Herderite
By Bob Farrar
The Mineral of the Month for September is be herderite. Herderite is of little economic importance, but it is an interesting mineral and is popular among collectors.Herderite consists fluoro-hydroxyl phosphate of calcium and beryllium, CaBe(PO4)(OH,F). In most specimens, hydroxyl predominates over fluorine; these specimens are more properly referred to as hydroxyl-herderite. “Herderite” technically refers to specimens in which fluorine predominates, but most specimens of both are simply labeled as herderite. Herderite crystallizes in the monoclinic system. Well-formed crystals are common, but are often intergrown and complex in form. Monoclinic symmetry is usually visible, but crystals may also appear orthorhombic or wedge-shaped. Crystal faces are often rounded rather than flat as in most crystals. Fibrous, radiating aggregates are also known. Herderite may be colorless, white, light brown, yellow, lavender, blue, or green. Other physical properties include a hardness of 5.0 to 5.5, a specific gravity of 2.9 to 3.0, and a greasy to glassy luster. Herderite also exhibits thermoluminescence; i.e., it glows when heated, but not to the point of incandescence. Crystal form, thermoluminescence, and occurrence usually distinguish herderite.
Herderite typically occurs in granitic pegmatites. True herderite is rare, being known as crystals up to 5 cm from Austria, and pale green gemstones from Minas Gerais, Brazil. Hydroxyl-herderite is much more common. Well-formed crystals are known from many localities in Brazil. For example, crystals up to 35 cm were found at the Sapo Mine, and violet crystals up to 15 cm were found at the Xanda Mine. Hydroxyl-herderite is also found in the pegmatites of northern Pakistan, near Gilgit. Other worldwide localities include Russia, Germany, and France. In the United States, the best-known localities for hydroxyl-herderite are in Maine. Crystals up to 5 cm were found at the Fisher Quarry in Topsham, though most Maine crystals are smaller, in the millimeter range. The Bennett Quarry was a prolific producer in the 1970s. Other notable Maine localities include the Havey, Tamminen, Waisanen, and Emmons Quarries. Another New England locality is North Groton, New Hampshire. Other United States localities include several pegmatites in southern California, including the Himalaya Mine and Chihuahua Valley.
Herderite typically occurs in granitic pegmatites. True herderite is rare, being known as crystals up to 5 cm from Austria, and pale green gemstones from Minas Gerais, Brazil. Hydroxyl-herderite is much more common. Well-formed crystals are known from many localities in Brazil. For example, crystals up to 35 cm were found at the Sapo Mine, and violet crystals up to 15 cm were found at the Xanda Mine. Hydroxyl-herderite is also found in the pegmatites of northern Pakistan, near Gilgit. Other worldwide localities include Russia, Germany, and France. In the United States, the best-known localities for hydroxyl-herderite are in Maine. Crystals up to 5 cm were found at the Fisher Quarry in Topsham, though most Maine crystals are smaller, in the millimeter range. The Bennett Quarry was a prolific producer in the 1970s. Other notable Maine localities include the Havey, Tamminen, Waisanen, and Emmons Quarries. Another New England locality is North Groton, New Hampshire. Other United States localities include several pegmatites in southern California, including the Himalaya Mine and Chihuahua Valley.
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