NOELBENSONITE, A NEW BAMN SILICATE OF THE LAWSONITE STRUCTURE TYPE, FROM WOODS MINE, NEW-SOUTH-WALES, AUSTRALIA

Noelbensonite, a new mineral, is the barium manganese analogue of laws onite. It is described from the Woods ornamental rhodonite mine, 30 km NNE of Tamworth, New South Wales, Australia, where it occurs as aggre gates of blocky to sometimes lamellar crystals ranging from a few micr ometres to (rarely) 100 mu m in length. It replaces NaMn amphibole, na mansilite, and pectolite, and also occurs as tiny monomineralic veinle ts 0.05-0.25 mm thick. Rare euhedral crystals are dominated by {100} a nd {011}, with {011} (01(1) over bar) = 68 degrees. The mineral is ort horhombic, space group apparently Cmcm; a = 6.325(1), b = 9.120(1), c = 13.618(1) Angstrom, V = 785.6(1) Angstrom(3), with a : b : c = 0.694 : 1 : 1.493. Noelbensonite is brittle, fracture irregular, Mohs hardn ess about 4, cleavage and twinning not observed, colour dark brown, st reak paler yellow-brown, lustre earthy on some veinlet surfaces to bri lliantly vitreous, calculated density 3.87 g/cm(3), refractive indices alpha = 1.82(1), beta (calculated from 2V) = 1.835(10), gamma = 1.85( 1), biaxial negative 2V(alpha) = 46 degrees(3 degrees), strong dispers ion r > nu, straight extinction to plane of flattening, {100}, alpha p arallel to c, beta parallel to b, gamma parallel to a with pleochroism in very thin sections: alpha = orange yellow, beta = orange, gamma = brownish orange, absorption gamma > beta > alpha. The average of 23 el ectron microprobe analyses (wt.%) is SiO2 26.02, Al2O3 0.17, TiO2 0.01 , Fe2O3 0.19, Mn2O3 34.76, CaO 0.31, Na2O 0.14, BaO 29.08, SrO 1.51, H 2Ocalc 7.87, total 100.06, leading to the simplified formula BaMn23+[S i2O7](OH)(2) . H2O. Up to 15% Sr and 9% Ca substitute for Ba in the la rge-cation sites. The strongest lines in the X-ray powder diffraction pattern are [(I-obs) d(obs)/Angstrom hkl] (100) 4.85 111; (50) 4.557 0 20; (59) 4.322 021; (77) 3.416 113,004; (80) 2.869 202; (47) 2.849 114 ; (82) 2.729 024; (45) 2.543 132; (48) 2.428 222; (38) 2.255 223,041. The name is for William Noel Benson (1885-1957), in honour of his clas sic researches in the New England Fold Belt and of his tenure of the C hair of Geology at the University of Otago.