琵琶湖湖底段丘における砂質団塊充填物質としての 藍鉄鉱・菱鉄鉱－菱マンガン鉱

抄録

Clastic sandy nodules are found on a submerged terrace south of Oki-shima Islet in Lake Biwa, southwest Japan. Vivianite and siderite-rhodochrosite crystals occupy interstitial spaces between the clastic sand grains in the nodules. There are three types of nodules recognised: (1) vivianite only, (2) siderite-rhodochrosite only and (3) coexisting vivianite and siderite-rhodochrosite. The vivianite contains little manganese, and its compositional variation is very small. Some parts of the siderite are enriched with manganese, showing compositions of rhodochrosite. The vivianite and siderite-rhodochrosite in the sediments might be formed during incipient diagenesis in a reduction environment below the oxidative environment of the lake bottom. These interstitial minerals play a role of depositing iron, manganese, phosphorus, carbon and related elements in the sediments.