Monday, November 15, 2010

3a. The Evils that beset mineral collections: Temperature changes

In the last installment, I discussed temperature effects, but not temperature change effects.

The largest effect happens in situations where a collection is kept in places that can get below the freezing temperature of water. There is an effect called freeze-thaw disaggregation, which is seen in nature at rock outcrops: water seeps into the rocks and expands as it freezes, prying the rock apart. Minerals with fluid inclusions, or ones that are inadvertently wetted down (does your basement flood?) might suffer from such effects.

The other extreme is rare as an environment, but can occur to specimens with fluid inclusions when viewed through a microscope with hot lights, or while being photographed with hot lights. Some fluid inclusions contain gas bubbles (e.g., carbon dioxide bubbles), and these expand as a specimen is heated. In one anecdotal case, a gemstone exploded when held in someone's mouth.

Temperature cycling can also cause a mineral specimen to disaggregate. Each mineral has its own thermal expansion coefficient, which measures how much it expands as it gets hotter and contracts as it gets cooler. Anisotropic minerals (corundum is a good example) have different thermal expansion coefficients in different directions, and can even contract in some directions while being heated.
A matrix mineral specimen is an aggregation of different grains, often with varying orientation. The grains will typically push against each other as temperature climbs, and pull apart as temperature falls. This might lead to delicate specimens falling apart.

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