Chlorite Vermiculitization and Pyroxene Etching in an Aeolian Periglacial Sand Dune, Allen County, Indiana

Abstract

Abstract--Weathering has etched and deeply-dentieulated the constituent orthopyroxenes, and chlorite has been transformed to vermiculite in the upper 3 m of an aeolian, periglacial sand dune formed in northeastern Indiana about 13,000 b.p. Pyroxene weathering begins with the development of cleavageparallel etch pits on {010} and ( 100 } surfaces. These pits coalesce and eventually crop out on basal surfaces as denticulations. The mean denticulation size increases logarithmically toward the surface, and the denticulation size of the orthopyroxenes is a quantifiable feature of the weathering process. Ferruginous pendants, microboxworks of iron oxides, and other indications of iron redeposition within the orthopyroxene microenvironments were not observed.Chlorite in the dune has been weathered to vermiculite. The parent chlorite is a high-Fe variety, and the transformation to vermiculate does not involve the development of a chlorite/vermiculite intermediary phase. Fe 2 § is oxidized as part of the transformation process and this iron is retained in the sediment as discrete goethite and as crystalline and noncrystalline coatings on the dune grains. The vermiculite from depths shallower than 64 cm is only partly expandable and is completely collapsed by K-saturation or heat treatment. This is a hydroxy-A1 vermiculite and its formation is typical of intense weathering under the acid conditions prevalent at the dune surface.

Vermiculitization of Phlogopite in Metagabbro, Central Turkey

“…Negligible expansion as indicated by the basal spacings at 9.4 and 14 ]1 with ethylene glycol and glycerol, incomplete contraction on heating, expansion and/or an increase in intensity of basal spacings of (002) and (004) by glycerol solvation following Mg-saturation, and contraction on heating at 550~ following K-saturation in these samples are consistent with the hydroxy-Al interlayer in the two-layer hydrate and dehydrated types of vermiculite (April and Newton, 1983;Buurman et al, 1988;Barnhisel and Bertsch, 1989;Douglas, 1989;Argast, 1991).…”

Occurrence and origin of soils with andic properties in Calabria (southern Italy)

“…Pyroxene crystals were moderately to strongly weathered, with a typical linear parallel pattern and/or elongated etch pits aligned along the longer crystallographic axis and the cleavage planes, and showed peculiar denticulated (multi-needle) margins on the basal faces (e.g. Argast, 1991;Velbel and Losiak, 2010). Due to the small size of pyroxene crystals and to their state of weathering, it was only possible to obtain reliable analysis for the minerals of SPI and TAU (Table 6a).…”

Weathering of Spodumene to Smectite in a Lateritic Environment

“…However, not all pyriboles weather to clay minerals by topotactic transformation. A number of studies have shown that weathering of hornblende may proceed by congruent dissolution along crystallographic directions and precipitation of clay minerals and iron oxides from solution with no crystallographic relationship persisting between hornblende and clay minerals (Berner and Schott, 1982;Anand and Gilkes, 1984;Velbel, 1989, Argast, 1991. Topotaxial transformation of pyriboles to layer silicates is presumably facilitated if minimum adjustments in structure and composition are involved (Gilkes et aL, 1986).…”