High-Gradient Magnetic Concentration of Chlorite and Hydroxy-Interlayered Minerals in Soil Clays

Identification and characterization of chlorite and hydroxy-interlayered layer-silicate minerals (HIM) common to many soils of the Atlantic Coastal Plain would benefit if the minerals could be concentrated and isolated by a suitable technique that would not alter them chemically or mineralogically. High-gradient magnetic separation (HGMS) meets these requirements and was used in this study as a pretreatment in the analysis of soil clays. Deferrated coarse clay (2–0.2 µm) of soil material from several Ultisols on the Coastal Plain of North Carolina was subjected to HGMS using magnetic fields ranging from 0.04 to 0.89 T. Resulting fractions were analyzed by x-ray diffraction, differential thermal analysis, Mössbauer spectroscopy, and total chemical analysis. Iron-substituted Ti oxides, including rutile, ilmenite, anatase, and pseudorutile were found in the fractions separated at low magnetic field; chlorite was found at higher field strengths; HIM was separated at medium to high fields; kaolin and quartz plus HIM were found in the tailings. Mössbauer spectroscopy showed the chlorite to contain Fe²⁺ indicating that it is inherited primary chlorite. The chlorite is trioctahedral and constitutes <10 g kg⁻¹ of the coarse clay; the HIM is dioctahedral and appears to make up >650 g kg⁻¹ of the coarse clay. Structural formula of the HIM calculated from total chemical analysis of Casaturated clay and assuming a negative charge of 49 per unit cell layer is (Ca₀.₀₇K₀.₂₆Na₀.₁₅)(Si₇.₃₈Al₀.₆₂) (Al₂.₈₇Fe₀.₆₃Mg₀.₅₀)O₂₀(OH)₄Al₁.₈₆(OH)₅(H₂O)₀.₅₈, suggesting a phengitic muscovite or celadonite precursor. The cation exchange capacity (CEC) based on Ca content in the total analysis is about 15 cmolc kg⁻¹ of HIM. The HGMS was effective in simplifying the mineralogical composition of materials studied, based on Fe content of the components.