Crystal-Melt Partitioning

Abstract

Definition 7 The transition elements are defined as having an electronic 8 configuration of partially filled d-or f-orbitals. As a group 9 they have a range of valence states and are responsible for the 10 variety of colors and magnetic properties in many natural 11 minerals and glasses. The multivalent nature of transition 12 elements makes their distribution in fluids, gases, and min-13 erals in natural geologic systems especially sensitive to tem-14 perature and redox state. 15 Abundance and Distribution 16 Only transition metals with unfilled d-shells are considered in 17 this chapter. There are 29 such metals, subdivided by their 18 position in the rows of the periodic table as first row (Sc – Zn), 19 second row (Y to Cd), and third row (Hf – Hg) transition 20 elements. The first row transition elements are the most abun-21 dant in the Sun, chondrites, and the bulk Earth compared to 22 the other two groups. Iron is the most abundant transition 23 element in the bulk Earth, maximized in concentration by 24 nucleosynthetic processes that formed all elements (Table 1). 25 The transition metals can be classified as siderophile 26 (affinity for metal), lithophile (combine readily with oxygen), 27 or chalcophile (pairing with sulfide). The classification of 28 transition metals as chalco-, sidero-, or lithophile, however, 29 varies substantially with oxidation state. For example, Fe is 30 siderophile at conditions of the core, lithophile in most of the 31 mantle and crust, and yet is a common constituent of many 32 sulfide minerals and melts. Some transition metals (Cd, Hg) 33 are even atmophile given their volatility at certain conditions. 34 Behavior 35 The valence states of transition metal ions control their sub-36 stitution into crystal structures and their solubility in solu-37 tions. Chemical fractionation during formation of the early 38 Earth segregated metallic Fe to the core. Partial melting to 39 form magmas has sequestered some of the transition elements 40 (Cr, Ni, Fe, Co) that are more compatible in silicates or oxides 41 of the mantle, or the crust (Sc, Ti). Exsolution of sulfides from 42 magmas or hydrothermal fluids has focused anomalously 43 high concentrations of other transition elements into ore 44 deposits of the crust (Cu, Zn, Ni). 45 Valence State and Crystal Chemistry 46 The valence states for transition metal ions can vary from 1 + 47 to 8 + , though 2 + and 3 + are the most common valence states in 48 minerals at terrestrial conditions. In common rock-forming 49 minerals of the crust and mantle, the first row transition 50 elements substitute mostly on octahedrally coordinated sites, 51 less so on cubic sites (Shannon, 1976). Substitution on tetra-52 hedral sites is rare. With the exception of Fe, no common 53 rock-forming minerals are dominated by the concentration of 54 any given transition element. Some accessory minerals are 55 dominated by transition elements: Ti in perovskite (CaTiO 3), 56 rutile (TiO 2), and ilmenite (FeTiO 3); Fe and Cr in spinel group 57 minerals; Mn in oxides; and Cu-Zn-Ni in sulfides.