John Carlos Baez on Nostr: But wait! It turns out there's a rare form of feldspar cualled celsian that's ...

But wait! It turns out there's a rare form of feldspar cualled celsian that's contains *barium* as a substitute for calcium. It's made of barium aluminosilicate. And it has screw axes and glide planes, so its space group is not a semidirect product! It’s an extension of ℤ³ by the point group P = ℤ/2 × ℤ/2 that gives a nonzero element of the cohomology group H²(P, ℤ³)!

So, barium comes to the rescue for mathematicians who want feldspars with nontrivial cohomology! 🎉

(6/n, n = 6)