Nickel, gallium and germanium
Nickel, gallium and germanium are the three elements whose concentrations, plotted against each other, classically define the chemical groups of iron meteorites.
From the 1950s onward, analyses showed that the trace elements gallium (Ga) and germanium (Ge) are spread unevenly among iron meteorites but cluster tightly within related sets. Plotting Ga or Ge (in parts per million, on logarithmic axes) against nickel (in weight per cent) separates irons into distinct fields. The original Roman-numeral groups I to IV were defined on Ga and Ge content; later subdivisions gave names such as IIAB, IIIAB, IVA and IVB.
The method works because Ga and Ge are moderately volatile, siderophile (metal-loving) elements, so their abundances reflect the bulk chemistry of each parent body's core, while trends of nickel and other elements within a group record fractional crystallisation of that core. Modern classifications add iridium, gold, arsenic, cobalt and other elements, and many workers use the IAB complex rather than a single group for non-magmatic irons.
Examples include Hoba (IVB, very high Ni and extremely low Ga and Ge), Gibeon (IVA), Cape York (IIIAB), Sikhote-Alin (IIAB) and Canyon Diablo (IAB). Values are measured by instrumental neutron activation analysis or laser-ablation inductively coupled plasma mass spectrometry on clean metal.
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