The carbonates of iron and manganese may be precipitated when the solutions containing them are protected from oxidation by a reducing agent, such as decaying organic matter, or by being far removed from the air. Carbonate of manganese, however, is a much more stable compound than carbonate of iron, and the oxidizing conditions are often sufficiently strong to cause the deposition of iron as hydrous sesquioxide and not strong enough to change the manganese from its carbonate form. It is not uncommon, therefore, to have iron deposited in one place as hydrous sesquioxide, and manganese carried further on and deposited as carbonate, or even under special conditions deposited as carbonate with the hydrous sesquioxide of iron. Fresenius[21] has shown that the warm springs of Wiesbaden, which contain iron and manganese among their other mineral constituents, deposit iron in the form of hydrous sesquioxide, while manganese is carried on further in solution and deposited as carbonate. In this behavior, therefore, we have the first striking difference in the deposition of iron and manganese, and it will be further discussed later on. The sulphides of iron and manganese differ very much in their nature and mode of occurrence. Iron is frequently deposited as sulphide, but manganese rarely occurs in that form, and when it does it is always in very small quantities. Iron forms several sulphides in nature: pyrite (FeS₂), marcasite (FeS₂),[22] pyrrhotite (Fe₁₁S₁₂), troilite (FeS) and numerous other more complex compounds unnecessary to enumerate here. Pyrite is the commonest form of iron sulphide, and occurs in rocks of all ages, from Archean to Recent. It is formed in nature by the action of soluble sulphides or sulphuretted hydrogen on soluble salts of iron, and also by the reduction of sulphate of iron by organic matter or other reducing agents. Manganese forms two[23] sulphides, alabandite (MnS) and hauerite (MnS₂). Both minerals are very rare, and so unstable that they rapidly oxidize on exposure. Alabandite is the less rare form, and usually occurs as a subordinate constituent of certain metalliferous veins or allied deposits.