PARAFRANSOLETITE, A NEW DIMORPH OF FRANSOLETITE FROM THE TIP TOP PEGMATITE, CUSTER, SOUTH-DAKOTA

Parafransoletite, Ca3Be2(PO4)2(PO3OH)2.4H2O, is a triclinic dimorph of fransoletite from the Tip Top Pegmatite near Custer, South Dakota. Cell parameters are a = 7.327(1) angstrom, b = 7.696(1) angstrom, c = 7.061(1) angstrom, alpha = 94.903(5)-degrees, beta = 96.820(5)-degrees, gamma = 101.865(5)-degrees, and Z = 1; group P1BAR. The strongest X-ray powder diffraction lines are [d in angstrom (I)(hkl)] 3.032(100)(201,112BAR,112BAR,211BAR), 3.619(80)(111), 2.595(60)(112), 7.52(50)(010), 2.334(40)(220, 301BAR). Parafransoletite is a secondary mineral found on fracture surfaces in beryl along with roscherite, montgomeryite, robertsite, mitridatite, whitlockite, and englishite. It occurs as colorless to white spear-shaped blades up to 0.4 x 0.1 x 0.04 mm, as sheaf- and bow tie-like aggregates, and as compact radial sprays to 2 mm in diameter. Individual crystals are flattened on {010}, elongate parallel-to [100], and exhibit the forms {010}, {011}, {131BAR}, {131BAR}, and {144BAR}. The luster is vitreous for individual crystals, silky for aggregates. The Mohs hardness is about 2 1/2, and no cleavage was observed. The density is 2.54(1) g/cm3 (meas), 2.56 g/cm3 (calc). Optically, the mineral is biaxial positive with alpha = 1.562(2), beta = 1.564(2), gamma = 1.588(2); Euler angles are phi = 98-degrees, theta = 85-degrees, psi = -13-degrees; 2V (calc) = 33-degrees. Parafransoletite, like most beryllophosphate minerals, results from the reaction of beryl with P-bearing hydrothermal solutions. The formation of these minerals is probably highly dependent upon pH.