Monday, June 27, 2011

The evils that beset mineral collections: 13. Sulfur, Pyrite, and Other Minerals

Minerals that damage other minerals?  Read on, oh Viewer...

A standard way to display minerals is in Dana Order: by anion group starting with native elements, sulfides and sulfosalts, and simple oxides; and ending with framework silicates. A major problem with this display procedure is that it puts elemental sulfur and elemental silver in proximity.  (Note that alphabetical display order also has this problem, though.)

What's so bad about that? Sulfur has a high vapor pressure, meaning that sulfur releases gas at room temperature and pressure. (It has a pronounced smell.)  In a small enclosed place, concentrated sulfur vapors react with metallic elements and sulfides, such as native silver and native copper, to create surface tarnish layers. This makes the tarnished minerals less attractive and can lead to their decomposition.

Pyrite and marcasite have a different problem.  These minerals decompose in humid conditions, producing sulfuric acid. A microbe is thought to be involved. Some museums store these minerals isolated in evacuated plastic bags to prevent them reacting with their surroundings: dissolving labels, boxes, and so on. Another reason for isolation however is to prevent specimens from contaminating each other, if a microbe is involved.

Finally, some radioactive minerals are alpha emitters, and as such can create radiation damage in other minerals if left in very close proximity (touching) for long periods of time. This process often happens in nature (producing for instance smoky quartz), and could possibly happen in collections left alone for decades; however, I know of no examples of this.

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