Mineral specimen mortality

Speaking of mimetite, one of the most famous stories involves the wonderful Tsumeb mimetite which appeared on the cover of the Tsumeb Issue in 1977. The crystals are from the pocket of 1972 which yielded all of what are regarded as the world's finest specimens of the species. The cover specimen, owned by German gem dealer Gerhard Becker, had water-clear yellow crystals of the finest quality. One day, however, Gerhard's wife decided to help her busy husband by washing the dust off of some of his specimens. She took the mimetite to the kitchen sink and carefully immersed it in hot water, as she had probably done for many of his gemstone specimens. In an instant the water-clear crystals turned milky and nearly opaque with internal fractures! Gerhard was, of course, crushed. "But," he says wistfully, "It's still a pretty nice specimen."

Many minerals are heat-sensitive to some degree. Cerussite will crack if it gets too cold. Large, clear crystals of Spanish fluorite can be heard cracking under tap water. Kurnakovite, in large crystals, will fracture badly if immersed in water of slightly different temperature.

Heat sensitivity is normally the result of a combination of factors: (1) low thermal conductivity and (2) high coefficient of thermal expansion. It is for this same reason that Pyrex ovenware is resistant to sudden temperature changes; it is relatively pure silica glass with high strength and an extremely low thermal expansion coefficient. It doesn't expand or contract much with changes in temperature.

A somewhat different case involved a Brazilian aquamarine. Back in the early 1970's the Detroit Gem and Mineral Show was held at the Light Guard Armory, a large gymnasium with high walls and a row of windows in places near the ceiling. Exhibit cases were set up in a different floor pattern each year, just for fun, and many fine specimens were placed on public view there during the show's heyday. One year an exhibit was set up by David Wilber which included some fine miniatures, particularly an aquamarine crystal from Espirito Santo. Brazil. Crystals from this locality had, as a distinguishing feature, many little blebby inclusions suspended in the gemmy blue interior like bubbles in carbonated water. The inclusions were also transparent, as if containing only gas or water. They certainly contained something, as showgoers soon discovered.

As the afternoon came on Saturday, the first full day of the show, a bright October sun cast squares of light on the show floor from the high windows. As the sun followed its path through the sky, the squares of light slowly moved across the show floor like a giant sundial. Across the row of exhibit cases the sunbeams fell until they shone directly onto the Espirito Santo aquamarine. Stresses began to build up inside the crystal as the inclusions heated up. Finally it exploded with a bang, in a hail of fragments. Apparently the increased temperature raised the gas pressure or the vapor pressure of the included liquid until the crystal was not strong enough to contain it. Dave had to sell the pieces as gem rough.