It is shown that several different basic iron phosphates have been confused since earliest times under the name dufrenite. The history and nomenclature of the subject is reviewed in detail. The specific name dufrenite is here re-defined to apply to a particular mineral of the dufrenite-complex, and the new name rockbridgeite is proposed for the other common member of this complex.
Both dufrenite proper and rockbridgeite are shown to be basic phosphates of ferrous and ferric iron that ordinarily are found in a more or less oxidized condition, analogous to vivianite. Dufrenite has the probable formula Fe"Fe"'4(PO4)3.(OH)8 · 2H2O and while crystals are seemingly orthorhombic the substance is probably monoclinic. Hirschberg, Thuringia, may be taken as the type locality and seven other localities are described. Rockbridgeite has the probable formula Fe'Fe"'6(PO4)4(OH)8 and is monoclinic or triclinic. Midvale, Rockbridge County, Virginia, may be taken as the type locality and eight other localities are described. X-ray diffraction patterns, optical and physical properties and other characterizing data are given for both dufrenite and rockbridgeite. Dufrenite may be dimorphous with beraunite. Rockbridgeite may be isostructural with chenevixiie but not with chalcosiderite. The earliest stage of alteration of dufrenite and rockbridgeite is marked by conversion of Fe" to Fe"' with accompanying slight leaching of P2O5 but without any marked change in the x-ray diffraction pattern; this stage is followed by a more or less complete removal of P2O5 with the formation of hydrous iron oxide and the destruction of the crystal structure.
Laubmannite, a second new but rare species separated from the drufrenite-complex, has the formula Fe"3Fe"'6(PO4)4(OH)12 and is isostructural with andrewsite, the copper analogue. Laubmannite occurs at Shady, Polk Co., Arkansas, and probably at the Nitzelbuch mine, Amberg district, Bavaria. Characterizing x-ray, optical and other data are given for this species and also for two other probably new but unnamed dufrenite-like minerals. Beraunite is shown to be, like dufrenite and rockbridgeite, a more or less oxidized basic ferrous-ferric phosphate and not a straight ferric phosphate as hitherto considered. A new occurrence of beraunite at Middletown, New Jersey, is described. Five new chemical analyses are reported, including one each of beraunite, laubmannite, and dufrenite and two of rockbridgeite. The reported analyses of other minerals in the dufrenite-complex that cannot definitely be shown to belong to either dufrenite or rockbridgeite are discussed.
An iron phosphate from Wheal Phoenix, Cornwall, described by Kinch and Butler to which the name sjögrenite was later applied by Krenner in the belief that it differed from other dufrenite-like minerals, is shown to be identical with ordinary dufrenite. Krenner almost certainly mistook chalcosiderite for Kinch and Butler's mineral, and the name sjögrenite should not be adopted in his meaning as urged by Quensel but be retained for the carbonate-hydroxide to which it is presently applied.