The new minerals carlsonite (IMA2014-067), (NH4)5Fe33+O(SO4)6·7H2O, and huizingite-(Al) (IMA2015-014), (NH4)9Al3(SO4)8(OH)2·4H2O, formed from a natural fire in an oil-bearing shale near Milan, Ohio. Carlsonite crystals are yellow to orange-brown thick tablets, flattened on {001}, or stout prisms, elongated on [110], up to about 0.5 mm in size. The mineral has a tan streak, vitreous luster, Mohs hardness of 2, brittle tenacity, irregular fracture, perfect {001} cleavage, calculated density of 2.167 g/cm3, and is easily soluble in H2O. Carlsonite is optically biaxial (−), α = 1.576(1), β = 1.585(1), and γ = 1.591(1) (white light). Huizingite-(Al) crystals, typically intergrown in light greenish yellow drusy aggregates, are tabular to bladed, flattened on {100}, up to about 0.25 mm in maximum dimension. The mineral has a white streak, vitreous luster, Mohs hardness of 2½, brittle tenacity, irregular fracture, no cleavage, calculated density of 2.026 g/cm3, and is easily soluble in H2O. Huizingite-(Al) is optically biaxial (+) with α = 1.543(1), β = 1.545(1), and γ = 1.563(1) (589.6 nm light). Raman and infrared spectroscopy was conducted on both minerals. Electron microprobe analyses provided the empirical formulas [(NH4)4.64Na0.24K0.12]Σ5.00Fe3.053+O(SO4)6.93H2O and [(NH4)8.76Na0.22K0.02]Σ9.00(Al1.65Fe1.343+)Σ2.99(OH)1.98(H2O)4.02(SO4)8.00 for carlsonite and huizingite-(Al), respectively. Huizingite compositions with Fe > Al were noted. Carlsonite is triclinic, P1¯, a = 9.5927(2), b = 9.7679(3), c = 18.3995(13) Å, α = 93.250(7)°, β = 95.258(7)°, γ = 117.993(8)°, V = 1506.15(16) Å3, and Z = 2. Huizingite-(Al) is triclinic, P1¯, a = 9.7093(3), b = 10.4341(3), c = 10.7027(8) Å, α = 77.231(5)°, β = 74.860(5)°, γ = 66.104(5)°, V = 948.73(9) Å3, and Z = 1. The five strongest lines in the X-ray powder diffraction pattern of carlsonite are [dobs in Å(I) (hkl)]: 9.23(100)(002); 8.26(40)(100,011); 7.57(43)( 1¯11,1 1¯1,011); 4.93(23)( 1¯1¯1, 1¯20); and 3.144(41)(multiple). Those for huizingite-(Al) are: 8.82(60)(100); 5.04(69)(121); 3.427(100)( 2¯2¯1); 3.204(68)( 2¯11); and 3.043(94)(2 1¯2,312).

The crystal structures of carlsonite (R1 = 0.030) and huizingite (R1 = 0.040) are bipartite, each consisting of a structural unit and an interstitial unit. For carlsonite, the structural unit is a [Fe33+O(H2O)3(SO4)6]5 cluster and the interstitial complex is [(NH4)5(H2O)4]5+. For huizingite-(Al), the structural unit is a [(Al,Fe3+)3(OH)2(H2O)4(SO4)6]5−cluster and the interstitial complex is [(NH4)5(H2O)4]5+. In the carlsonite cluster, three FeO6 octahedra share a common vertex, while in the huizingite-(Al) cluster, three (Al,Fe)O6 octahedra form an abbreviated corner-linked chain. The cluster in carlsonite is the same as that in metavoltine, while the huizingite-(Al) cluster is unique. The range of Lewis basicity of the structural unit in carlsonite is 0.23–0.11 valence units (v.u.) and in huizingite-(Al) it is 0.20–0.12 v.u.; the corresponding Lewis acidities of the interstitial complexes in these structures are 0.13 and 0.14 v.u., respectively. A characteristic Lewis acid strength of 0.13 v.u. is suggested for NH4+ when it is in its most typical coordinations of 7 to 8. The close structural relationship between carlsonite and metavoltine and the similarity of their powder diffraction patterns suggests that carlsonite may have misidentified as metavoltine in some NH4-rich mineral assemblages. The new heteropolyhedral cluster in the structure of huizingite-(Al) is of interest because its existence may provide insights into the structural and paragenetic relations among hydrated ferric sulfate minerals. In particular, it may exist as a complex in aqueous solutions or in solid-state transformations involving the formation and/or breakdown of sideronatrite-style [Fe3+(SO4)3]3− chains. We surmise that it may be a more commonly formed mineral than its abundance would indicate and that its rarity may reflect a narrow stability range, and so a transitory existence.

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