David K. Joyce Minerals

Arsenic – Darkening Upon Exposure

By David K. Joyce

A while back I was trimming some pieces of native arsenic from the occurrence at Port Alberni, British Columbia. I was surprised when I first started trimming them to find that the arsenic had a metallic luster inside the black, markedly non-metallic exterior. I’d always read and heard that arsenic had a metallic luster but had never really seen any specimens that HAD a nice metallic luster.

I noted a couple of months later that the metallic luster had largely disappeared and that the arsenic had been differentially darkening over that period to give it a banded look. Over a bit more than a year, the arsenic totally darkened and is now almost totally black with only a slight trace of brownish banding.

I decided to try to document this darkening and a series of digital images to give a “time-lapse” study effect in order to see the darkening of a fresh fragment of arsenic from the same location. Here are the images:

Arsenic Darkening Day 1 Arsenic Darkening Day 2 Arsenic Darkening Day 9 Arsenic Darkening Day 15 Arsenic Darkening Day 25 Arsenic Darkening Day 50 Arsenic Darkening Day 65 Arsenic Darkening Day 95 Arsenic Darkening Day 385

You’ll notice that there is a band of silvery mineral, at the top that, in the beginning, looked sort of like the arsenic but did not darken over time. That mineral is lollingite.

The methodology was sloppy. I have no idea what light intensity or camera exposure details were used each time. Hopefully, you get the idea, though?

I wondered why the arsenic would darken differentially? Slight impurities contained in the arsenic?

I am grateful to Franz Bernhard, of Austria, for the work he did to help understand this phenomenon. Details of the work are revealed in a thread in mindat.org at the following link: http://www.mindat.org/forum.php?read,6,255517,272978 . This helped greatly to explain the differences in darkening. Thank you Franz!

In summary, the differences in darkening are caused by small amounts of antimony in different layers of the arsenic. See the above link for details.