[adrian_b]

Gold is difficult to oxidize, but once oxidized it has some of the biggest ions, which stay easily in solution if no reducing agent is present.

The ion Au(I) has about the same size as the ions of potassium (which are exceeded in size only by cesium, rubidium, thallium and radium).

The ion Au(III) has a more normal size, but it is still relatively big, similar to the trivalent ions of the rare earths.

The big size of the gold ions is one of the reasons why its combinations with small ions, like oxide and sulfide, are unstable, so you cannot find such minerals in nature.

On the other hand, the gold ions form stable compounds with bigger ions, like telluride. Therefore there are many minerals where gold is combined with tellurium (unlike silver and copper, which combine with the smaller sulfur).

Nevertheless, on Earth tellurium has an abundance almost as low as gold, even if tellurium is abundant in the Solar System. The reason is that tellurium is easily vaporized, so less of it has condensed when the Earth has formed and a good part from what has condensed initially has been lost later, when the Earth has been heated by many asteroid impacts during its early history.

While tellurium is rare because it went up, being lost as vapors, gold is rare because it went down and most of it is dissolved in the iron core of the Earth. Because both tellurium and gold are very rare at the surface of the Earth, the chances of them meeting together in amounts great enough to form a mineral are very low.

The result of this scarcity of tellurium on Earth is that most of the gold can be found as native gold and only a smaller fraction is found in compounds with tellurium. Had tellurium not been lost from Earth, the amount of native gold would have been very small, similarly with the much smaller amounts of native silver and copper that exist versus the amounts available in sulfide minerals.

[djtango]

Thanks for sharing this - excellent content. I've been out of the game for a long time now but isn't this just the case that Gold is too soft as an ion to mix well with stuff like oxides?

Cs(I) should be larger than Au(I) but it seems to form a comparatively stable oxide Cs2O. But yes Tellurium is also a nice soft element so AuTl have a good affinity for one another.

Was unaware of their chemistry, it doesn't even ring a bell tbh I wonder if I had ever encountered it before. I did enjoy studying the Post Transition Group Metals back in the day

[adrian_b]

Yes, as I have said, size is only one of the reasons of incompatibility with oxide ions.

As you say, gold has a much higher electronegativity than cesium and rubidium, i.e. not much lower than that of silicon, which makes it a "soft" ion, and that reduces the stability of any compound with oxide or hydroxide or fluoride ions. On the other hand, the incompatibility with the "softer" sulfide is mostly caused by the size ratio.