Natural and anthropogenic schwertmannites from Towada-Hachimantai National Park, Honshu, Japan

Schwertmannite, a recently described iron hydroxysulfate mineral (ideal formula Fe8O8(OH)6SO4), forms naturally as a fluffy brownish-yellow precipitate in Lake Matsuo-Goshikinuma, Iwate Prefecture, Japan, during part of the distinctive annual geochemical cycle of the lake. The dominant inflow to the lake is anoxic ground water which drains disseminated pyritic deposits and enters through a vent in the lake floor. The lake water typically has pH≈3 and contains ∼125 mg/l soluble-S and ∼30 mg/l soluble-Fe. From approximately September to June each year the lake circulates completely, dissolved oxygen levels are relatively high, iron-oxidizing bacteria (Thiobacillus ferrooxidans) are abundant, and the lake is turbid brown as schwertmannite is formed. Samples have been identified and characterized by X-ray powder diffraction, X-ray fluorescence analysis, and by extraction with oxalate and dithionite reagents. Freshly precipitated samples are predominantly schwertmannite; samples that have remained in contact with the lake water for months or more show partial transformation to goethite, though the transformation is incomplete in material ∼30 year in age. Schwertmannite was also found to be formed in the oxidation processes employed by two plants that treat drainage from disused pyrite mines in Japan.