• journal_title：The Canadian Mineralogist
• Contributor：Mark A. Cooper ; Neil A. Ball ; Frank C. Hawthorne ; Werner H. Paar ; Andrew C. Roberts ; Elizabeth Moffatt
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.3.865
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：3
• firstpage：865
• section：Articles

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 2½–3; georgerobinsonite is brittle with an uneven fracture. The calculated density is 6.23 g/cm³. The indices of refraction are α ≈ 2.07, β > 2.11, γ > 2.11; 2V(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) Å, V 606.5(3) ų, 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 Pb_4.09 (Cr⁶⁺_1.73S⁶⁺_0.24)_∑1.97 O₈ (OH)_1.98 F_0.90 Cl_1.12. 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 MoKα 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.