Ferro-papikeite, ideally NaFe22+(Fe32+Al2)(Si5Al3)O-22(OH)(2), a new orthorhombic amphibole from Nordmark (Western Bergslagen), Sweden: Description and crystal structure

Ferro-papikeite, ideally NaFe22+(Fe32+Al2)(Si5Al3)O-22(OH)(2), is a new mineral of the amphibole supergroup from the Filipstad Municipality, Varmland County, Central Sweden, where it occurs in a medium-grade felsic metavolcanic rock. Ferro-papikeite is pale brown with a translucent luster, has a colorless to very pale-brown streak, and shows no fluorescence under long-wave or short-wave ultraviolet light. Grains are subhedral, 0.4-3.0 mm in size, and show well-developed {210} cleavage. It has a Mohs hardness of-6 and is brittle with a splintery fracture, has the characteristic perfect {210} cleavage of orthorhombic amphiboles, intersecting at similar to 56 degrees, and the calculated density is 3.488 g/cm(3). In transmitted plane-polarized light, ferro-papikeite is moderately pleochroic X = very pale brown, Y = Z = honey brown; X < Y = Z. Ferro-papikeite is biaxial (+), alpha = 1.674(2), beta = 1.692(2), gamma = 1.716(2), 2V(meas) = 86.2(9) and 2V(calc) = 88.3 degrees, dispersion is r < v, weak. The orientation is: X parallel to a, Y parallel to b, Z parallel to c. Ferro-papikeite is orthorhombic, space group Pnma, a = 18.628(4), b = 17.888(4), c = 5.3035(11) angstrom, V = 1767.2(6) angstrom(3) , Z = 4. The strongest ten X-ray diffraction lines in the powder pattern are [d in angstrom(I) (hkl)]: 8.255(100)(210), 3.223(39)(440), 3.057(68)(610), 2.824(28)(251), 2.674(41)(351), 2.572(56) (161,621), 2.549(38)(202), 2.501(50)(261,451), 2.158(25)(502), and 1.991(31)(661). Chemical analysis by electron microprobe gave SiO2 36.50, Al2O3 22.24, TiO2 0.09, FeO 31.54, MnO 0.65, MgO 5.48, CaO 0.08, Na2O 2.35, F 0.22, H2Ocalc 1.85, O=F -0.09, sum 100.91 wt%. The formula unit, calculated on the basis of 24 (O+OH+F) with (OH) = 2 apfu and Fe3+ = 0.13 apfu (determined from the distance) is (A)(Na0.70Ca0.01)(B+C)(Mg1.25Fe3.902+Mn0.082+Al1.62Fe0.133+Ti0.014+)(Sigma 6.99) (T)(Si5.60Al2.40)(Sigma 8)O-2 (OH1.89F0.11)(2). The crystal structure of ferro-papikeite was refined to an R-index of 3.60% using 2335 unique observed reflections collected with MoKa X-radiation. Al-[4](3+) is ordered over the four T sites as follows: T1B > T1A > T2B >> T2a, Al-[6](3+) is completely ordered at M2, and Fe2+ is strongly ordered at M4. The A site is split with Na+ strongly ordered at Al. End-member ferro-papikeite is related to end-member gedrite, square Mg-2(Mg3Al2)(Si6Al2)O-22 (OH)(2), by the substitutions Na+ ->square, Fe2+ -> Mg, and Al3+ -> Si4+. The description of ferro-papikeite as a new species further emphasizes the compositional similarities between the monoclinic calcium amphiboles and the orthorhombic magnesium-iron-manganese amphiboles.