Calcite - Specimen

Locality: Linwood Mine, Buffalo, Scott County, Iowa, U.S.A.

Description: This calcite grew inside a cave that was sealed away for hundreds of millions of years. The Linwood Mine sits on the Iowa bank of the Mississippi River, just east of Buffalo, cut into Middle Devonian limestone laid down on the floor of a warm, shallow sea. Long before the Pennsylvanian period, groundwater moving along fractures and bedding planes dissolved caves and passages in this limestone. Geologists call these buried caves paleokarst. Some of them were up to 270 metres long.

Most of the larger caves later filled with river sediment. Many of the smaller openings in the lower Cedar Rapids limestone stayed open, though, and mineral-rich water slowly lined them with crystals. Calcite is the main mineral there, and it grew in several generations, switching back and forth between sharp, pointed crystals and blunter, blocky ones. Marcasite, barite and pyrite formed in the same pockets. Sphalerite is uncommon, and chalcopyrite and quartz are rare. Scientists see the same pattern of several calcite generations in the famous Upper Mississippi Valley zinc-lead district.

People have quarried limestone at this site since at least 1897. Linwood began open-pit work in 1918. Around 1956, demand for high-purity limestone and the thick rock covering the best beds pushed the operation underground. Today Linwood is one of the largest underground limestone mines in the United States. It runs room-and-pillar workings on two levels, mines about 32 acres a year, and produces stone for lime, chemicals and aggregate. Because these crystal pockets were opened recently and deep underground, away from surface weathering, they are unusually well preserved.

The mine is still active, and it isn't open to the public for collecting. Crystals are saved only when miners open a pocket during production, so specimens reach the market now and then rather than steadily. Collectors also find it hard to get Linwood calcite without dings on the crystals. The minerals of the mine were described in a 1992 article in The Mineralogical Record by Paul Garvin and David Crawford. Some Linwood calcite glows a soft purple-magenta under midwave ultraviolet light, and some crystals carry tiny marcasite inclusions.

This piece shows the mine's blocky rhombohedral calcite, with transparent to translucent silvery-grey crystals clustered across limestone matrix.

Chemical Composition: CaCO3

Quantity: 1pc
With this product, you will get the actual stone in the photograph.