Glauconite << GLAW kuh nyt >> is a bright green mineral that looks like tiny flakes of the mineral mica, or small lumps of clay. Nearly pure glauconite, called greensand, is used as a water softener. Glauconite deposits occur in Colorado, New Jersey, and Wisconsin. Chemically, glauconite is a hydrous (water-containing) silicate of potassium and iron. It may have been formed originally from biotite (a dark-colored mica), but other minerals, and even organic matter, may change into glauconite. Glauconite is slowly forming from seawater.
Glauconite is valuable to geologists for dating rocks and fossils because it contains potassium. Natural potassium always contains a number of radioactive atoms. Radioactive potassium breaks down at a known and constant rate to a gas called argon. By measuring the ratio of potassium to argon in a glauconite sample, geologists can determine the sample’s age.
