Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer, and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, Byzantine empire and the Nottingham alabasters of Medieval England. Mohs scale of mineral hardness, based on scratch Hardness comparison, defines hardness value 2 as gypsum. It forms as an evaporite mineral and as a hydration product of anhydrite.
Iowa Gems and Minerals in Your Pocket

From the spiky teeth of a geode containing sparkling quartz crystals, the rich browns and golds of smoky quartz and goethite needles on calcite, and the coral-like branches of plumose barite to the abstract reds and whites of polished agate cabochons, world-class mineral crystals are harvested from the rocks of the Hawkeye State. Collecting these high-quality crystals requires access to active mines, pits, and quarries, and individual collectors are rarely allowed entrance to these facilities. With information about each specimen s type, source, size, and current location, Paul Garvin and Anthony Plaut s "Iowa Gems and Minerals in Your Pocket" provides access to the glittering, gleaming world of Iowa crystals.

Most, if not all, of Iowa s gems and minerals are products of crystallization in underground cavities that filled with water containing dissolved chemicals. The famed Iowa geodes (Iowa s state rock) are products of a complex process of replacement and cavity-filling in the Warsaw Shale. Armored by a rind of tough chalcedonic quartz, these spheroidal masses, which range up to more than a meter across, weather out of the host rock and accumulate along streams in the southeastern part of the state. During the Pleistocene Epoch, large masses of glacial ice rafted the ultra-fine-grained variety of quartz called Lake Superior agates, which had previously weathered out of their host rocks, southward into Iowa. They can be found in the gravels that have accumulated along major streams in the eastern half of the state.

Iowa s long record of mining lead, coal, gypsum, and limestone contains a rich history; the forty-seven mineral specimens in"Iowa Gems and Minerals in Your Pocket"make up a fascinating illustrated guide to that history. Carefully lit and photographed to reveal both maximum detail and maximum beauty, each specimen becomes a work of art.

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