Tuesday, December 14, 2010

4. The Evils that beset mineral collections: Humidity

Some minerals are found in wet environments, and dry out.
Some minerals are found in dry environments, and pick up water.
Some minerals are water-soluble.
Some water-soluble minerals can dissolve and re-precipitate as the humidity in the air around them changes.
Some minerals react with the water in the air to form acid.
All these pose problems.
Let's look at some examples:
  • Hydrous borate minerals, such as kurnakovite, and hydrous carbonates like hanksite, grow in saturated brines. When removed from their brines, they develop white surface alteration. Some mineral specimens are coated with clear lacquer to prevent their surfaces from reacting; others are wrapped in plastic, or kept in closed plastic bags or glass jars. Lacquer coating should be noted on the records accompanying a mineral specimen, and disclosed if and when it changes owners.
  • Opals may dry out, leading to cracking or "crazing." Opals are mineraloids, not minerals, and have variable amounts of water, as well as varying degrees of structural organization. Many museums display opals with an open container of water in their cases, to prevent further drying out.
  • Some attractive uranium minerals, like autunite and zeunerite, are not stable in typical human-friendly environments, and naturally dehydrate (here, to meta-autunite and meta-zeunerite respectively). Most mineral collectors just accept this change as inevitable.
  • Many marcasites and some pyrites decompose in wet environments, producing sulfuric acid. This may be due to bacterial action. Some museums store these minerals in individual evacuated and sealed plastic bags to fight decomposition.

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