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 less than or equal to 0.061 were single-phase Mn-g oethites; samples with higher Mn-s values contained another Mn-bearing phase (probably jacobsite). Mn-hematites were prepared by dehydroxyla tion of corresponding Mn-goethites at 500 degrees C. Orthorhombic a an d b unit cell dimensions of Mn-goethites changed in a linear manner wi th Mn-s, but not at rates predicted by the Vegard law. Hexagonal unit cell dimensions of Mn-hematites did not vary with Mn-s. Mossbauer para meters isomer shift (IS), quadrupole splitting (QS), and hyperfine fie ld (B-hf) were measured at 293 and 15 K. For all single-phase Mn-goeth ites and Mn-hematites (Mn-s less than or equal to 0.061), magnetic spl itting was observed at both temperatures. At 293 K, small but systemat ic decreases in B-hf were observed with increasing Mn substitution; IS and QS were not dependent on Mn-s. Mn substitution strongly lowered t he Morin transition temperature of hematite. At 15 K, the Morin transi tion was not present for Mn-s > 0.020(4). The saturation magnetization of Mn-goethites and Mn-hematites (Mn-s less than or equal to 0.061) w as the expected zero (within error) for antiferromagnetic goethite and for hematites obtained from dehydroxylation of goethites. Mn-goethite s 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 similar to 700 nm ban d in spectral data for Martian bright regions, but association of it w ith Mn3+ is not a unique interpretation. Comparison of laboratory and Martian spectral data implies that Mn-s < 0.032 for the Mn3+ content o f Martian hematites.