Monthly Archives: November 2014

Bridgmanite – Silicate Perovskite

In the issue from the 28th of November the magazine Science reported on the successfull characterization of a natural sample of the ‘most abundant’ solid phase of the earth.

It is: magnesium iron silicate (Mg,Fe)SiO3, which crystallizes in the fameous perovskite-analogue structure of CaTiO3:

Bridgmanite_b

 

So, we have tilted and twisted corner-connected SiO6 octahedra (orange) and in the centers of each 8 such octahedra the cation Mg or Fe is located with occupancies of 70 and 30 %, respectively. The space group is Pnma (#62, setting 1, orthorhombic crystal system) and the cell constants are a = 5.02 Å, b = 6.90 Å, and c = 4.81 Å.

But what (the hell) is the reason that this rather unspectacular structure is worth a story in Science?

Well, although being the most abundant phase of the earth it is kind of concealed, namely in the lower earth mantle, which begins in a depth of 660 km under the ground and where pressures of 250,000 atm exist. Nobody is able to dig a hole of such depth. But some tiny chunks of Bridgmanite came into existence also on the earth crust, when in 1879 a meteorite crashed with a huge amount of energy the earth ground in Australia. Only now, the team of Oliver Tschauner of the University of Nevada was able to fully structurally characterize this mineral.

The structure itself was not a surprise as high-pressure experiments with artificial silicate perovskites already lead to the assumption that this mineral has this structure. But now it is proved!

And only when a mineral is fully characterized from an natural occuring sample it can be named! The scientist named it Bridgmanite after the inventor of high-pressure crystallography Percy W. Bridgman (1882 to 1961).

Read the full story here:

http://www.sciencemag.org/content/346/6213/1057

 

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Chapter 7 available!

Dear Students,

Chapter 7 of our course is now available. This is the final chapter of this course and it will be all about nets and topology.

We will show you how to use three programs for the analysis of framework structures: TOPOS, Systre and 3dt.

We hope that you will upload some of the pictures you will have created upon completion of this course to our flickr group.

And once more, we invite you to like our facebook page:
http://www.facebook.com/crystalmooc

Even after this course ends we will provide interesting, crystallographic news on this page. And we also would like to keep you informed about our future plans for another online course.

For one more time: see you in class!

Michael and Frank

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Chapter 6 starts today!

Dear Students,

in this week we will introduce a very special class of crystalline materials, which are called Metal-Organic Frameworks or short MOFs. Research interest in this kind of materials has intensified immensely over the last decade. As MOFs are also the field of study of our research group, telling you something about these very special crystals is a matter of heart for us.

MOFs are comprised of inorganic and organic secondary building units. We will take a look at different variations of how these secondary building units can be assembled together and we will introduce the principles of classification of these network-like assemblies.

We provide several crystal structures of MOFs as VESTA files, in order for you to be able to get familiar with the structural features of MOFs at the atomic level. (You can find a tutorial on how to obtain and install VESTA in Unit 2.7)

Enjoy the beauty of porous crystals!

Michael and Frank

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Chapter 5 activated!

Dear Students,

today Chapter 5 of our course has been activated!

We hope, you are all doing well!

In this week we will practise the handling with the International Tables a little further. But in contrast to staring only at circles and commas we will additionally look at real world crystals and some of their symmetry and physical properties. And you can be curious about the story of one of the most thrilling scientific discoveries of the last century: the discovery of quasicrystals by Dan Shechtman.

Looking forward to meet you at class and in the forum!

Frank and Michael

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Error correction for the Space group list poster

There was a small error in the Space group list poster, which was caused by a stupid ‘Copy&Paste’ mistake: The example of the crystal structure with the space group Fdd2 was labeled  – as the example right above on the poster – with ‘Cobaltite’, but it has to be named,  of course, ‘Edenharterite’ instead.

Thanks to Melanie Müller, who brought this error to my intention!

Furthermore, I decided to include the space group numbers on the poster in order to provide a better and faster ‘navigation’ of the viewers’ eye 🙂

The corrected version can be downloaded here!

 

Chapter 4 released!

In this chapter, we focus on symmetry operations that separate crystal structures from (other) macroscopic objects: glide planes and screw axes.

After that we are ready to climb up the last step of our “ladder of crystal classification”. We will open the “crystallographers’ bible” and will explain what psalm are important to crystallographers!

best wishes!
Frank and Michael

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