Showing papers on "Hematite published in 1975"

The Formation of Hematite from Amorphous Iron(III)Hydroxide

Abstract: The formation of hematite from amorphous Fe(III)hydroxide in aqueous systems at pH 6 and 70°C, both with and without oxalate, was followed by kinetic measurements, electron microscopy, i.r. spectroscopy and thermal analysis. In the absence of oxalate, small amorphous particles coalesce into aggregates which eventually become single crystals of hematite. When oxalate is present, crystal growth is much faster and does not proceed through the intermediate stage of aggregation. Aggregates, when formed, consist of groups of single crystals. It is suggested that oxalate accelerates the nucleation of hematite crystals by acting as a template, the Fe-Fe distance in Fe-oxalate ions being similar to that in hematite.

An electron microscopy study of naturally occurring oxidation produced precipitates in iron-bearing olivines

Abstract: Iron-bearing olivine grains naturally altered by oxidation were examined in the transmission electron microscope to determine the precipitate phases and their crystallographic and morphological relationships to the host. Precipitate complexes heterogeneously nucleated on dislocations were composed of Si-rich, Mg-rich/Si-rich, and Fe-rich regions corresponding to α- tridymite, enstatite, and magnetite and/or hematite, respectively. The tridymite and magnetite (hematite) occurred as rod-like interleaved fingers, while the enstatite was more equidimensional. The crystal orientations of the precipitate phases with respect to the host structure, listed in Table 2, were well defined, but, in general, could not be simply related to the close packing of oxygen planes. Iron-rich (001) planar precipitates occasionally nucleated homogeneously in the host as well as heterogeneously on dislocations. Oxygen diffusion does not appear to be the rate-controlling process for precipitate nucleation and growth as precipitation kinetics in fresh olivine oxidized at 900 ° C would indicate an oxygen diffusivity of 10−8 cm2/sec, a value 10 orders of magnitude faster than determined previously.

Mineralogy and Chemistry of the Patapsco Formation, Maryland, Related to the Ground-Water Geochemistry and Flow System: A Contribution to the Origin of Red Beds

Abstract: Red and drab-colored sediments of the Patapsco Formation were studied to determine if color changes, clay mineralogy, and iron content of the sediments are related to the present ground-water geochemistry (Back and Barnes, 1965) and the present ground-water flow system (Mack, 1962) The red color of the sediments becomes darker in the direction of the ground-water flow, particularly in local discharge areas This color change is related to the amount and kind of iron oxides in the sediments Hematite and goethite coexist in most of the red and mottled samples; hematite is more abundant than goethite in red sediments but is rarely found in drab sediments Large amounts of amorphous or poorly ordered iron oxyhydroxides in these sediments indicate that much of the iron has been introduced by diagenetic processes Lower iron oxide values prevail toward the center of the outcrop belt, and higher values prevail in the recharge area Detrital kaolinite and illite are the most abundant clay minerals Vermiculite and mixed-layer illite-smectite are almost always found in red colored sediments and are probably products of post-depositional diagenesis The mineralogic and chemical variations correlate regionally with the observed ground-water flow pattern and with observed changes in Eh and dissolved iron content of the ground water These results suggest that fluctuations in the ground-water flow system in conjunction with Eh and pH conditions caused precipitation of iron hydroxides that, on aging, have crystallized as goethite and (or) hematite

Alpha iron (III) oxide crystals and derivatives

Abstract: Elongated, polycrystalline particles of alpha iron oxide, characterized by an open, porous structure and consisting of hematite microcrystallites containing from 2 to 4 percent of sulfate are prepared by the reaction of lithium hydroxide and ferric sulfate in weakly acidic to neutral aqueous solution under at least autogenous pressures at temperatures in the range of 150°-350° C The particles are readily converted to gamma iron oxide of good magnetic properites and also have utility as pigments and catalysts

Ferrite thin films for high recording density

Abstract: Ferrite thin films were prepared in serial process of vacuum evaporation of iron on an alumina substrate, oxidation of iron surface by alkali treatment, conversion to hematite at a high-temperature in air and reduction to ferrite at a low-temperature in hydrogen atmosphere. In the conversion to the hematite above 900°C, the solution of the alumina to the hematite was taken place to give rise to the firm adhesion of the film to the substrate. Magnetic properties of the ferrite thin films hardly depended on the film thickness in the range of 0.6 μm to 1.5 μm. Hc was 400-500 Oe and Br was 3000-2300 G, depending on the temperature of the annealing in air. In recording characteristics, these films considerably exceeded the conventional γ-Fe 2 O 3 coating medium.

A Mössbauer study of the reduction of hematite in hydrogen at 535° C

Abstract: The Mossbauer effect has been used to examine the phases produced by hydrogen reduction of hematite to α-iron at 535° C. No evidence has been found for spinels other than that of essentially stoichiometric magnetite; nor has there been any evidence of wustite formation, in contrast to recent Russian experiments on the reduction of iron oxides at temperatures well below the eutectoid transition point of 570° C.

Thermal Transformation of Lepidomelane

Abstract: Lepidomelane, an iron-rich biotite, was heated in air or in vacuum, or by electron bombardment, and the processes of dehydration and transformation were studied by means of X-ray diffractometry, optical microscopy, and electron microscopy. By heat treatment, vacancies are at first formed by the evaporation of water molecules and alkali ions, and they move and condense to form holes, which act as preferential nucleation sites for new phases. Other preferential sites are edges of exposed silicate sheets along microcracks, and these are decorated by a newly formed maghemite- like mineral whose a is 10.20 A (on a hexagonal cell), so that the sites can be clearly seen even under the reflection microscope. By heating at higher temperatures or prolonged heating, various phases, olivine, hematite, magnetite, and leucite are formed, depending upon the conditions of dehydration.