Archaeal fructose-1,6-bisphosphatase and related enzymes of inositol monophosphatase family (FBPase class IV). These are Mg++ dependent phosphatases. Members in this family may have both fructose-1,6-bisphosphatase and inositol-monophosphatase activity. In hyperthermophilic archaea, inositol monophosphatase is thought to play a role in the biosynthesis of di-myo-inositol-1,1'-phosphate, an osmolyte unique to hyperthermophiles.