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A siderophile (or siderophile element) is an element that is soluble in molten iron. This affects where it is found: if sometime in its history, it was in contact with molten iron, it is likely to be mixed in.
For example, one siderophile, iridium is rarely found on Earth, though common in meteorites. It is presumed that the bulk of the iridium that the Earth had at the time of its formation immediately dissolved in Earth's iron and is now in the Earth's iron core. For this reason, the presence of iridium found at one particular upper layer around the Earth is taken as evidence of a major meteor impact sometime in the past, presuming the meteor brought the iridium.
This solubility can depend upon pressure and temperature.