Artroeite, PbAlF_3 (OH)-2, a new mineral from the Grand Meef mine, Graham County, Arizona: Description and crystal stricture

摘要

Artroeite, PbAlF_3(OH)_2, space group PI, a = 6.270(2), b = 6.821(3), c = 5.057(2) A, alpha = 90.68(2), beta = 107.69(2), gama = 104.46(2)deg , V= 198.6(2) A3, Z = 2, is a new mineral from the Grand Reef mine, near Klondyke, Graham County, Arizona. It occurs as colorless Haded crystals associated with quartz, fluorite, galena, anglesite, and an as yet undescribed mineral of composition PbCa_2Al(F,OH)9. Artroeite has a Mohs hardness of about 2.5, a measured density of 5.36(2) g/cm~3, and a calculated density of 5.47 g/cm~3. It exhibits a perfect {100} cleavage and a good {010} cleavage. Optically it is biaxial (-) with a = 1.629(1), beta = 1.682(2), and gama = 1.691(2). Dispersion is strong, r > v. The six strongest powder diffraction lines are {d(I,hkl)} 4.42_(100)(101), 3.221_(40)(101), 2.595_(70)(121,021), 2.190_(65)(201,012,030), 2.030_(50)(022), 2.015_(40)(230) A. The structure was solved by direct methods and refined to R = 0.022 using X-ray difiractometer data (1096 independent reflections). In the structure, edge-sharing dimers of AlF_3(OH)_3 octahedra link together by bonds to Pb atoms to form approximately close-packed layers parallel to (101). The layers are linked to one another by one Pb-F bond gild two H bonds per formula unit. Pb is coordinated to 6 F and 3 O atoms. Three F Jigands are associated with the same Al octahedral face and correspond to much longer bonds. It is probable that the lone pair electrons are situated on that side of the Pb atom. The structure is compared with those of acuminite and tikhonenkovite.