Amorphous Aluminum Hydroxide Formed at the Earliest Weathering Stages of K-Feldspar

Abstract

Abstract--Weathering products formed on the surface of K-feldspar in Yakushima Island, Japan were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). XRD confirmed that the weathering products were composed mainly of gibbsite and halloysite. SEM, TEM and EDX clearly showed formation of amorphous aluminum hydroxide exhibiting 2 distinct habits: 1) curled fibrous or circular forms less than 0.02 p,m in diameter; and 2) a spherical habit less than 1.0 p~m in diameter. The fibrous aluminum hydroxide exhibited curled fibrous forms or circular forms less than 0.02 p.m in diameter and gave a diffuse electron diffraction halo. EDX indicated that the material consisted mainly of A1 and very small amounts of Si and Fe. The spherical aluminum hydroxide also gave similar EDX and electron diffraction characteristics to the fibrous material. These fibrous and spherical aluminum hydroxides must be formed as a metastable phase in the earliest weathering stages, and transformed into a stable phase of gibbsite and halloysite as the reaction proceeded.

Weathering microenvironments on feldspar surfaces: implications for understanding fluid-mineral reactions in soils

“…Casey et al, 1989;Muir et al, 1989;Hellmann et al, 1990). Kawano and Tomita (1994) found that a tens of nm thick leached layer on albite grains hydrothermally reacted with distilled water had subsequently crystallized to Na-smectite and mica via a non-crystalline and fibrous material. Tazaki (1986) described comparable processes during natural weathering of microcline perthites from Brazil whereby 'primitive-clay precursors' were inferred to have been produced by crystallization of a leached layer.…”

“…Parham, 1969;Eggleton and Buseck, 1980;Eggleton, 1987); (3) topotactic replacement of feldspar; and (4) replacement of a 'leached layer' produced by chemical weathering of feldspar (e.g. Tazaki, 1986;Kawano and Tomita, 1994). Coatings formed by mechanisms 3 and 4 would be expected to have a low porosity and permeability and a coherent interface with the feldspar grain and so will potentially have a greater impact on weathering rates than porous aggregates of clay minerals that are likely to form by mechanisms 1 and 2.…”

Mineral weathering in trachydacitic-derived soils and saprolites involving formation of embryonic halloysite and gibbsite at Mt. Amiata, Central Italy

“…Kawano and Tomita (1996) also found gibbsite and halloysite in the early stages of weathering of K-feldspar. This study is particularly relevant to the Mt.…”

Surface alteration of plagioclase during acid dissolution

“…In investigations of the products of naturally weathered potassium feldspar, Kawano and Tomita detected a surface precipitate of amorphous aluminium hydroxide with fibrous and spherical particle morphology. 16 …”