Weathering of Iron-Bearing Minerals in Soils and Saprolite on the North Carolina Blue Ridge Front: II. Clay Mineralogy

Abstract

Abstract--The mineralogy of the clay fraction was studied for soils and saprolite on two Blue Ridge Front mountain slopes. The clay fraction contained the weathering products of primary minerals in the mica gneiss and schist parent rocks. Gibbsite is most abundant in the saprolite and residual soil horizons, where only chemical weathering has been operable. In colluvial soil horizons, where both physical and chemical weathering have occurred, the clay fraction consists largely of comminuted primary phyllosilicates--muscovite, chlorite, and possibly biotite--and their weathering products: vermiculite, interstratified biotite/vermiculite (B/V), and kaolinite. The clay-size chlorite contains Fe 2 § as indicated by MSssbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe 3 § Vermiculite in colluvial soils and, especially, surface horizons is weakly hydroxy-interlayered. The kaolinite in the clay fraction resulted at least partly from the comminution of kaolinized biotite in coarser fractions.The hematite content ranged from 0 to 8% of the clay fraction and strongly correlates (r = .95) with dry clay redness, as measured by the redness rating: RR = (10 -YR hue) x (chroma) + (value). The hematite is largely a product of the weathering of almandine; thus, the soil redness is dependent on the amount of almandine in the parent materials and its degree of weathering in the soils. Goethite (13-22% of the clay fraction) imparts a yellow-brown hue to soils derived from almandine-free parent rocks. The release of Fe in relatively low concentrations during the weathering of Fe-bearing primary minerals, particularly biotite, appears to have promoted the formation of goethite.

Weathering of Iron-Bearing Minerals in Soils and Saprolite on the North Carolina Blue Ridge Front: I. Sand-Size Primary Minerals

“…The apparent resistance of chlorite to chemical weathering allows its comminution even into the clay fraction (Graham et al, 1989). The persistence of primary chlorite throughout soil profiles was observed by Bain (1977) in weakly developed till-derived soils in Scotland.…”

Vermiculitization of Phlogopite in Metagabbro, Central Turkey

“…The origin of the trioctahedral variety of vermiculite remains unclear (Justo et al, 1986;de la Calle and Suquet, 1988;Zhelyaskova-Panayotova et al, 1992 whereas the dioctahedral variety is unquestionably accepted as the product of low-temperature surface weathering (Gruner, 1939;Hathaway, 1955;Barshad and Kishk, 1969;April et al, 1986;Graham et al, 1989;Moon et al, 1994). Experimental studies produced vermiculite as an alteration product of micas Toksoy-Krksal, Ttirkmeno~lu, and GOnciao~lu Clays and Clay Minerals owing to the effects of low-alkali solutions (Komarneni and Roy, 1981).…”

Composition, properties and formation of Pseudogley on loess along a precipitation gradient in the Pannonian region of Croatia

“…Chemical weathering of chlorites in soils may result in its complete destruction or in its transformation to vermiculite, smectite or related interstratified minerals (Ryan et al, 2008). However, some studies showed that chlorite was resistant to chemical alteration through the soil, which allowed its comminution into the clay fraction (Graham et al, 1989). Based on the results obtained, we presume faster chlorite destruction in profile 1 with the lowest pH values (less soil clay minerals) and more pronounced transformation of chlorite to other mineral phases in profiles 2 and 3 (more soil clay minerals).…”