Earth Blue – a guest blog by Professor David Dobson
By Ruth Siddall, on 7 May 2019
Professor David Dobson was UCL Slade School Scientist in Residence for 2017-2018. Intrigued my the media coverage that the development of the new YIn Mn Blue pigment made in 2017, David was moved to make his own blue and think more about blue minerals in the Earth. David has recently been interviewed in Science Magazine by Kai Kupferschmidt.
David writes …
We live on the blue planet. Blue is so common in our everyday experience that we don’t even notice it. The sky is blue due to light scattering and water absorbs short wavelengths of the visible spectrum making it a pale blue. But blue minerals are rare; so much so that in medieval and renaissance time blue pigments were reserved for God and the saints. Most mineral colouration comes from small amounts of transition metal impurities in the mineral structure. This class of element can exist in several different electrical charge states and the hopping of electrons from one transition metal ion to another causes absorption of light in the visible spectrum and hence colour.
Iron, with allowed charges of 2+ or 3+, is the most common transition metal and so most minerals display the colours associated with electron hopping between 2+ and 3+ iron – red or brown when 3+ dominates and green when 2+ dominates. But deep in the Earth’s interior, at pressures of 180 to 230 thousand atmospheres the most common mineral, ringwoodite, is a rich royal blue. Once again, water is responsible, at least in part. In this case water is incorporated into ringwoodite as protons (H+ ions) and it substitutes for the main cations, Mg2+ or Si4+. In order for a stable substitution in a crystal lattice the charges must balance – you can’t replace one silicon (Si4+) ion for just one proton because the crystal would be left with an excess negative charge which would blow it apart. Instead the proton is accompanied by an iron ion to make a [Fe3+H+] substitution on the silicon site. This pushes the iron into a much smaller site than it usually occupies, surrounded by only 4 oxygen (O2-) ions rather than the usual 6 oxygens. This in turn changes the energy of charge transfer electron hopping transitions between iron 2+ and 3+ ions, making ringwoodite blue rather than brown. This [Fe3+H+] substitution is such a good fit in the silicon site that, if all the ringwoodite in the Earth had as much water as possible in its structure (and that is a BIG if), there could be as much as 4 time the entire volume of the oceans locked up as structurally bound water in the Earth’s mantle and Earth’s interior would be as blue as its exterior.
Here in UCL Earth Sciences we are attempting to develop synthetic structures which mimic the unusual ferric iron structure of ringwoodite but which are stable at atmospheric pressure. So far we have shown that we can make blue pigments from iron-bearing oxides and are now investigating how much Fe3+ the structures can take before they become unstable. That will determine just how blue we can make them. The prospects are bright…blue.
Ringwoodite synthesised at 20 GPa and containing 10% iron
David’s new blue, with about 0.3% iron
Three blues created by Fe2+-Fe3+ charge transfer: vivianite (in the centrifuge vial), (on the left) my Fe-bearing zinc germanate with Fe from 0 to 0.3% and (on the right) a Fe-dopes zinc silicate.