The Enigma of Naturally Occurring Quasicrystals
For decades, the scientific community held a firm belief that crystals must be periodic—meaning their internal structure repeats in a predictable, grid-like pattern. This fundamental assumption was shattered by the discovery of quasicrystals, materials that possess an ordered but aperiodic structure. Unlike traditional crystals, quasicrystals do not repeat their patterns in a simple translation, yet they maintain a long-range order that defies traditional crystallography.
This article explores the nature of these fascinating materials, the extreme conditions required for their formation in nature, and the broader scientific implications of their discovery.
The Nature of Aperiodicity
At its core, a quasicrystal is an aperiodic structure. While a standard crystal (like table salt or diamond) is composed of atoms arranged in a repeating unit cell that tiles the space perfectly, a quasicrystal's arrangement is ordered but never repeats exactly.
This distinction is critical because it allows for symmetries that were previously thought to be mathematically impossible in nature. For instance, five-fold symmetry—often seen in pentagons—cannot tile a 2D plane without leaving gaps. However, quasicrystals can achieve this through a non-repeating, ordered sequence, challenging our understanding of how matter organizes itself at the atomic level.
How Quasicrystals Form in Nature
Because the conditions required to create these structures are so specific and rare, quasicrystals are not commonly found in the same way as quartz or feldspar. Their formation typically requires instantaneous, extreme high temperatures followed by rapid cooling.
Two primary natural occurrences have been identified:
Space Debris Impacts: When meteorites strike the Earth, the immense pressure and heat generated by the impact can force atoms into these aperiodic arrangements.
Lightning Strikes: When lightning hits sand, the resulting heat can create unique glass-like structures, sometimes containing quasicrystalline phases.
Characterizing these materials is particularly challenging for scientists. Because they lack a repeating unit cell, traditional chemical formulas cannot be easily applied. Describing a prototypical section of a quasicrystal is difficult because it is impossible to do so without repeating elements in a way that doesn't align with traditional crystallography.
Beyond the Mineral World
The discovery of quasicrystals has opened doors to further research into other non-traditional structures. Discussions among researchers have highlighted related phenomena, such as:
Clathrates: In the aftermath of nuclear explosions, researchers have identified materials called clathrates—cagelike chemical lattices that trap other atoms inside them, representing another departure from simple periodic crystals.
Paracrystalline Viruses: In the biological realm, "natural" paracrystalline viruses (such as the Invertebrate Iridescent Virus) exist, which can turn isopods blue and subsequently kill them, demonstrating that aperiodic or near-periodic order can manifest in biological entities.
The Scientific Journey
The quest to understand quasicrystals was not merely a theoretical exercise; it was a high-stakes scientific hunt. As detailed in the work of Paul Steinhardt, the journey moved from theoretical predictions to experimental verification, and eventually to the literal hunting of these materials in meteor fields in eastern Russia.
Furthermore, the intersection of mathematics and chemistry continues to evolve. Some researchers have even explored the possibility that the patterns within these aperiodic structures have a relationship with the distribution of prime numbers, suggesting that the mathematical beauty of quasicrystals extends far beyond the physical material itself.