Monohydrocalcite from Shiowakka occurs as a constituent of calcite-dominant sinter precipitated in cold (14.5-17.8°C) saline (Mg²⁺ ?? 4, 000mg/l, Cl^- ?? 4, 000mg/1) spring water with pH=6.67 and Mg²⁺/Ca²⁺=0.64. The idiomorphs under SEM are trigonal bipyramid with (Ca_0.979 Fe²⁺0.013Mg_0.005(Na₂)_0.003) as the cationic counterpart, where the relation Fe2+>Mg in mole ratio is new to this mineral in contrast to Mg>Fe²⁺ in all the known monohydrocalcite analyses. The X-ray powder diffraction pattern is indexed on a hexagonal cell with ?? =10.566Å and ?? =7.565Å, where a subcell with ?? = ?? /√3 is prominent. The closer occurrence with algae indicates the precipitation favoured by its existence.