Identification of Mixite Minerals - an SEM and Raman Spectroscopic Analysis

Two mixites from Boss Tweed Mine, Tintic District, Juab Vounty, Utah and Tin Stope, Majuba Hill, Pershing Co., Nevada have been analysed by SEM with EDX analysis and by Raman spectroscopy. The SEM images show the mixite crystals to be elongated fibres up to 200 μm in length and 2 μm in width. Detailed images of the mixite crystals show the mineral to be composed of bundles of fibres The EDX analyses depend on the crystal studied, however the Majuba mixite gave analyses which matched the formula BiCu6(AsO4)3(OH)6.3H2O). Raman bands observed in the region 880 to 910 cm-1 and in the 867 to 870 cm-1 region are assigned to the AsO stretching vibrations of (HAsO4)2- and (H2AsO4)- units. Whilst bands at 803 and 833 cm-1 are assigned to the stretching vibrations of uncomplexed (AsO4)3- units. Intense bands are observed at 473.7 and 475.4 cm-1 are assigned to the ν4 bending mode of AsO4 units. Bands observed at 386.5, 395.3 and 423.1 cm-1 are assigned to the ν2 bending modes of the HAsO4 (434 and 400 cm-1) and the AsO4 groups (324 cm-1). Raman spectroscopy lends itself to the identification of minerals on host matrices and is especially useful for the identification of mixites.