The new lattices ACC and MCC found by Fields, Conway, Sloane, and me... do they occur in chemistry? My answers: The radioactive element Protactinium has a tetragonal crystal structure within 1% of ACC; it has a/c=1.212 instead of ACC's 1.2247. For MCC, I have been unable to find any chemical element with this structure although perhaps under unusual conditions (high pressure, temperature?) it happens. Widening my search to all inorganic compounds I found A.Leclaire, M.M.Borel, A.Grandin, B.Raveau: A novel form of molybdenum (V) phosphate: delta-K Mo2 P3 O13, Zeitschrift fur Kristallographie 188 (1989) 77-83 http://rruff.info/doclib/zk/vol188/ZK188_77.pdf If I understand their claims aright, the potassium atoms in this stuff form a lattice apparently within 5% or less (in all geometric respects) of being an MCC lattice.