Metastable cubic FeS was synthesized by the sulfidation of powdered metallic Fe by aqueous H2S. Crystals of cubic FeS and troilite formed below 92°C between pH 2 and 6, with cubic FeS precipitating early in the 4–85-h-long experiments and then disappearing by conversion to mackinawite during aging. Above 92°C, troilite and/or mackinawite were the only products. Above pH 6, mackinawite was the only product.
Cubic FeS and troilite formed by reactions between aqueous H2S and Fe2+, but mackinawite formed by reactions involving solid phases such as Fe metal or previously formed cubic FeS. The short persistence of metastable cubic FeS makes it unlikely that this phase will be found in a natural occurrence.