The invention involves the formation of a sulfided stable iron (II) compound from an iron (II) oxide and/or hydroxide and where the molar ratio of sulfur to iron (II) is greater than 1. Preferably these oxides and/or hydroxides are present as nanoparticles in the 5-10 nanometer range. It has been discovered that such particles can be formed at lower cost and with fewer impurities by using ferrous carbonate (FeCO3) from siderite as compared to known processes from various iron salts such as sulfates and chlorides.