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 micrometer) 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, Mossbauer 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. Mossbauer 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 Ca-saturated 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)4 Al1.86(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 cmol(c) kg-1 of HIM. The HGMS was effective in simplifying the mineralogical composition of materials studied, based on Fe content of the components.