Editing Planetary core (section)

===Differentiation===
[[Planetary differentiation]] is broadly defined as the development from one thing to many things; homogeneous body to several heterogeneous components.<ref name="Merriam Webster 2014">{{cite dictionary |dictionary=Merriam Webster |title=differentiation |date=2014 |url=http://www.merriam-webster.com/dictionary/differentiation }}</ref> The [[hafnium-182]]/[[tungsten-182]] isotopic system has a [[half-life]] of 9 million years, and is approximated as an extinct system after 45 million years. [[Hafnium]] is a [[lithophile element]] and [[tungsten]] is [[siderophile element]]. Thus if metal segregation (between the Earth's core and mantle) occurred in under 45 million years, [[silicate]] reservoirs develop positive Hf/W anomalies, and metal reservoirs acquire negative anomalies relative to undifferentiated [[chondrite]] material.<ref name="Wood, Walter and Jonathan 2006" /> The observed Hf/W ratios in iron meteorites constrain metal segregation to under 5 million years, the Earth's mantle Hf/W ratio places Earth's core as having segregated within 25 million years.<ref name="Wood, Walter and Jonathan 2006" /> Several factors control segregation of a metal core including the crystallization of [[perovskite]]. Crystallization of perovskite in an early [[magma ocean]] is an [[oxidation]] process and may drive the production and extraction of iron metal from an original silicate melt.