Tvedalite occurs as cream white to pale beige spherulites up to 3 mm in diameter in vugs in a nepheline syenite pegmatite of the Vevja quarry in Tvedalen, Brunlanes, Vestfold County, Norway. The tvedalite spherulites are always overgrown by a crust of chiavennite. Microprobe and wet chemical analyses as well as thermogravimetric data give the chemical formula (Ca,Mn)4Be3Si6O17(OH)4·3H2O. The relative proportions of large divalent cations vary irregularly from rim to core. The two most extreme compositions are (in percent of the non-Be divalent atoms) Ca50Mn46Fe4 and Ca80Mn18Fe2. Crystallographic data were obtained by a combination of X-ray powder and electron diffraction methods. The mineral is orthorhombic, a = 8.724(6), b = 23.14(1), c = 4.923(4) Å, V = 993.8(9) Å3, Z = 2. The strongest reflections [d in Å, (I/I0), (hkl)] of the powder diffraction pattern are 11.6(93)(020), 5.80(68)(040, 130), 3.87(75)(060), 3.16(74)(250, 151), 2.889(75)(080,260,310), 2.837(100)(241),2.494(58)(081,261). Cleavage is perfect {010}. Dmeas = 2.541(6) g/cm3, Dcalc = 2.554 g/cm3. Mohs hardness is 4½. The average refractive index is n = 1.604.

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