Iron sand - Sasetu (English spelling)

Iron minerals distributed in sedimentary layers. The main sources of iron minerals are igneous rocks such as granite and andesite. These source rocks are weathered and decomposed, and mineral particles, fragments, and chemical components are transported by river water. Magnetite and ilmenite, which are contained as secondary components in rocks, have high chemical resistance and mechanical durability against weathering, and are hard and dense. In the process of transport by river water, magnetite and ilmenite are separated from other minerals and rock fragments with different densities and particle sizes, and are deposited and concentrated in the sediments as iron sand. Iron sand is divided into diluvial iron sand, alluvial iron sand, and beach iron sand. Diluvial iron sand is deposited on mountain slopes not far from the weathered area, and is generally small in scale. Iron sand deposited on riverbeds is called alluvial iron sand. Iron sand that has been swept away by waves on the coast is called beach iron sand, and is distributed widely. Iron sand found along the coast of Uchiura Bay (Funka Bay) in Hokkaido, the east coast of Aomori Prefecture, and the Ariake Sea in Kyushu originates from andesitic volcanic rocks and is accompanied by ilmenite. Iron sand found on the mountainside of the San'in region originates from granite in the Chugoku region and contains few impurities such as titanium and vanadium. Iron sand was the raw material for the ancient "tatara-fuki" ironmaking method. Iron sand that contains a lot of ilmenite is used as a raw material for titanium. Tin sand and gold sand are also formed by the same process as iron sand, but the important conditions for the formation of placer ores are that the mineral is stable against weathering and has a high specific gravity.

[Takeuchi Hisakine and Kaneda Hiroaki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

堆積(たいせき)層中に分布する鉄鉱物。鉄鉱物の供給源はおもに花崗(かこう)岩や安山岩などの火成岩類である。これら源岩が風化作用を受け分解され、鉱物粒子や破片、それに化学成分などが河川水の作用により運搬される。岩石中に副成分として含まれる磁鉄鉱やチタン鉄鉱は、風化に対して化学的抵抗性も機械的耐久性も強く、硬度および密度も高い。河川水による流水運搬の過程で、磁鉄鉱やチタン鉄鉱は密度も粒子の大きさも違うほかの鉱物や岩片から選別され、砂鉄として堆積物中に沈積、濃集する。砂鉄は洪積砂鉄、沖積砂鉄、海浜(かいひん)砂鉄に分けられる。洪積砂鉄は風化を受けた場所からあまり遠くない山の斜面などに堆積したもので、一般に規模は小さい。川床などに堆積したものは沖積砂鉄とよばれる。海岸で波により淘汰(とうた)された砂鉄は海浜砂鉄とよばれ、分布規模が大きい。北海道内浦湾（噴火湾）沿岸、青森県東海岸、九州有明(ありあけ)海などの砂鉄は、安山岩質火山岩を源岩とし、チタン鉄鉱を伴う。山陰地方の山腹に分布する砂鉄は、中国地方の花崗岩類を源岩とするもので、チタンやバナジウムなどの不純物は少ない。砂鉄は昔の「たたら吹き」製鉄の原料であった。チタン鉄鉱の多い砂鉄はチタン原料となる。砂錫(さすず)や砂金なども砂鉄と同様な作用でできたものであるが、砂鉱を形成する重要な条件として、風化に対し安定で、比重が大きい鉱物であることがあげられる。

［武内寿久祢・金田博彰］

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