NATURAL AND ANTHROPOGENIC SCHWERTMANNITES FROM TOWADA-HACHIMANTAI NATIONAL-PARK, HONSHU, JAPAN

Schwertmannite, a recently described iron hydroxysulfate mineral (idea l formula Fe8O8(OH)(6)SO4), forms naturally as a fluffy brownish-yello w precipitate in Lake Matsuo-Goshikinuma, Iwate Prefecture, Japan, dur ing part of the distinctive annual geochemical cycle of the lake. The dominant inflow to the lake is anoxic ground water which drains dissem inated pyritic deposits and enters through a vent in the lake floor. T he lake water typically has pH approximate to 3 and contains similar t o 125 mg/l soluble-S and similar to 30 mg/l soluble-Fe. From approxima tely September to June each year the lake circulates completely, disso lved oxygen levels are relatively high, iron-oxidizing bacteria (Thiob ncillus ferrooxidans) are abundant, and the lake is turbid brown as sc hwertmannite is formed. Samples have been identified and characterized by X-ray powder diffraction, X-ray fluorescence analysis, and by extr action with oxalate and dithionite reagents. Freshly precipitated samp les are predominantly schwertmannite; samples that have remained in co ntact with the lake water for months or more show partial transformati on to goethite, though the transformation is incomplete in material si milar to 30 year in age. Schwertmannite was also found to be formed in the oxidation processes employed by two plants that treat drainage fr om disused pyrite mines in Japan. (C) 1998 Elsevier Science B.V.