New Minerals from the Redmond Mine, North Carolina, USA: I. Redmondite, Hydroredmondite, and Sulfatoredmondite, Three Minerals Containing the Novel [Pb₈O₂Zn(OH)₆]⁸⁺ Structural Unit

Abstract

The new minerals redmondite (IMA2021–072), [Pb₈O₂Zn(OH)₆](S₂O₃)₄, hydroredmondite (IMA2021–073), [Pb₈O₂Zn(OH)₆](S₂O₃)₄·2H₂O, and sulfatoredmondite (IMA2021–089), [Pb₈O₂Zn(OH)₆](SO₄)₄·6H₂O, were found in the Redmond mine, Haywood County, North Carolina, USA, where they occur together in a highly unusual secondary assemblage that comprises a variety of rare Pb-Zn-Cu sulfates, thiosulfates, and carbonates. The properties of redmondite are as follows: colorless to shades of brown, transparent equant rhombs to about 1 mm; white streak; adamantine luster; brittle; Mohs hardness 2; conchoidal fracture; no cleavage; 5.757 g/cm³ calculated density; biaxial with α = 1.96–1.97, β = 1.97–1.98, γ = 1.97–1.98; extreme dispersion. The properties of hydroredmondite are as follows: colorless, transparent prisms, blades, and tablets up to about 1 mm; white streak; adamantine luster; brittle; Mohs hardness 2; curved and stepped fracture; good cleavages on {101} and ⁠{101}; 5.124 g/cm³ calculated density; biaxial (+) with α = 1.830(5), β = 1.880(calc), γ = 1.950(calc), 2V_meas = 83(1)°. The properties of sulfatoredmondite are as follows: colorless, transparent wedge-shaped crystals to about 2 mm; white streak; adamantine luster; brittle; Mohs hardness 2; curved, irregular fracture; no cleavage; 5.173 g/cm³ calculated density; biaxial (+), α = 1.780(5), β = 1.850(calc), γ = 1.860(calc), 2V_meas = 40(3)°; moderate r > v dispersion. Electron probe microanalyses provided the empirical formulae: Pb_8.16Zn_1.00S⁶⁺_3.97S²⁻_3.97O₂₀H_5.79, Pb_8.00Zn_1.00S⁶⁺_4.00S²⁻_4.00O₂₂H_9.98, and Pb_8.00Zn_0.99S⁶⁺_4.00S²⁻_0.89O_29.11H_17.98 for redmondite, hydroredmondite, and sulfatoredmondite, respectively. Redmondite is monoclinic, P2₁/c, a = 9.1672(4), b = 10.6576(4), c = 14.0620(10) Å, β = 101.173(7)°, V = 1347.83(12) ų, and Z = 2. Hydroredmondite is monoclinic, P2₁/n, a = 12.5991(9), b = 9.2819(4), c = 12.9774(9) Å, β = 90.443(6)°, V = 1517.57(16) ų, and Z = 2. Sulfatoredmondite is monoclinic, C2/m, a = 17.294(2), b = 7.3668(9), c = 12.7271(18) Å, β = 110.622(9)°, V = 1517.5(4) ų, and Z = 2. The crystal structures of redmondite (R₁ = 0.0346 for 2061 I > 2σI), hydroredmondite (R₁ = 0.0346 for 3022 I > 2σI), and sulfatoredmondite (R₁ = 0.0344 for 1661 I > 2σI) are all based on the same [Pb₈O₂Zn(OH)₆]⁸⁺ structural unit.

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