When Magnetite is not Fe3O4

It has to be over ten years since I signed up to receive The Alchemist Newsletter from ChemWeb.com.   Over that period the newsletter has notified me of many interesting developments in many different fields.   Each issue of the newsletter provides a series of well written, short summaries of recent developments with links to the articles or press releases.  I’d encourage anyone with an interest in chemistry to sign up.   Not only is it interesting, but it’s free.

An issue of The Alchemist Newsletter from December was particularly enlightening as a summary with the  deceptive  title “Rust never sleeps” details developments in determining the atomic structure of the surface of magnetite .   Interestingly Magnetite, which I think of solely as the magnetic iron ore, has an increasingly prominent role in catalysis, hydrogen production, spintronics, and drug delivery systems.    Further, the  properties of magnetite are governed by missing iron atoms in the sub-surface layer.  The surface of magnetite is not Fe3O4 but instead  is Fe11O16.

The Alchemist Newsletter links to the following press release:
http://www.eurekalert.org/pub_releases/2014-12/vuot-tfd120314.php

That press release mentions:
“Perhaps the most surprising property of the magnetite surface is that single atoms placed on the surface, for instance gold or palladium, stay perfectly in place instead of balling up and forming a nanoparticle. This effect makes the surface an extremely efficient catalyst for chemical reactions...

Instead of a fixed structure of metal atoms with built-in oxygen atoms, one rather has to think of iron-oxides as a well-defined oxygen structure with little metal atoms hiding inside. Directly below the outermost atomic layer, the crystal structure is rearranged and some iron atoms are absent.

It is precisely above such places of missing iron atoms that other metal atoms attach. These iron-vacancy-sites are regularly spaced, and so there is always some well-defined distance between gold or palladium atoms attaching to the surface. This explains why magnetite surfaces prevent these atoms from forming clusters.”

If you are like me one of the things that you remember from your first geology course is that Magnetite has the formula Fe3O4 and that Hematite has the formula Fe2O3.   It’s a bit of a shock to think that the formulas change at the surface.

That article made me ask myself which mineral formulas I could still remember.   There were thirty-one in total (and only five were native elements).

Christopher Brett
Perth, Ontario

Journal Reference:

R. Bliem, E. McDermott, P. Ferstl, M. Setvin, O. Gamba, J. Pavelec, M. A. Schneider, M. Schmid, U. Diebold, P. Blaha, L. Hammer, G. S. Parkinson. Subsurface cation vacancy stabilization of the magnetite (001) surface. Science, 2014; 346 (6214): 1215 DOI: 10.1126/science.1260556