A NEW ANHYDROUS AMPHIBOLE FROM THE HOSKINS MINE, GRENFELL, NEW-SOUTH-WALES, AUSTRALIA - DESCRIPTION AND CRYSTAL-STRUCTURE OF UNGARETTIITE, NANA2(MN-2(2)-3(3)+)SI8O22O2(MN)

Ungarettiite is a new amphibole species from the Hoskins mine, near Gr enfell, New South Wales, Australia. It occurs with Mn-bearing oxides, silicates, and carbonates in a stratiform schist associated with metaj asper, metabasalt, and metasiltstone. Ungarettiite is brittle, H = 6, D-meas = 3.52 g/cm(3), D-calc = 3.45 g/cm(3). In plane-polarized light , it is strongly pleochroic, X = orange red, Y similar to Z = very dar k red; X Lambda a = -2 degrees (in beta acute), Y = b, Z Lambda c = 17 degrees (in beta obtuse), with absorption X < Y less than or equal to Z. Ungarettiite is biaxial positive, alpha = 1.717(2), beta = 1.780(4 ), gamma = 1.800(2); 2V = 51(2)degrees, dispersion r < v. Ungarettiite is monoclinic, space group C2/m, a = 9.89(2), b = 18.04(3), c = 5.29( 1) Angstrom, beta = 104.6(2)degrees, V= 912(1) Angstrom(3), Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I, hkl)]: 2.176(10,171), 3.146(9,310), 2.544(9,($) over bar 202), 1.447( 9,($) over bar 3.11.1), 3.400(8,131), 1.656(8,461), 8.522(7,110), 2.29 9(7,($) over bar 171), 2.575(6,241), and 2.047(6,202). Analysis by a c ombination of electron microprobe and crystal-structure refinement giv es SiO2 50.66, Al2O3 0.04, TiO2 0.03, Fe2O3 0.50, FeO 0.00, Mn2O3 24.3 5, MnO 12.42, MgO 1.46, ZnO 0.10, CaO 0.18, Na2O 9.13, K2O 0.76, F not detected, sum 99.63 wt%. The formula unit calculated on the basis of 24 O atoms is (K0.15Na0.82)(Na1.97Ca0.03)(Mn1.662+Mg0.34- Mn2.963+Fe0. 063+Zn0.01)(Si7.99Al0.01)O22O2 and is close to the ideal end-member co mposition of NaNa2(Mn22+Mn33+)Si8O22O2. The crystal structure of ungar ettiite was refined to an R index of similar to 1.5% using MoK alpha X -ray intensity data. Site-scattering refinement shows that the M1, M2, and M3 sites are occupied dominantly by Mn. The [M-O] distances are M 1: 2.03; M2: 2.17; M3: 2.01 Angstrom, compatible with the site occupan cies M1 similar to M3 similar to Mn3+, M2 similar to Mn2+. All bonds t o the O3 anion are very short, and a bond valence analysis indicates t hat the O3 site is occupied by a divalent anion: O2-, as suggested by the overall electroneutrality requirement of the structural formula.