The Tetravalent Manganese Oxides : Clarification of their Structural Variations and Relationships and Characterization of their Occurrence in the Terrestrial Weathering Environment as Desert Varnish and other Manganese Oxide Concentrations

Citation

Potter, Russell Marsh (1979) The Tetravalent Manganese Oxides : Clarification of their Structural Variations and Relationships and Characterization of their Occurrence in the Terrestrial Weathering Environment as Desert Varnish and other Manganese Oxide Concentrations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/k2s0-9177. https://resolver.caltech.edu/CaltechTHESIS:09192022-193330791

Abstract

A number of structural problems in the mineralogy of the tetravalent manganese oxides have been addressed by infrared spectroscopy in conjunction with X-ray diffraction and chemical analysis. The first pyrolusite with proven orthorhombic symmetry is reported. Examination of pyrolusites with different degrees of orthorhombic distortion supports the view that this distortion is the result of micro-pores within the crystal. Pyrolusite infrared spectra exhibit variations which cannot be correlated to orthorhombic distortion, ramsdellite intergrowths, or other mineral impurity. The continuous structural variation of the nsutites from a rarnsdellite end-member to a pyrolusite end-member has been confirmed although much of the variation in synthetic nsutites appears to be related to crystalline order. Ramsdellite has a single, crystallographically-ordered type of water, which appears to be an integral part of the structure. The only hydrous component of romanechite is water, which is located in the channels in a specific crystallographic site. The only hydrous component of lithiophorite is hydroxide ion, which is oriented perpendicular to the cleavage planes. Birnessite appears to have a layer structure on the basis of its infrared spectrum. Its identity with proposed synthetic analogs is confirmed. Todorokite is a valid mineral species which is not analogous to any synthetic phases or to any alteration products of them. It appears to have a layered structure. Rancieite probably has a layer structure related to birnessite.

It is possible to distinguish manganese oxides of different structural groups from one another by their spectra in the mid-infrared region, which is sensitive to the hydrous components and the manganese octahedral framework. Because of its sensitivity to short range order, infrared spectroscopy is often superior to X-ray diffraction for the determinative mineralogy of the manganese oxides, which often occur in a finely-particulate, poorly-crystalline state. Spectra in the region 4000 cm⁻¹ to 200 cm⁻¹ are presented for well-characterized manganese oxide samples in order to form a basis for identification of manganese oxide mineralogy. The following oxides are included: aurorite, birnessite, braunite, buserite, chalcophanite, coronadite, cryptomelane, groutite, hausmannite, hollandite, lithiophorite, manganite, manganosite, manjiroite, marokite, nsutite, partridgeite, pyrolusite, quenselite, ramsdellite, rancieite, romanechite (psilomelane), todorokite, and woodruffite.

This data base has been applied to the study of the mineralogy of manganese oxide concentrations of the terrestrial weathering environment. Through the integrated application of a variety of infrared spectroscopic, X-ray diffraction, electron optic, and chemical techniques, the characteristic mineralogy of desert varnish has been identified as bimessite, hematite, and mixed-layer illite-montmorillonite clay minerals. Clay minerals comprise greater than 70 percent of the varnish, the oxides constitute t he bulk of the remainder and are in intimate physical association with the clays. An abrupt change in chemistry, mineralogy, and morphology exists at the varnish-rock interface. The origin of the material is external to the rock which it coats. The clays are most likely transported by wind or water. The oxides appear to be transported by water. Clay minerals are necessary for varnish formation, probably through their influence on oxide concentration or deposition.

Morphological distinctions among desert varnish, manganese dendrites, river deposits and other manganese oxide concentrations of the terrestrial weathering environment have a sound basis in differences in their mineralogy. The manganese oxide in manganese dendrites is either romanechite or a hollandite group mineral. These are mixed with varying amounts of silicate minerals, which are a passive substrate for the oxide deposition. Manganese stream deposits are generally birnessite with minor amounts of silicate minerals; one nsutite stream deposit has been identified. Crack deposit mineralogy resembles that of manganese dendrites. Cave and subglacial deposit mineralogy resembles that of manganese stream deposits. No dendrite has been found to consist of pyrolusite. Hydropsilomelane concretions, which carry chemical remanent magnetization in sediments in Baja California, Mexico, consist of siltstone matrix minerals cemented by a manganese oxide having the chalcophanite structure. The high concentration of magnesium presumed to occur in the interlayer position of this mineral extends the known range of substitution in chalcophanite-structure minerals.

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