The name frondelite is given to the mineral of composition Mn″Fe4′″(PO4)3(OH)5 isostructural and isomorphous with rockbridgeite, the ferrous iron analogue. The type locality of frondelite is the Sapucaia pegmatite, Municipio of Conselheiro Pena, Minas Gerais, Brazil, where it occurs in association with triphylite and other iron-manganese phosphates. It occurs in brown botryoidal masses with a radiating fibrous structure. It is orthorhombic with cleavages (100) excellent; (010) good; (001) fair. Hardness 4.5; sp. gr. 3.476; α = 1.86, β = 1.88, γ = 1.893; absorption in shades of orange Z>Y>X; 2V medium; dispersion r> v strong. Orthorhombic, tentative space group B221 or B2212 (D25);aO = 13.89, b0= 17.01, c0 = 5.21 Å.
The frondelite-rockbridgeite series is represented by frondelite, Mn″Fe4′″(PO4)3(OH)5 from the Sapucaia pegmatite, Brazil, manganoan rockbridgeite (Fe″Mn″)Fe4″′(P04)3-(OH)5, from the Fletcher quarry, North Groton, New Hampshire, and rockbridgeite Fe″Fe4″′(PO4)3(OH)5 from Rockbridge County, Va. Ferrous iron may oxidize to ferric iron. At Fletcher quarry green rockbridgeite (Fe″, Mn″)Fe4′″(PO4)3(OH)5 oxidizes readily to brown rockbridgeite, (Fe″′,Mn″)Fe4′″(PO4)3(OH)5.
The variation of unit cell and optical properties are discussed. Three new analyses are given.