GEORGEROBINSONITE, Pb4(CrO4)2(OH)2FCl, A NEW CHROMATE MINERAL FROM THE MAMMOTH - ST. ANTHONY MINE, TIGER, PINAL COUNTY, ARIZONA: DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Georgerobinsonite, Pb 4 (CrO 4 ) 2 (OH) 2 FCl, is a new chromate mineral species from the Mammoth – St. Anthony mine, Tiger, Pinal County, Arizona. It occurs as minute intergrowths of thin tabular orange crystals less than 0.1 mm across with {001} dominant and minor {010} and {110}, associated with caledonite, a cerchiaraite-related mineral, cerussite, diaboleite, Cr-bearing leadhillite, matlockite, murdochite, pinalite, wulfenite and yedlinite, in vugs in a silicified matrix. Georgerobinsonite is orange-red, transparent and has a pale orange streak and an adamantine luster, and it does not fluoresce under ultraviolet light. No cleavage or parting was observed. The Mohs hardness is 2½–3; georgerobinsonite is brittle with an uneven fracture. The calculated density is 6.23 g/cm 3 . The indices of refraction are α ≈ 2.07, β > 2.11, γ > 2.11; 2 V (obs) = 84(2)°; it is strongly pleochroic, α orange, γ yellow; optical orientation: X = a , Y = c and Z = b , and the dispersion is strong with r > v . Georgerobinsonite is orthorhombic, space group Pmmn , a 7.613(2), b 11.574(3), c 6.883(2) A, V 606.5(3) A 3 , Z = 2, a:b:c = 0.6578:1:0.5947. The strongest six lines in the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are: 2.131(100)(232), 3.308(80)(012), 3.195(80)(211, 220), 6.371(60) (110), 3.357(60)(031, 201) and 3.143(60)(102). Chemical analysis with an electron microprobe gave CrO 3 14.79, PbO 77.99, SO 3 1.64, F 1.47, Cl 3.39, H 2 O (calc.) 1.52, sum 99.42 wt.%; the valence states of Pb, Cr and S, and the amount of H 2 O, were determined by crystal-structure analysis. The resulting empirical formula, on the basis of 12 (O, F, Cl, OH) anions, is Pb 4.09 (Cr 6+ 1.73 S 6+ 0.24 ) ∑1.97 O 8 (OH) 1.98 F 0.90 Cl 1.12 . The crystal structure of georgerobinsonite was solved by direct methods and refined to an R 1 index of 2.0% based on 574 observed reflections collected on a four-circle diffractometer with Mo K α X-radiation. There are two Pb 2+ cations coordinated by eight and nine anions, and both show prominent lone-pair stereoactive behavior. The Pb polyhedra share faces and edges to form layers parallel to {001} that are linked into a heteropolyhedral framework by (CrO 4 ) groups, with hydrogen bonding between OH and Cl anions. The mineral is named after Dr. George Willard Robinson (b. 1946), mineral curator, researcher, teacher and field collector.