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Nickel belongs to the transition metal family and shows close chemical similarities with Fe and Co. Even more than these elements, it is concentrated in silicated Fe-Mg minerals by octahedral substitution with Fe2+ ions. Thus, the mean mineral/ matrix partition coefficient of Ni is 14.0,5.0 and 2.6 for basaltic whole rocks, for olivine, and orthopyroxene and clinopyroxene, respectively. The high Ni contents in olivine (3000–4500 ppm), spinel (3000–3500 ppm) and orthopyroxene minerals (650–1000 ppm) emphasize its high affinity for the first formed minerals during fractional crystallization of magmas. This is due to the electronic configuration of the Ni2+ ion: the coordinating octahedron of oxygen atoms is more stable with Ni2+ than with Fe2+ or Mg2+ atoms in the centre (Burns 1970). Ni cannot be defined as a rare element in the universe (in decreasing order of abundance, it takes the 14th place, before Na, K or Mn), but the average crustal content of 75 ppm emphasizes its lowlithophile characteristics. Ni is more abundant in materials of mantellic origin, especially in ultrabasic rocks with a world average content of 1450 ppm; the content being as high as 3000 ppm in dunites (Turekian 1978). |
