By Bob Farrar
The Mineral of the Month for the September of MGS is eosphorite. Named for the Greek word for “dawn-bearing,” from its color. Eosphorite is not a particularly common mineral, nor is it of any economic or gemological importance. However, it is an interesting mineral that deserves the attention of collectors. Eosphorite is a member of the phosphate group, consisting of phosphate of iron, manganese, and aluminum with the formula (Mn,Fe)Al(PO4)(OH)2·H2O. In eosphorite, the manganese predominates over iron. When iron predominates, the mineral is childrenite. It crystallizes in the orthorhombic system. Crystals are usually small, but on rare occasions can be several inches long. They are typically flattened and rectangular in shape, and are commonly found as subparallel bundles or spherical masses of radiating needles. Eosphorite is usually some shade of brown or orangish brown, but can be pink, yellow, black, or colorless. Other physical properties include a hardness of 5, a specific gravity of 3.1, and front pinacoid cleavage. The appearance and color of eosphorite usually distinguish it from other minerals found in its typical occurrence, pegmatites. Eosphorite is a secondary mineral of phosphate-rich granitic pegmatites. It typically forms in the later stages of the development of the formation through the alteration of primary phosphate minerals. As such, eosphorite crystals are often found on top of other crystals in cavities in the pegmatite. Eosphorite is perhaps best developed in the pegmatites of Brazil. It is found in numerous localities in Minas Gerais. Exceptional crystals up to 19 cm were found at the João Modesto Mine. The Poço D'Anta Mine near Itinga has produced numerous specimens. Crystals in the 2 – 3 cm range have been found at Parelhas, in the Brazilian state of Rio Grande do Norte. Besides Brazil, noteworthy worldwide localities include Kazakhstan and Afghanistan. In the United States, the most important localities of eosphorite are in New England. Its type locality is Branchville, CT. However, it is best developed in Maine, particularly in the pegmatites of Plumbago Mtn., near Newry. Other New England localities include Rumford, ME, and the Palermo Mine in North Groton, NH. A well-known locality outside of New England is the Foote Mine, at Kings Mtn., NC. Eosphorite may not be as familiar to collectors as some other minerals, but it is worth learning more about. While not especially abundant, it is common enough that modest specimens can be had for a reasonable price, and it makes an interesting addition to any collection.