Yominum sulfide

Yominum sulfide is apparently a chemical, with a molecular formula of K4Ym3(SO73Es2), that presumably forms crystalline structures.

When Spock was performing a memory test, one of the questions asked him what the molecular formula for yominum sulfide was. 

As the formula is known to be inconsistent with those of known sulfides, the substance may not even exist&mdash;and the very memory-test question itself might have been a trick question in that case.

硫化ヨミニウム