Evanichite, Pb6Cr3+(Cr6+O4)2(SO4)(OH)7FCl, from Tiger, Arizona, USA, the First Mineral Containing Both Cr3+ and Cr6+

Abstract

Abstract A new mineral species, evanichite (IMA 2022-033), ideally Pb6Cr3+(Cr6+O4)2(SO4)(OH)7FCl, has been discovered from the Mammoth-Saint Anthony mine, Tiger, Arizona, USA. It occurs as a single group of subparallel short prismatic crystals on top of a quartz matrix. Individual crystals are up to 0.70 × 0.20 × 0.20 mm in size. Associated minerals include georgerobinsonite, bobmeyerite, pinalite, diaboleite, boleite, leadhillite, caledonite, cerussite, calcite, baryte, and fluorite. Evanichite is orange-brown to red-brown in transmitted light, transparent with a white streak and an adamantine luster. It is brittle and has a Mohs hardness of 2½−3 with perfect cleavage on {100}. No twinning was observed. The calculated density is 5.878 g/cm3. Evanichite is insoluble in water or hydrochloric acid. An electron probe microanalysis yielded an empirical formula [based on 21 (O+F+Cl) apfu] of Pb5.91(Cr3+0.81Fe3+0.11)Σ0.92(Cr6+O4)2(S1.07O4)(OH)7F0.99Cl1.05, which can be simplified to Pb6(Cr3+,Fe3+)(Cr6+O4)2(SO4)(OH)7FCl. Evanichite is trigonal with space group P3 and unit-cell parameters a = 7.7651(14), c = 9.6199(17) Å, V = 502.3(2) Å3, and Z = 1. Its crystal structure consists of two unique Pb polyhedra [Pb1 coordinated by (5O + 3OH) and Pb2 by (5O + 2OH + F + Cl)], one M3+(OH)6 octahedron (M = Cr3+ + Fe3+), two Cr6+O4 tetrahedra (Cr1 and Cr2), and one SO4 tetrahedron. These polyhedra form four different layers parallel to (001). Layer 1 consists of edge- and corner-shared Pb1 polyhedra, layer 2 of isolated Cr1 and Cr2 tetrahedra, layer 3 of edge- and corner-shared Pb2 polyhedra, and layer 4 of isolated Cr3+(OH)6 octahedra and SO4 tetrahedra. These layers are stacked along [001] in the sequence layer 4 + layer 3 + layer 2 + layer 1 + layer 4. Both Pb1 and Pb2 cations are coordinated to anions with four short Pb–Φ (Φ = O, OH, F, or Cl) distances between 2.37 and 2.60 Å on one side and four (for Pb1) or five (for Pb2) long Pb–Φ distances ranging from 2.84 and 3.19 Å on the other side, indicating that both of them are lone-pair stereoactive. Evanichite is the first mineral that contains both Cr3+ and Cr6+, as well as the first chromate-sulfate compound with (CrO4)2− and (SO4)2− occupying distinct crystallographic sites.