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請問白鐵也是合金嗎？還是另一種金屬？跟鐵有什麼不同？

Marcasite - Wikipedia

The mineral marcasite, sometimes called white iron pyrite(即白鐵礦), is iron sulfide (硫化鐵) (FeS2). Marcasite is often mistakenly confused with pyrite(黃鐵礦), but marcasite is lighter and more brittle(更輕且更易碎). Specimens of marcasite often crumble and break up due to the unstable crystal structure, and it is this crystal structure that is the main difference between marcasite and pyrite(白鐵礦樣本相當易碎，原因是他的晶體結構相當不穩定，這也是和黃鐵礦最大的不同處。). Though marcasite has the same chemical formula as pyrite, it crystallizes in a different crystal system, thereby making it a separate mineral. In jewelry, pyrite used as a gem is improperly termed "marcasite". This is wholly incorrect, as marcasite is never used as a gem.(黃鐵礦被當作寶石使用，但白鐵礦則否，因此"marcasite"這個名字是個誤用，白鐵礦從未作為寶石使用。)


Two halves of a ball of radiating marcasite from France.Marcasite can be formed as both a primary or a secondary mineral.

As a primary mineral it forms nodules, concretions and crystals in a variety of sedimentary rock, such as at Dover, Kent, England, where it forms as sharp individual crystals and crystal groups, and nodules (similar to those shown here) in chalk. It can also be found in low-temperature hydrothermal veins.

As a secondary mineral it forms by chemical alteration of a primary mineral such as pyrrhotite or chalcopyrite. On fresh surfaces it is pale yellow to almost white and has a bright metallic luster. It tarnishes to a yellowish or brownish color and gives a black streak. It is a brittle material that cannot be scratched with a knife. The thin, flat, tabular crystals, when joined in groups, are called "cockscombs."

Marcasite may go through a condition known as "pyrite decay", in which a specimen slowly disintegrates into a white powder. Little is known about this detrimental condition. It only affects certain marcasite specimens seemingly at random, while other specimens remain unaffected. When a specimen goes through pyrite decay, the marcasite reacts with moisture in the air, the sulfur combining with water to produce sulfuric acid that attacks other sulfide minerals and mineral labels. It is most important to remove an afflicted specimen from other minerals to prevent this "disease" from spreading.

Some research has suggested bacteria may aid and accelerate this process by literally 'eating' the marcasite. What is known is that samples with a rough surface tend to decay faster than those with bright, shiny faces, probably due to the greater surface area to react with water in the air, and also it's clear that samples kept in a dry environment (low humidity) are less likely to decay.

Other minerals often found associated with marcasite are pyrite, galena, sphalerite, fluorite and calcite.(經常伴隨白鐵礦生成的礦物有黃鐵礦、方鉛礦、閃鋅礦、瑩石，以及方解石。)