LUMSDENITE, NaCa3Mg2(As3+V24+V105+As65+O51)center dot 45H(2)O, A NEW POLYOXOMETALATE MINERAL FROM THE PACKRAT MINE, MESA COUNTY, COLORADO, USA

Lumsdenite (IMA 2018-092), ideally NaCa3Mg2(As3+V24+V105+As65+O51)center dot 45H(2)O, is a rare new polyoxometalate mineral from the Packrat mine, Gateway district, Mesa County, Colorado, USA. Crystals of lumsdenite occur as blades up to 0.2 mm in length, commonly growing in sprays. The crystals are dark green blue, with a green-blue streak. The mineral occurs on asphaltum, associated with montroseite- and corvusite-bearing sandstone. Other secondary minerals found in close association with lumsdenite are gypsum, huemulite, rosslerite, and at least two other potentially new minerals. Lumsdenite is optically biaxial (-), with alpha 1.617(2), beta 1.651(5), and gamma 1.675(5) in white light. The pleochroism scheme for lumsdenite is X = greenish yellow, Y = dark greenish blue, Z = greenish blue; X << Z < Y. The mineral is triclinic, P<(1)over bar>, with alpha 10.3490(5), b 17.6263(9), c 23.2556(16) angstrom, alpha 82.208(6), beta 88.351(6), gamma 81.702(6)degrees, V 4158.8(4) angstrom(3), and Z = 2. The strongest four powder diffraction lines for lumsdenite are [d(obs) angstrom(I)(hkl)]: 9:35(100)(101, (1) over bar 01), 14.86(80)(011), 17.30(44)(010), and 10.22(32)(100). The atomic arrangement of lumsdenite contains the novel polyoxometalate heteropolyanion [As3+V124+,5+As65+O51] structural unit in lumsdenite, [As(3+)V(2)(4+)5(10)(+)As(6)(5+)O(51)](11-), which has previously been found in four other minerals from the Packrat mine. The charge of the structural unit is balanced by the charge of the [NaCa3Mg2(H2O)(31)center dot 14H(2)O](11+) interstitial complex. The name lumsdenite is for the location of the mine at the head of Lumsden Canyon.