The true formula of abhurite is Sn21Cl16(OH)14O6. A stability constant for the phase has been determined at 298.2 K. For the reaction Sn21Cl16(OH)14O6 (s) + 26H+ (aq) ⇋ 21Sn2+ (aq) + 16Cl− (aq) + 20H2O (1), log KH+ (298.2 K) is equal to -39.9 (7). This value is used to assess the relative stability of abhurite in the natural environment and to evaluate its modes of occurrence in relation to other secondary Sn(II) and Sn(IV) species.