Reflectivity and other physicochemical properties of Mn-substituted goethites and hematites

Goethite-bearing samples with values of Mn_s (Mn/(Mn+Fe) mole fraction) up to 0.206 were synthesized by precipitation from alkaline solution. Samples with Mn_s<=0.061 were single-phase Mn-goethites samples with higher Mn_s values contained another Mn-bearing phase (probably jacobsite). Mn-hematites were prepared by dehydroxylation of corresponding Mn-goethites at 500°C. Orthorhombic a and b unit cell dimensions of Mn-goethites changed in a linear manner with Mn_s, but not at rates predicted by the Vegard law. Hexagonal unit cell dimensions of Mn-hematites did not vary with Mn_s. Mössbauer parameters isomer shift (IS), quadrupole splitting (QS), and hyperfine field (B_hf) were measured at 293 and 15 K. For all single-phase Mn-goethites and Mn-hematites (Mn_s<=0.061), magnetic splitting was observed at both temperatures. At 293 K, small but systematic decreases in B_hf were observed with increasing Mn substitution; IS and QS were not dependent on Mn_s. Mn substitution strongly lowered the Morin transition was not present for Mn_s>0.020(4). The saturation magnetization of Mn-goethites and Mn-hematites (Mn_s<=0.061) was the expected zero (within error) for antiferromagnetic goethite and for hematites obtained from dehydroxylation of goethites. Mn-goethites with Mn_s>0.061 were magnetic because of the presence of strongly magnetic jacobsite. For reflectivity spectra, bands resulting from Mn³⁺ were centered near 454 and 596 nm for Mn-goethites and near 545 and 700 nm for Mn-hematites. There is evidence for a ~700 nm band in spectral data for Martian bright regions, but association of it with Mn³⁺ is not a unique interpretation. Comparison of laboratory and Martian spectral data implies that Mn_s<0.032 for the Mn³⁺ content of Martian hematites.