Alterite and Magnesioalterite, Two New Oxalate Minerals, from the Vermillion Cliffs, Coconino County, Arizona, USA

Abstract

Abstract Two new organic minerals, alterite and magnesioalterite, ideally Zn2Fe3+4(SO4)4(C2O4)2(OH)4·17H2O and Mg2Fe3+4(SO4)4(C2O4)2(OH)4·17H2O, respectively, were discovered in carbonaceous petrified wood from an unnamed uranium prospect, the Vermillion Cliffs, Coconino County, Arizona, USA. Associated minerals include gypsum, alunogen, natrojarosite, sulfur, celestine, and quartz. Both alterite and magnesioalterite are yellowish green in transmitted light and transparent with white streak and vitreous luster. They are brittle and have a Mohs hardness of ∼1.5; cleavage is perfect on (001). No parting or twinning was observed. The measured densities are 2.18(4) and 2.17(3) g/cm3 for alterite and magnesioalterite, respectively. Optically, alterite is biaxial (+), with α = 1.545(5), β = 1.565(5), γ = 1.635(5), 2Vmeas. = 56(2)°, 2Vcal. = 58°. Magnesioalterite is also biaxial (+), with α =1.520 (5), β = 1.578 (6), γ = 1.610 (5), 2Vmeas. = 74(2)°, 2Vcal. = 76.5°. Both new minerals are insoluble in water, but slowly dissolve in hydrochloric acid. An electron microprobe analysis, together with data from an Elemental Combustion System for C, yielded the empirical formula (based on 45 O apfu) (Zn0.84Fe2+0.57Mg0.48Mn0.14)Σ2.03Fe3+4.00(S0.99O4)4(C2O4)2(OH)4·17H2O for alterite and (Mg0.74Zn0.60Fe2+0.58Mn0.09)Σ2.01Fe3+4.00(SO4)4.00(C2O4)2(OH)4·17H2O for magnesioalterite, both of which can be simplified to (Zn,Fe,Mg,Mn)2Fe3+4(SO4)4(C2O4)2(OH)4·17H2O and (Mg,Zn,Fe2+,Mn)2Fe3+4(SO4)4(C2O4)2(OH)4·17H2O, respectively. The measured δ13C ‰ value for the carbonaceous petrified wood on which the minerals were found is −23.1 and for alterite and magnesioalterite is 0.2. Alterite and magnesioalterite constitute a complete solid solution. They are monoclinic with the same space group, C2/c. The unit-cell parameters are a = 16.7656(15), b = 9.4074(7), c = 25.351(3) Å, β = 108.258(5)°, V = 3797.1(6) Å3 for alterite and a = 16.7696(5), b = 9.4020(2), c = 25.3466(8) Å, β = 108.2520(10)°, V = 3795.28(18) Å3 for magnesioalterite. The crystal structures of alterite and magnesioalterite are characterized by four-membered clusters of corner-sharing Fe3+(O5OH) octahedra. These clusters are linked by SO4 tetrahedra along the b axis and by the oxalate groups (C2O4)2− along the a axis to form sheets parallel to (001). Between the sheets are layers of M2+(H2O)6 octahedra (M = Zn2+, Mg2+, Fe2+, and Mn2+) and three symmetrically distinct H2O molecules that are not bonded to any non-H cations. The linkage between the sheets and the layers is achieved by hydrogen bonds, accounting for the good cleavage parallel to (001). Alterite and magnesioalterite are two of five double-salt minerals with hydrated sulfate-oxalates, after coskrenite-(Ce), Ce2(SO4)2(C2O4)·8H2O; levinsonite-(Y), YAl(SO4)2(C2O4)·12H2O; and zugshunstite-(Ce), CeAl(SO4)2(C2O4)·12H2O. They are also the most hydrated among the 34 oxalate minerals reported thus far. Noticeably, both alterite and magnesioalterite contain a significant amount of Fe2+ substituting for Mg and Zn, pointing to the likelihood for the existence of a Fe2+-analogue of alterite.