Twinning and intertwined microcrystals in an intriguing, yet elusive, mineral

After a hundred years with no solution, the seemingly simple but actually very complex mineral kaliophilite, KAlSiO4, is finally revealed by electron crystallography.

now in a position where many enigmatic mineral structures can be accurately determined by electron crystallography.
Twinning and superstructures, with not only two but numerous intergrowing crystals, are notoriously hard to solve. In a very informative electron micrograph [see Fig. 2 of Mugnaioli et al. (2012)], a typical part of the mineral is shown. Although the sample is less than 100 Â 200 nm, it is obvious that there are several small crystals intergrowing. Even without any knowledge in crystallography, it should be clear that it must be very difficult to solve the atomic structure of such a compound. And, for those who know the pros and cons of single crystal and powder X-ray crystallography, it should be evident why this mineral has resisted being solved, in spite of numerous attempts, some even before W. H. Bragg and W. L. Bragg invented X-ray crystallography in 1912. The crystal size is smaller than what is needed even for X-ray powder diffraction. Now, Gemmi and his group and several experienced mineralogists have at last solved the atomic structure of this mineral. It was possible by electron crystallography. For one thing, the very much stronger interaction of electrons than that of X-rays with matter, makes it possible to use extremely small samples for the crystallographic analysis. It is clear from the above-mentioned figure that although the mineral is heavily twinned, small areas of some tens of unit cells (i.e. a few tens of nanometres) may in fact be single crystals.
Electron crystallography has the unique advantage over X-ray and neutron crystallography that both real and reciprocal space can be directly observed. Electron microscopy data, unlike any type of diffraction data, contain the crystallographic structure factor phases.
In addition to the contribution to the development of electron crystallography, the mineral kaliophilite (KAlSiO 4 ), studied here, is of general interest for several reasons. It is one of a dozen minerals found with almost the same stoichiometric composition, but with varying degrees of superstructure. The present is by far the most complicated of them all, with a unit cell 27 times larger than the smallest in the series. These compounds are built up by chains and rings of AlO 4 and SiO 4 tetrahedra. The rings are mainly 6-rings, but they can vary in how the tetrahedra point upwards (U) or downwards (D). A most intriguing pattern in kaliophilite, making up just one unit cell, is shown in Fig. 1. The present mineral is the first with three different such schemes. Is this a portal into a world of infinite variations of these (and other!) minerals? It will certainly prompt X-ray crystallographers to keep an eye on what goes on in electron crystallography. In a wide number of fields, also outside mineralogy and pharmaceuticals, the many structures being quickly and accurately solved by electron crystallography should encourage the use of not only singlecrystal and powder X-ray and neutron diffraction, but also electron diffraction. scientific commentaries 952 Sven Hovmö ller An intriguing, yet elusive, mineral IUCrJ (2020). 7, 951-952

Figure 1
A unit cell of kaliophilite in space group P3, a = 27.06 and c = 8.56 Å ;. Si tetrahedra are shown in yellow, Al tetrahedra are shown in sky blue, Na cations are shown in blue and K cations are shown in red. Reproduced from Mugnaioli et al. (2020).