%Chromium

There are several oxidation states of chromium (Cr); the commonest form in the natural environment is the trivalent Cr³⁺, which forms the most stable compounds, but hexavalent Cr⁶⁺ is of importance because of its relative mobility, bioavailability, oxidising potential and toxicity. The main chromium mineral is chromite, in which Fe can be replaced by Mg. Chromite has a high melting point and crystallises directly from magma; therefore concentrations of chromium in basic and ultrabasic rocks such as serpentinite can reach 2000–3000 mg kg^-1. Chromium is present as a minor constituent of other minerals, where it can replace Fe³⁺ and Al³⁺; in this context it gives rubies and emeralds their colours and, in fact, the element gets its name from the Greek 'chroma'. Anthropogenic sources include refractory brick production, coal combustion, electroplating facilities and electric arc furnaces in steel works. Most organisms, including bacteria, plants and animals, take up chromium mainly in the relatively rare but bioavailable Cr⁶⁺ form.