What is it?
So far we’ve had a crystal structure starting with every letter of the alphabet, except one. Xenotime is not some kind of bizarre extra-terrestrial timekeeping system, but is in fact a mineral. Ironically, it was supposed to be spelled with a “k”, after the ancient greek kenós (κενός) which means “vain”, but an early misprint replaced the “k” with an “x”, which stuck. The main component of xenotime is yttrium orthophosphate. It also contains relatively high levels of uranium and thorium impurities, which make it very useful for U-Th-Pb isotopic dating of sedimentary rocks.
What does it look like?
Where did the structure come from?
The structure was determined by M. Strada and G. Schwendimann in 1934, and can be found in the Open Crystallography database (#1011143).