The 2H and 3R polytypes of sabieite, NH4Fe3+(SO4)2, from a natural fire in an oil-bearing shale near Milan, Ohio

Abstract

The mineral sabieite, NH4Fe3+(SO4)2, was found in 2011 along the banks of the Huron River near Milan, Ohio, where it formed as the result of a natural fire in an oil-bearing shale. The mineral is directly associated with pyracmonite, tschermigite, and voltaite and occurs as colorless, pale pink, tan, and yellow hexagonal tablets. The streak is white. Crystals are transparent with vitreous luster. Mohs hardness is 2½, tenacity is brittle, fracture is irregular, and cleavage is perfect on {001}. The measured density is 2.65(2) g/cm3. The mineral is optically uniaxial (-) with indices of refraction ω = 1.657(3) and ε = 1.621(5) (white light). The empirical formula (based on 2 S apfu) is [(NH4)0.73(H3O)0.22K0.04Na0.01]Σ1.00(Fe3+0.95Al0.02Mg0.01)Σ0.98(SO4)2. Powder diffraction showed crystals to be combinations of the 2H and 3R polytypes. The structure of the 2H polytype was solved and refined from single-crystal data yielding R1 = 0.0694 for 509 Fo > 4σ(F) reflections. The 2H polytype has space group P63 and cell parameters a = 4.83380(17), c = 16.4362(9) Å, V = 332.59(2) Å3, and Z = 2 and the 3R polytype has space group R3̄ and cell parameters a = 4.835(2), c = 24.496(15) Å, V = 495.9(5) Å3, and Z = 3. The sabieite polytypes (including the original sabieite from Sabie, South Africa, which is the 1T polytype) have glaserite-like structures with layers consisting of Fe3+O6 octahedra that share each of their vertices with SO4 tetrahedra. NH4 groups occupy 12-coordinated sites in the interlayer region, bonding to 6 O atoms in each of the adjacent layers. In the 1T polytype, successive layers have identical configuration and orientation, providing a one-layer repeat sequence. In the 2H polytype, alternate layers are flipped (or rotated), in a two-layer repeat sequence. In the 3R polytype, successive layers are shifted relative to one another, in a three-layer repeat sequence. The different orientations of adjacent layers in the structures of the 2H and 3R polytypes result in significant changes in the linkages between the (NH4)O12 and Fe3+O6 polyhedra.