lovely image is a photograph of a laboratory plasma at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.