Tuesday 17 March 2015

A second naturally occurring quasicrystal from the Khatyrka Meteorite.

When atoms join together to form molecules of more than two atoms they adopt fixed shapes, as the molecular bonds are held at set angles to one-another. For example water (H2O) molecules have a ‘v’ shape as the two H-O bonds are held at an angle of 120° to one-another rather than 180°. Crystals are formed because certain atoms arrange themselves naturally into large complex molecules in which these bond-angles become extended into plains of symmetry. Conventional crystals have rotational symmetries of two, three, four or six; where a crystal with a rotational symmetry of two will show the same arrangement of atoms if rotated through 180°, a crystal with a rotational symmetry of three will show the same arrangement of atoms if rotated through 120°, etc.  While crystals do not always reveal these symmetries to the naked eye, this does explain the dramatic symmetrical structures often seen in naturally occurring crystals, and how gem-cutters are able to cut jewels into distinctive shapes.

Quasicrystals are structures in which atoms arrange themselves into regular patterns with more complex structures, typically lacking true rotational symmetry, but having approximate rotational symmetries forbidden to regular crystals, such as seven or ten. Such structures were first theorized in the 1960s, and have been created artificially in the laboratory since the 1980s, but only a single naturally occurring quasicrystal has ever been discovered, in a rock sample from the  Khatyrka Ultramafic Zone of the Koryak Mountains in Chukotka in the Russian Far East, later identified as a carbonaceous chondrite (stony meteorite).

In a paper published in the journal Scientific Reports on 13 March 2015, a group of scientists led by Luca Bindi of the Dipartimento di Scienze della Terra at the Università di Firenze describe a second naturally occurring quasicrystal from the Khatyrka Meteorite, from a sample recovered by a 2011 expedition to the Koryak Mountains.

The original crystal had a chemical composition of Al63Cu24Fe13 (i.e 24 copper and 13 iron atoms for every 63 Aluminium atoms), and an icosahedral rotational symmetry (i.e. an approximately 20-fold symmetry), which it was theorized could only have formed at very high temperatures and pressures, followed by rapid cooling (which would have prevented the structure recrystallizing into a more conventional structure). Other minerals from the sample, and subsequently recovered samples of the Khatyrka Meteorite, have also suggested a high temperature and pressure creation followed by rapid cooling, and an isotope derived date of about 4.5 billion years ago for the formation of this material – early in the history of the Solar System.

The new quasicrystal has a chemical composition of Al71Ni24Fe5 (i.e. 24 nickel and 5 iron atoms for every 71 atoms of aluminium) and a decagonal symmetry (i.e. 10 rotational symmetry planes). All aluminium-dominated quasicrystals produced to date have required high temperatures and pressures to form, rapid cooling to stabilize and a low oxygen environment. Aluminium-nickel-iron quasicrystals with decagonal have been formed in laboratory since 1989, and can only form at temperatures of between 1120 and 1200 K.

The top panel shows micro CT-SCAN 3D-images (at different rotations) of the whole Grain 126. The brighter and the darker regions are Cu-Al metals and meteoritic silicates, respectively. The bottom panel shows a SEM-BSE image of Al71Ni24Fe5 quasicrystal (QC) in apparent growth contact with ‘‘olivine’’ (‘‘Ol’’). The surface of the quasicrystal appears to exhibit growth steps. The image also contains sodalite (Sod). Bindi et al. (2015).

Both the known Khatyrka quasicrystals show a high degree of structural perfection, which suggests rapid heating to a temperature of in excess of 1000 K, and possibly as high as 1500 K, under conditions of high pressure, followed by rapid cooling. This suggests that the most likely explanation for these quasicrystals is that they are shock-induced structures, formed in a collision between two bodies in the early Solar System.

 High-resolution transmission electron microscopy (HRTEM) image showing that the real space structure consists of a homogeneous, quasiperiodic and ten-fold symmetric pattern. Bindi et al. (2015).

See also...

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