BARWOODITE, Mn2+6(Nb5+,□)2(SiO4)2(O,OH)6, A NEW MEMBER OF THE WELINITE GROUP FROM GRANITE MOUNTAIN, ARKANSAS

The new mineral barwoodite (IMA2017-046), Mn-6(2+)(Nb5+,square)(2)(SiO4)(2)(O,OH)(6), was found in miarolitic cavities at the Big Rock quarry (also known as the 3M quarry), Granite Mountain, Little Rock, Pulaski County, Arkansas, USA. It is interpreted as crystallizing from pegmatitic fluids. The mineral occurs as brownish-red hexagonal plates up to 3 mm in diameter. Crystals are transparent with vitreous luster and light orange streak. The tenacity is brittle, the Mohs hardness is approximately 31/2, the fracture is curved, and there is one perfect cleavage on {001}. The calculated density for the empirical formula is 4.227 g/cm(3). Barwoodite is optically uniaxial (-) with omega = 1.873(3) and epsilon = 1.855(5) (white light) and is nonpleochroic. Electron microprobe analyses (WDS mode) provided the empirical formula Mn-6(2+)(Nb0.945+Fe0.183+Mn0.113+square(0.77))(Sigma 2)(SiO4)(2)[O-3.58(OH)(2.42)](Sigma 6). The six strongest X-ray powder diffraction lines are [d(obs) <(A)overcircle>(1)(hkl)]: 3.125(95)(11 (1) over bar), 2.858(56)(021), 2.688(57)(210), 2.349(81)(300), 1.7930(100)(212), and 1.5505(75)(140, (32) over bar1). Barwoodite is trigonal, P3, a 8.2139(10), c 4.8117(4), V 281.14(7)(3), and Z = 1. The structure determination for barwoodite (R-1 = 0.0246 for 746 reflections with I-o > 2 sigma I) shows it to be isostructural with welinite, franciscanite, and orebroite. Together, these four minerals comprise the welinite group.