ALBERT

Description: 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 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 Vegrad law. Hexagonal unit cell dimensions of Mn-hematites did not vary with Mn(s). Moessbauer parameters isomer shift (IS), quadrupole splitting (QS), and hyperfine field (B(sub hf)) were measured at 293 and 15 K. For all single-phase Mn-goethites and Mn-hematites (Mn(s) less than or equal 0.061), magnetic splitting was observed at both temperatures. At 293 K, small but systematic decreases in B(sub hf) were observed with increasing Mn substitution; IS and QS were not dependent on Mn(s). Mn substitution strongly lowered the Morin transition temperature of hematite. At 15 K, the Morin transition was not present for Mn(s) greater than 0.020(4). The saturation magnetization of Mn-goethites and Mn-hematites (Mn(s) less than or equal 0.061) was the expected zero (within error) for antiferromagnetic goethite and for hematites obtained from dehydroxylation of goethites. Mn-geothites with Mn(s) greater than 0.061 were magnetic because of the presence of strongly magnetic jacobsite. For reflectivity spectra, bands resulting from MN(3+) were centered near 454 and 596 nm for Mn-goethites and near 545 and 700 nm for Mn-hematites. There is evidence for a approximately 700 nm band in spectral data for Martian bright regions, but association of it with Mn(3+) is not a unique interpretation. Comparison of laboratory and Martian spectral data implies that Mn(s) less than 0.032 for the Mn(3+) content of Martian hematites.

Description: The objective was to synthesize Ti-substituted goethites and hematites and to characterize them using x-ray diffraction, thermogravimetric analysis, coulimetry, optical and Mossbauer spectroscopies, and magnetic data in order to determine the evidence of Ti substitution for Martian spectral data. Evidence is provided that up to 4 mole percent can be incorporated into the goethite structure during the low temperature synthesis. Because the spectral data in the visible and near-IR are not sensitive to the degree of Ti-substitution in goethites and hematites, such data cannot be used to infer the presence or absence of Ti substitution in Martian hematites and goethites.