Before the discovery of X-ray diffraction, the most powerful tool for analysing minerals was the polarized light microscope, which, despite providing valuable morphological data, was unable to deliver accurate information about the structural arrangement of atoms within crystals. The very first crystal structures to be determined by X-ray crystallography were those of minerals, and with the invention of X-ray powder diffraction in 1916–1917 (Milestone 4), structural mineralogy had its boom in just a couple of years.

When William Henry Bragg and R. E. Gibbs started to study quartz, many other simpler structures had already been disentangled, but quartz kept baffling scientists because of its complexity. It was only in 1925 that the structures of α- and β-quartz became known. This marked the beginning of extensive work on silicates by many researchers, with the main input coming from the Braggs' school.

As the number of crystal structures being determined kept growing, the need emerged to rationalize some theoretical principles to interpret the data. In 1926, Victor Goldschmidt distinguished between atomic and ionic radii, and postulated some rules for atom substitution in crystal structures. Inspired by his work, Linus Pauling realized that those principles were not always sufficient to describe the structure of complex ionic crystals and formulated a new set of rules of his own. These rules accounted for the importance of coordination polyhedra and were first put in practice in the study of zeolites.

This was a very fruitful period for structural mineralogy. In 1928, Felix Machatschki, who was working with Goldschmidt, showed that silicon could be replaced by aluminium in feldspar structures, an observation reinforced by the work of William Taylor some years later.

Finally, in 1930, with all the information gathered thus far, William Lawrence Bragg put together the first comprehensive classification of silicates, describing their structure in terms of grouping of SiO4 tetrahedra, isolated as in olivine; or in chains, rings or sheets as in diopside, beryl or mica, respectively; or in frameworks as in zeolites and feldspars.

Gypsum crystals inside the Cave of Crystals in Naica, Mexico. Credit: © JAVIER TRUEBA/MSF/SCIENCE PHOTO LIBRARY

Meanwhile, mineralogists had turned their attention to the study of crystal defects and imperfections. As these microstructures can be related to the natural processes involved in their formation, these studies provide useful hints for understanding the growth environment in which many natural minerals are found.

Nowadays, X-ray crystallography remains a valuable tool in Earth and planetary science. The structure and behaviour of minerals under extreme conditions, such as those found in the deep Earth, are routinely investigated using high-pressure crystallography. And with the X-ray spectrometer installed in NASA's rover Curiosity, the composition and past environmental conditions of the surface of Mars is being uncovered.