Tantalowodginite, (Mn0.5vacancy0.5)TaTa2O8, a new mineral species from the Emmons pegmatite, Uncle Tom Mountain, Maine, U.S.A.

Tantalowodginite is a new mineral found in the Emmons granite pegmatite dike in Oxford County, Maine, U.S.A. It occurs as anhedral masses (0.5-12 cm) in the pegmatite core composed of K-feldspar, quartz, almandine, and schorl. Rarely, it occurs as crystals (0.2-1 cm) in miarolitic cavities associated with muscovite and fluorapatite. Tantalowodginite is rimmed with either black wodginite or columbite-(Mn). It is orange-red to deep red in color, semitransparent with a vitreous to sub-adamantine luster, has a yellowish-tan streak, is brittle with a conchoidal fracture, and shows a distinct {100} cleavage. The Mohs hardness is 5.5. Calculated density is 7.87 g/cm3 and the measured density is 7.61(1) g/cm3. Tantalowodginite is biaxial positive (þ) and has a 2V angle of @ 708 with strong dispersion. It exhibits weak to moderate pleochroism: orangish-yellow parallel to cleavage and greenish-yellow perpendicular to cleavage on (010). The measured Zc is 5-128. The birefringence is strong to extreme; interference colors are very high-order tints in golden yellow. The refractive index (RI) is greater than 2.00 and the calculated mean RI is 2.24. Tantalowodginite is non-fluorescent under 254 nm (short wave) and 366 nm (long wave) ultraviolet light. The average chemical analysis of six electron microprobe analyses is Li2O 0.54, MnO 6.23, FeO 0.23, TiO2 0.01, SnO2 8.14, Nb2O5 3.97, Ta2O5 80.75, total 99.88. The simplified formula is (Mn0.5A0.5)TaTa2O8. X-ray diffraction data show that tantalowodginite is monoclinic, space group C2/c. The refined unit-cell parameters are a 9.542(1) A? , b 11.488(2) A? , c 5.128(1) A? , and b 91.13(1)8, with Z¼4. In the wodginite structure there are three octahedrally coordinated sites. The A- and Bsites form zig-zag chains along z via edge sharing. Within these chains, the A- and B-sites alternate within the same plane. The C-sites form chains via edge sharing that lie in a different plane and connect the A-B chains by sharing apexes alternately with the A and B polyhedra. The strongest measured X-ray powder diffraction lines are [d in A? , (I/I0), (hkl)]: 7.332 (20) (110), 4.741 (20) (200), 3.838 (30) (021), 3.667 (100) (220), 3.000 (100) ð221Þ, 2.957 (100) (221), 2.883 (30) (040), and 1.778 (30) (260). The type specimen is deposited in the mineralogical collection of the Maine Mineral and Gem Museum, 99 Main Street, Bethel, Maine, U.S.A.