Mineral facies developed by weathering of a meta-gabbro, Loire-Atlantique (France)

Abstract

A study of the weathering products developed from a plagioclase amphibolite (metagabbro) was carried out using a petrographic microscope, mineral separation, X-ray diffraction, electron microprobe and wet chemical analysis methods. The weathering profile was considered as having two parts: one where the structure of the source rock had persisted and the other where an essentially argillaceous material was present in a chaotic arrangement. It was apparent that rock structures governed the types of chemical systems active in forming clay minerals. These systems can be grouped into four types: 1. (1) Grain boundary reactions. These are the earliest manifestations of weathering. Amphibole—plagioclase boundaries give rise to iron beidellite from the feldspars. 2. (2) Primary plasma zones: internal breakdown of the metamorphic minerals results in heterogeneous plasma. In the lower layers (relict rock structure), plagioclases are less affected than amphiboles. The latter form a nontronite-talc-iron-rich gel assemblage. In the upper layers (argillized), both plagioclase and amphibole are altered to a vermiculitic mineral with a composition governed by that of the host mineral: aluminous dioctahedral for plagioclases, and ferromagnesian trioctahedral for calcic amphiboles. These vermiculites are altered to ferri-beidellites in the adjacent plasma. 3. (3) Secondary plasma zones: in the general plasma structure of the argillized zone, a large-grain (often > 100 μm in length) trioctahedral vermiculite of highly variable composition is formed. 4. (4) Fissure zones: walls of the fissures are sometimes covered with clay cutans which are progressively thicker as porosity increases. Whatever the size or position of these cutans, they are always composed of an iron beidellite—iron gel assemblage. MgAlFe coordinates.