Magnetita e sua transformação para hematita em um solo derivado de esteatito

Abstract

SUMMARYThe main objective of this work was to characterize the magnetic minerals and to identify their pedogenic transformation on a steatite-forming soil of Minas Gerais, Brazil. The iron-rich spinel phase was characterized by chemical analysis, powder X-ray diffraction, Mössbauer spectroscopy, with and without an externally applied magnetic field of 6 tesla, and saturation magnetization measurements. Nearly stoichiometric and well-crystallized magnetite was the only magnetic mineral actually detected. The cubic unit cell parameter of the fresh rock magnetite was found to be a o = 0.8407(5) nm. Hematite (hexagonal cell; a = 0.5036(3) nm, c = 1.375(4) nm) was detected in the altered rock and in the sand-soil and silt-soil fractions. Magnetite is assumed to transform into hematite during pedogenesis through progressive oxidation of structural Fe 2+ to Fe 3+ . In partially oxidized magnetites, a relatively small proportion of Fe 3+ was interpreted as being uncoupled from the Fe 2+ -Fe 3+ charge transfer system, in octahedral sites of the spinel structure. Compositional formulae of magnetite with different degrees of non-stoichiometry are proposed. Ilmenite was found in minor proportions in the magnetically extracted portions from both rock and soil samples.Index terms: Iron-rich spinel, iron oxide, soapstone, magnetic soil, Mössbauer spectroscopy.(1) This work is part of the thesis of the first author, presented to the Departmento

Genesis and transformation of basic rock-derived soils with shiny ped faces under tropical conditions

“…The weathering product of ilmenite results from iron dissolution with relative titanium enrichment that results in leucoxene formation (Figure 5) (Deer et al, 1966;Santos et al, 2021). In turn, magnetite weathering through the gradual oxidation of structural Fe 2+ to Fe 3+ might lead to hematite formation during pedogenesis in a mineralogical pathway that differs from that of mafic systems, wherein magnetite can be transformed into hematite by oxidation (Santana et al, 2001;Camêlo et al, 2017). Weathering and iron loss were evidenced by hypocoating around both lithogenic oxides (Figure 5).…”

Crystallographic Changes Accompanying the Verwey Transition in a Magnetite from Steatite

“…The 298 K-XRD pattern (Figure 4) shows well defined reflections, with line-width at half height, 0.0832 o 2θ, for the more intense (311) reflection, which are characteristic for the cubic system Fd3m, corresponding to a determined unit cell dimension of a = 0.83959 (2) nm. This cell parameter is well comparable with the reported value for magnetite (JCPDS 24 card No.…”

“…Magnetite (ideal formula, Fe 3 O 4 ; cubic, space group Fd3m) is a ferrimagnetic iron oxide with an inverted spinel structure. 1,2 In nature, magnetite is often oxidized to hematite (αFe 2 O 3 , hexagonal, space group ), 3 a thermodynamically more stable ferric oxide, either directly or via an intermediate formation of maghemite (γFe 2 O 3 or, in the equivalent chemical structure of a ferric spinel, ; = vacancy), 4 which also crystallizes in space group Fd3m. The crystallochemical steps governing the transformation magnetite → maghemite are still a focus of attention and controversy.…”

Preparation and characterization of tin-doped spinel ferrite

“…The chemical and crystallographic mechanisms by which the oxidation of Fe 2+ to Fe 3+ in magnetite (Fe 3 O 4 ; cubic) leads to hematite (␣-Fe 2 O 3 ; rhombohedral hexagonal) or maghemite (␥-Fe 2 O 3 ; cubic or tetragonal) are not enough clear so far. In natural conditions, purer and well-crystallized bulk magnetite is directly transformed to hematite [1,2], whereas the oxidation of Fe 2+ in the crystalline structure of highly substituted, small-sized grains and less wellcrystallized iron-rich spinels leads first to maghemite prior the final conversion to hematite [3][4][5][6]. The conversion rate of magnetite to hematite seems to be somehow influenced by the climate, which prevailed during the rock weathering on the soil formation process [7].…”