IntroductionSiderophile elements are the high-density transition metals that tend to sink into the nucleus because they dissolve easily in iron either as solid solutions or in the molten state. The highly siderophilic elements (HSE) are made up of Os, Ir, Ru, Rh, Pt, Pd, Re and Au. Most siderophile elements have practically no affinity for oxygen: in fact the gold oxides are thermodynamically unstable with respect to the elements. They form stronger bonds with carbon or sulfur, but even these are not strong enough to separate with the chalcophile elements. Therefore, siderophile elements are bound through metallic bonds with iron in the dense layer of the Earth's core where pressures can be high enough to keep the iron solid. Manganese, iron and molybdenum form strong bonds with oxygen but in the free state (as they existed on primitive earth when there was no free oxygen) they can mix so easily with iron that they do not concentrate in the siliceous crust as true lithophiles do. elements. However, manganese ores are found in more or less the same sites as those of aluminum and titanium due to the great reactivity of manganese towards oxygen. Because they are so concentrated in the dense core, siderophile elements are known for their rarity in the Earth's crust. For this reason most of them have always been known as precious metals. Iridium is the rarest transition metal found within the Earth's crust, with a mass abundance of less than one part per billion. Mineable precious metal deposits usually form as a result of weathering of ultramafic rocks, but are not highly concentrated even compared to their crustal abundances, which are typically several orders of magnitude lower than their solar abundances. However, since… halfway through the article… opic variations compared to tholeiites or komatiites, which probably contain a larger contribution from peridotite in a pyroxenite-peridotite hybridized mantle source. High-precision Os-188/Os -188 data for hot spot volcanism are limited, but combined variations in long-term Re/O and Pt/O preserved in some mantle sources may reflect long-term fractionation of Re and Pt from the Os between the inner and outer core, or ancient sulfur segregation and lithological variation. The study of HSE in volcanic rocks from hot-spot Solar System bodies also provides information on planetary-scale processes, indicating that the Earth, Moon, Mars, and fully differentiated asteroids all have higher-than-average abundances of HSE in their mantles. expected from low pressure metals. silicate Partitioning.z 2013 Elsevier BV All rights reserved.DELMON.A, 1972, AMERICAN JOURNAL OF SCIENCE, V272, P805