For centuries, minerals were the only objects of study for crystallography. Today they are still important but the more ambitious challenges in this field are linked to worlds we do not know: extraterrestrial mineralogy, mineral processes in the deep interior of our planet, and the mineral chemistry of the initial states of the Earth and their possible role in the origin and infancy of life. The Curiosity rover exploring the surface of Mars carries on board the first X-ray diffraction device (CHEMIN) that has analysed Martian rock samples.
The first X-ray diffraction diagrams performed on Mars. They show the presence of gypsum and of clays, in other words showing a watery environment with neutral or lightly alkaline pH.
The exploration of outer space is an inevitable destiny of humanity. Getting to know the mineral composition of this outer world, including planets and moons, is the first vital step to future colonization. Designing and building new equipment that can develop crystallographic studies in those conditions is one of the challenges for 21st century science.
Meteorites, whether coming from the asteroid belt found between Mars and Jupiter, or whether provoked by impacts on other planets and moons, are made of different types of crystals. One of the challenges of mineralogy is to decode the information in the form, texture and structure of these crystals to reveal the history of our solar system.
We live on the Earth’s crust, a thin external layer of the Earth, which is only a few dozen kilometres thick. But the planet is an immense mineral sphere with a radius of almost 6,400 kilometres. In its depths, in the mantle and the core, mineral processes take place that directly affect the habitability and the ecosystems of the planet surface.
|The internal structure of Earth|
This deep crystalline world used to be the almost exclusive territory of science fiction. Its conditions of high pressure (millions of times the atmospheric pressure) and high temperature (up to 6500 ºC) made it practically inscrutable for experimental science. Today, diamond anvil cells enable us to reach these extraordinary pressures and at the same time follow the evolution of mineral transformations by X-ray diffraction. The information being obtained is completely changing our conception of the Earth’s interior and of its dynamics. The study of this fascinating world provides us with unpredictable discoveries and is another of the great scientific challenges of the future.
Putting a date on the moment when life appeared on planet Earth is crucial for understanding its origin and evolution as well as for understanding the actual geological evolution of our planet. Searching in the oldest rocks on Earth, palaeontologists have found microstructures from 3500 million years ago, which, due to their complex curve shapes, so different from typical crystalline forms, could be remains of the earliest life on Earth.
Biomorphs of silica and carbonate are self-organised crystalline materials created out of purely inorganic reactions that imitate forms of primitive life.
Surprisingly, it has been discovered that inorganic crystallization can create purely mineral structures that faithfully imitate primitive forms of life. Furthermore, they do so in chemical environments that most probably occurred on primitive Earth and that were very similar to those that created the ancient rocks that contain the supposed fossil remains.
Crystallization studies of self-organised mineral patterns are today a necessary tool for deciphering the authenticity of the possible remains of primitive life that are found in the oldest rocks on Earth, or in rocks of extraterrestrial origin. The two pictures above show stromatolitic structures created 2700 million years ago in Andalusians Hill, in the Tumbiana Formation, Australia.
Today there is no doubt that mineral reactions like serpentinization (the decomposition of the mineral olivine in the presence of water) are the source of carbon and simple organic molecules. Neither is there doubt that molecules like amino acids and lipids can be formed from non-biological reactions.
Partial decomposition of olivine by serpentinizatio.
However, the leap to structural and functional complexity that exists between these “simple” organic molecules created by mineral chemistry and the initial stages of earliest lift is enormous. A common idea is that the route that leads from one to the other must have been aided by efficient catalysers. It is thought that mineral crystalline structures were efficient catalytic substrates capable of empowering the self-assembly reactions of complicated organic structures.
One of the most explored groups of minerals is that of the clays. The surfaces of pyrite, the so-called iron-sulphur world, are the object of numerous studies.
Finally, the physical and chemical properties discovered in self-organised mineral patterns that are generated in extreme conditions make these structures a possible niche of interesting autocatalytic reactions.
The Earth is the only known planet in which matter has been able to self-organise into self-replicating structures with complex forms and behaviour, which we call life. How and where life originated is a mystery, probably the greatest mystery there is for us. The possible role of crystals and crystallization in the origin of life is also a mystery to be unravelled.