Many people may not be aware of this, but there is a fairly precise definition of what constitutes a mineral. A mineral is an element or chemical compound that is a periodic crystal, has an incommensurate structure, or is in an amorphous phase; all of these have to be formed naturally, as the result of a geological process. New research that appeared in the current issue of Science may force mineralologists to change that definition to include what are known as quasicrystals.
Crystals are solids with an atomic structure that has a long-range periodic order—that order can be described by a small group of atoms and a rotational symmetry operator that is repeated over long distances. In the early days of the modern study of crystals, researchers found that the only allowable types of symmetries have two-, three-, four-, and six-fold symmetry axes. Anything else, such as five- or seven-fold symmetry, or anything higher, is apparently forbidden.
But researchers have since described the quasicrystal, which contains a more subtle, long-range ordering phenomenon. In quasicrystals, the "atomic positions along each symmetry axis are described by a sum of two or more periodic functions whose wavelengths have an irrational ratio." This difference in how ordering is handled means that quasicrystals are not constrained by the symmetry restrictions placed on pure crystals. Whereas regular crystals are described by square or hexagonal tiling patterns, the more apt equivalent for quasicrystals is Penrose tiling.
Quasicrystals were first observed 25 years ago, when an alloy of aluminum and magnesium was rapidly quenched in a laboratory. In the intervening quarter century, many compounds have been identified as forming a quasicrystal, but all of the examples were created in a lab.
In an attempt to discover a naturally occurring quasicrystal, a team of researchers from around the globe focused their efforts on the mineral khatyrkite ((Cu,Zn)Al2). This mineral, along with its chemical relative cupalite ((Cu,Zn)Al), were picked as candidates because many of the known synthetic quasicrystals were metallic and contained aluminum. The group found what it was looking for in a mineral sample from the Museo di Storia Naturale of the Universit� degli Studi di Firenze that was obtained in the Koryak region of Russia (northeast of the Kamchatka peninsula). It contains what they claim to be the first naturally occurring quasicrystal ever identified.
Chemical analysis of the sample found a number of grains that were rich in aluminum, consistent with khatyrkite and cupalite, but lacking in zinc. It also identified a previously unknown material growing into some of the grains, an AlCuFe alloy. The structure of this alloy was similar to the beta phase of a synthetic CuAlFe alloy that is already well studied and known to be a quasicrystal. Further chemical testing found this specific alloy to have the composition Al65Cu20Fe15, very near to the ideal composition of i-AlCuFe. Powder X-ray diffraction revealed that the AlCuFe mineral grains had a unit cell structure of a face-centered icosahedral, the same as the known ideal quasicrystalline material.
Electron diffraction patterns from a high-resolution transmission electron microscope clearly showed the characteristic peaks of a lattice that exhibits two-, three-, and five-fold symmetry—the signature of an icosahedral quasicrystal. Applying an inverse Fourier transform to the image resulted in a real space representation of the material—shown above—with a clear fivefold symmetric pattern.
The researchers were obviously curious about how the material formed. Since it was embedded within a crystalline material, one would expect there to be stress or strain at the grain boundaries. But examination of the sample found no signs of strain at the edges of the quasicrystal, suggesting that it either formed without strain, or those signs were annealed away in the eons since it first was created.
Either way, this represents the first time that this sort of material has been found to be formed by natural processes, which leads the authors to question whether the definition of a mineral should be revised. If so, it might open a whole new class of compounds to the field of research.
Science, 2009. DOI: 10.1126/science.1170827