Contributions to the chemistry of tapiolite - manganotapiolite, a new mineral

LAHTI, SEPPO I.; JOHANSON, BO and VIRKKUNEN, MARJATTA 1983: Contributions to the chemistry of tapiolite manganotapiolite, a new mineral. Bull. Geol. Soc. Finland 55, 2, 101-109. Microprobe determinations and X-ray diffraction studies of tapiolite specimens from the granitic pegmatites of the Eräjärvi area, in Orivesi, southern Finland, revealed a new manganesedominant analogue of tapiolite. A small crystal, 1.5 m m long and 0.5 m m thick, from a narrow lithium pegmatite dyke shows some parts richer in Mn than Fe. The name manganotapiolite is used for this Mn-dominant member and ferrotapiolite for the common Fedominant member of the tapiolite group. The tapiolite crystal studied is prismatic, showing poorly developed {100} faces. Manganotapiolite is dark brown, strongly pleochroic from yellowish brown to reddish brown, but thicker fragments are opaque. In reflected light the colour is grey with a weak bireflectance changing from light brownish grey to grey. Internal reflections are strong with red brown colour. The reflectance values measured in air are: 16.0-15.7 (470 nm), 14.7-14.1 (546 nm), 15.0-14.8 (589 nm), 15.1-14.6 (650 nm), and in oil: 4.5-4.0 (470 nm), 3.9-3.4 (546 nm), 4.3-3.8 (589 nm), 4.3-3.8 (650 nm). Density (calc.) is 7.72 g/cm and VHN (100 g load) 711. Micro probe analyses on five points of one manganese-rich part gave: FeO 6.4, 6.2, 3.7, 3.9 and 3.2; MnO 6.7, 7.7, 8.6, 9.7 and 10.2; CaO 0.4, 0.2, 0.6, 0.2 and 0.4; Ta20r78.1, 77.6, 78.6, 79.8 and 76.3; Nb20* 7.1, 7.6, 7.2, 6.7 and 8.5; Ti0 2 0.1, 0.1, 0.0, 0.0 and 0.0; SnOr 1.0, 1.2, 1.3, 1.0 and 1.0; Sb2Oa 0.0, 0.0, 0.0, 0.0 and 0.1; Total 99.8, 100.6, 100.1, 101.4 and 99.7 wt. %. The empirical formula calculated from the mean values of these results (based on O = 6 and Z = 2) is: (Mn0 59 0.320.03)x0.94(i.720.27Sn004)Z2.03O6. The Fe/Mn ratio of the crystal, however, varies gradually and irregularly from 66/34 to 24/76. The ideal formula of the manganotapiolite unit cell is: (Mn, Fe)2 (Ta,Nb)4Oi2, where Mn+ > Fe 2 + and Ta > Nb. The X-ray single crystal and powder diffraction studies of the material including the five points analysed showed tetragonal symmetry with space group P42/mnm, a = 4.762 A and c = 9.272 Å and V = 210.26 A. The strongest lines in the X-ray powder diffraction pattern are: 4.24 (40) (101), 3.367 (100) (110), 2.592 (90) (103), 2.381 (60) (200), 1.754 (90) (213), 1.682 (60) (220), 1.504 (40) (320, 302) and 1.411 (40) (303, 116). Precession photographs show very strong reflections of a cell of the rutile type and twinning on {013}. The crystal exhibits small inclusions of native antimony, microlite and cassiterite.