(K,Ca,Na)(Fe,Mg,Al)8(Si,Al)12(O,OH)36 · nH2O
The ratio of ferric vs. ferrous iron varies greatly. An ideal end-member "ferrostilpnomelane" can be represented by K5Fe482+(Si63Al9)O165(OH)48.12H2O. An ideal end-member "ferristilpnomelane" (which is thought to result from oxidation after primary crystallization) can be represented by K5Fe483+(Si63Al9)O216.36H2O. Note that neither end-member composition has been reported yet.