Pyrite - O-tekko (English spelling) pyrite

It is one of the most common sulfide minerals found in various metal deposits formed at relatively low temperatures. It is found in low-temperature hydrothermal vein deposits, fumarolic deposits including kuroko deposits, copper-bearing iron sulfide deposits, as well as in muddy sedimentary rocks, coal, lake sediments under reducing conditions, and sediments around smokers (chimney-shaped structures formed by the accumulation of sediments from hydrothermal fluids) on the deep sea floor. Since it is formed under conditions with sufficient sulfur supply and low temperatures, it does not coexist with minerals that are formed under sulfur-poor conditions (such as arsenopyrite, pyrite, and saphroite). It is decomposed under surface conditions or by the action of bacteria, and eventually becomes sulfate compounds and so-called limonite (mineralologically known as goethite). It decomposes quickly when it contains ferrous sulfate solution as a liquid inclusion. It is polymorphous with marcasite, but its stability range is unclear. However, it transforms into pyrite when heated.

The most common coexisting minerals are chalcopyrite, sphalerite, and galena. Its morphology is highly varied, with cubes, octahedrons, dodecahedrons, and icosahedrons, as well as solids based on these, and some are known to show trigonal symmetry. The nickel-bearing variety is called bravoite (chemical formula (Fe,Ni)S ₂ ). To the naked eye, it has a brass-colored metallic luster, and is one of the hardest sulfides. It is found in many places in Japan, but the most famous are the Kishu Mine (closed) in Mie Prefecture and the Yoshiwara Mine (closed) in Fukuoka Prefecture, which have crystals with a side length of over 15 cm, and the Akatani Mine (closed) in Niigata Prefecture, which has trigonally deformed crystals. In recent years, it has been considered a source of pollution, and research into its trace components has progressed. Its English name comes from the Greek word for "flash."

[Akira Kato January 19, 2016]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

比較的低温生成の各種金属鉱床中に産するもっとも普通の硫化鉱物の一つ。低温熱水鉱脈鉱床、黒鉱鉱床を含む噴気性堆積(たいせき)鉱床、含銅硫化鉄鉱床などのほか、泥質堆積岩、石炭、還元性条件下の湖水堆積物、深海底のスモーカー（熱水からの沈殿物が堆積した煙突状の構造物）周辺堆積物中などに産する。十分な硫黄(いおう)の供給条件および低温条件下での生成になるため、硫黄に乏しい条件下で産する鉱物（たとえば砒(ひ)鉄鉱・紅砒ニッケル鉱・サフロ鉱など）とは共存しない。地表条件で、あるいはバクテリアなどの作用で分解され、最終的には硫酸化合物といわゆる褐鉄鉱（鉱物学的には針鉄鉱）になる。液体包有物として硫酸第一鉄溶液などを含んでいると分解が早い。白鉄鉱とは同質異像関係にあるが、その安定領域は明らかではない。しかし加熱すると黄鉄鉱に転移する。

　もっとも普通の共存鉱物は、黄銅鉱・閃(せん)亜鉛鉱・方鉛鉱など。形態は立方体・正八面体・正十二面体・正二十面体およびこれらを基調とした立体で、非常に変化に富み、三方晶系の対称を示すものも知られている。含ニッケル変種は黄鉄ニッケル鉱bravoite（化学式(Fe,Ni)S₂）とよばれる。肉眼的には真鍮(しんちゅう)色の金属光沢で、硫化物としては硬度の高い部類に属する。日本における産地はきわめて多いが、なかでも三重県紀州鉱山（閉山）や福岡県吉原鉱山（閉山）の一稜(りょう)の長さ15センチメートルを超えるものや、新潟県赤谷(あかたに)鉱山（閉山）の三方変形結晶の産出例は有名である。近年、公害源物質として重視され、微量成分の研究が進んでいる。英名はギリシア語の「閃光」に由来する。

［加藤　昭　2016年1月19日］

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