Reference:
▪ Cooper, M.A., Ball, N.A., Hawthorne, F.C., Paar, W.H., Roberts, A.C., Moffatt, E. (2011): Georgerobinsonite, Pb4(CrO4)2(OH)2FCl, a new chromate mineral from the Mammoth – St. Anthony mine, Tiger, Pinal County, Arizona: description and crystal structure. Canadian Mineralogist, 49, 865-876.

Abstract:
Georgerobinsonite, Pb4(CrO4)2(OH)2FCl, 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 2–3; georgerobinsonite is brittle with an uneven fracture. The calculated density is 6.23 g/cm3. The indices of refraction are 2.07, β > 2.11, > 2.11; 2V(obs) = 84(2)°; it is strongly pleochroic, orange, 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) Å, V 606.5(3) Å3, Z = 2, a:b:c = 0.6578:1:0.5947. The strongest six lines in the X-ray powder-diffraction pattern [d in Å(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 CrO3 14.79, PbO 77.99, SO3 1.64, F 1.47, Cl 3.39, H2O (calc.) 1.52, sum 99.42 wt.%; the valence states of Pb, Cr and S, and the amount of H2O, were determined by crystal-structure analysis. The resulting empirical formula, on the basis of 12 (O, F, Cl, OH) anions, is Pb4.09 (Cr6+1.73S6+0.24)1.97 O8 (OH)1.98 F0.90 Cl1.12. The crystal structure of georgerobinsonite was solved by direct methods and refined to an R1 index of 2.0% based on 574 observed reflections collected on a four-circle diffractometer with MoK X-radiation. There are two Pb2+ 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 (CrO4) 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.