Masutomilite, manganese analogue of zinnwaldite, with special reference to masutomilite–lepidolite–zinnwaldite series

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Abstract
A new mineral with the following formula is described: (K1.788Na0.148 Rb0.141)(Li2.536Mn2+0.985Fe2+0.183Fe3+0.061Al1.958Ti0.012) (Si6.646 Al1.354)·O19.634 (F3.159 (OH)1.202)=(K, Na, Rb)2.077(Li, Mn, Fe2+, Fe3+, Al, Ti)5.735(Si, Al)8.000·O19.639 (F, OH)4.361 as OH+O+F=24. It corresponds to manganese analogue of zinnwaldite and is expressed by a K2(Mn2+2−1Li2−3Al2) (Al2−1Si6−7O20) (F2−3 (OH)1−2). The name masutomilite is proposed for the material with Mn2+> Fe2+ in honor of Dr. Kazunosuke Masutomi for his contribution to minerals of Japan. X-ray powder diffraction and optical data indicate the mineral to be monoclinic, space group Cm or C2⁄m with a 5.253(8)Å, b 9.085(4)Å, c 10.108(8)Å, β 100.15(0.03)°, V 474.84(0.56)Å3 and Z=1. The precession method verified the space group. The strongest lines in the X-ray powder pattern are 10.10(72)(001), 3.64(43)(-112), 3.32(100)(003), 3.09(58)(112), 2.903(35)(-113, 031), 2.589(17)(200, -131), 1.989(46)(005), 1.653(23)(204, -135). It is pale purplish pink. Transparent. Cleavage {001} perfect. Hardness (Mohs) 2.5. Sp. Gr. (meas.) 2.94 and 2.96 (calc). Optically biaxial negative with α 1.534, β 1.569, γ 1.570, (−)2V 29°–31°, r>v very weak, X, Z colorless to pale purple, Y purple, absorption Xb=Y, X nearly perpendicular to (001), a∧Z 3°. The mineral occupies core part of a zoned single mica crystal up to 10 cm across and 1 cm thick found in a druse of granite pegmatite at Tanakamiyama, Otsu, Shiga Prefecture, Japan. The outer part of the crystal is a brown manganoan zinnwaldite. The associated minerals are topaz, schorl, albite and quartz. For comparison of a high manganese zinnwaldite with masutomilite, a mica from Tawara, Naégi district, Hirukawa-mura, Gifu Prefecture, Japan is also described. A complete solid solubility does exist for the masutomilite–lepidolite–zinnwaldite series. The relationship between chemical composition, unit-cell dimensions a or b, and optical properties for the masutomilite–lepidolite series is figured out.