Natural “amorphous” ferric hydroxide

A series of mixed iron and titanium oxide coprecipitates ranging in composition between 0 < Ti/Ti + Fe < 1 was synthesized and aged under varying conditions of pH, temperature and time in order to establish a working model for pedogenic titanium and titano-ferric oxides. X-ray powder diffraction (XRD), selective chemical dissolution, magnetic susceptibility, charge distribution and electron optical data indicate that the freshly prepared Fe-Ti oxides consist of an Fe-rich (Ti-ferrihydrite) phase (Ti/Ti + Fe ~< 0.70) having pH-dependent positive charge and a Ti-rich phase (Ti/Ti + Fe >~ 0.7) with permanent and pH dependent negative charge.Synthetic Ti-ferrihydrite and amorphous TiO2 were completely soluble in acid ammonium oxalate (2 hr extraction in the dark) whereas poorly crystalline anatase (width at half height, WHH > 2.0~ was partly oxalate soluble. NH4-oxalate soluble Ti was particularly high in soils developed under a cool montane climate (afro-alpine) and lower in soils of warmer subtropical climate, which contain anatase and futile.Several mixed Fe-Ti crystalline phases were identified after aging NH3 coprecipitates of Fe and Ti nitrate at 7WC and pH 5.5 for 70 days:(1) goethite and hematite in the composition range 0 < Ti/Ti + Fe ~< 0.20; at low Ti concentrations (<5 mole %) goethite was favored and/or hematite inhibited; (2) The results suggest that secondary or pedogenic Ti-Fe oxides can form by coprecipitation and crystallization in the weathering solution, and emphasize the essential role of water (as opposed to dry oxidation) in the alteration of primary titaniferous minerals.

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“…Furthermore, this conforms with the data of Schwertmann and Fischer (1973) for synthetic and natural ferrihydrites.…”

Section: Clays and Clay Mineralssupporting

confidence: 90%

Amorphous and Crystalline Titanium and Iron-Titanium Oxides in Synthetic Preparations, at Near Ambient Conditions, and in Soil Clays

Fitzpatrick

Roux

Schwertmann

1978

Clays and clay miner.

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“…The shape of hematite from the TRE soils in electron micrograph was of very fine particles of 5 to 8 nm in diameter, which was approximately one-tenth smaller than that obtained from other origins such as Terra Rossa (9), and was rather similar in morphology to ferrihydrite, precipitated ferric hydroxide (8). The sharpness of X-ray peaks in the hematite examined in this study, however, was not so degraded.…”

Section: Discussioncontrasting

confidence: 65%

“…The goethite in the LV A-I was morphologically alike to this mineral, but that in the PV A-III was not. The hematite particles from the TRE soils were rather similar in morphology to ferrihydrite, precipitated ferric hydroxide gel (8), although various hematites show small hexagonal plates, i.e., the size of artificially synthesized hematite is about 50 nm, and that of hematite from a Terra Rossa in Algeria about 70 nm (9), which are approximately ten times larger than that in this study.…”

Section: Resultscontrasting

confidence: 53%

Goethite and hematite in some solls from the amazon region

Kitagawa

1983

Soil Science and Plant Nutrition

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“…) occur in profile 3 (44-101 cm) and lesser amounts in profile 4 (34-52 cm). This material is most likely to be ferrihydrite (Schwertmann & Fischer 1973). The greater amounts present in profile 3 may be the result of free draining at depths where this material occurs.…”

Section: Sesquioxidesmentioning

confidence: 98%