The geological unit called Charentes basin is composed of Eocene and Oligocene deposits, laid above karstic limestone formations of the Campagnien, in the North of the Aquitaine Basin. The kaolin clays of Charentes belong to this mainly continental, tertiary formation often referred as “siderolithic”, of which the principal outcrop is situated in the South of the Charente Maritime department, 35 miles going North-East from Bordeaux city. The quarries are scattered along a 20 mile long, 7 mile wide, North - South band.
The clay concentrations of economic value are composed of a succession of clays, sands and pebbles. This torrential-stream deposit, close to enlaced rivers, laid to the deposition of sandy-clayey materials, with a variable iron content, coming from a lateritic weathering, of the French “Massif Central” granites ((Kulbicki, 1956; Marchadour, 1980; Dubreuilh et Patel, 1982). The presence of numerous lignite rich levels indicates that the deposit was performed in the presence of a lot of organic matter, leading to important pedogenetic and diagenetic possibilities of evolutions. These chemical and mineralogical evolutions (dissolution–crystallization) allow the neo-formation of kaolin and gibbsite, as well as iron sulphide (Delineau, 1994).
At their basement, highly enlaced and with channel shapes, those deposits often fill karstic depressions, leading to the formation of “clay wells”. The juxtaposition of features are some times without explanations using the deposition laws, probably in relation with post-sedimentary strain phenomena, eventually linked to substratum collapse” (Delineau 1994). In the upper part of the series, the deposits are more regular, with lateral extensions up to several hundreds of meters.
Those complex geometries, with structures smaller than 20 meters, lead to particularly difficult recognition, estimation and exploitation phases. To this complex geometry, one should notice important lithology variations. The AGS company uses no less than 24 description codes and 8 colour codes, for its samples description. Those classes are subdivided to take into account the grade in organic matters, iron, titanium, potassium, the colour, the aptitude to flow …
Lignite formations are relatively frequent in Charentes clay deposits. Their thickness changes between some decimeters in lenses, to metric scale in continuous forms. These organic materials had some influences on kaolin deposited layers. Some of the observed influences are as follow: In gathered samples close to these organic materials, clays generally do not contain mica minerals, and especially in the neighborhood of Cuisian lignite, kaolinite is very well-ordered and the clay does not contain swelling clays with hydrazine. Occurrence of gibbsite is always associated with these well-ordered kaolinites. Normally occurrence of hyper-aluminous clays due to the existence of gibbsite is one of the interesting subjects in the history of these kaolins. This causes many discussions about the origin of this mineral. The existence of gibbsite has been mentioned in the studies of Languine and Halm (1951), Caillere and Jourdain (1956) , Kulbickie (1956), Dubreuilh et al. (1984) and Delineau (1994)
Generally, kaolin deposits have been covered with colored sequences of sand. In some quarries, we can observe red, green and some times black sands. The black color might be due to the existence of pyrite and organic materials. Sometimes fossil woods (floated branches and trunks of trees) can be found and with the coarse size of pebbles (several millimeters) are evidence of a high energy transportation. This type of sand can have some influences on the leaching by mineral and organic acids produced by pyrite and organic materials, of the lower kaolin deposits. Thiry has found that generally these kaolins contain rather well-ordered kaolinite. Obviously, the level of crystallization can control technical properties of kaolinite as well as the structural impurities. The high energy current can interrupt the continuity of the settled layers of kaolin and reduce the simplicity of the estimation methods.
Gibbsite is not stable in presence of quartz and it will be changed into kaolinite minerals, so gibbsite has formed after the deposition and we can called it neo-formation gibbsite (Thiry 1984). Now, the main question is about gibbsite formation in the middle of kaolin series. Due to the pH of leaching, a dissolution of Al2O3 or SiO2 can occur (podzol or laterite profile) The first theory tries to describe this with podzol profiles: it assumes the leaching of silica from minerals and accordingly the gibbsite formation from leached kaolin. We thus should find the hyper-aluminous materials, containing gibbsite in the lower series of kaolin. On the other hand, a second theory proposes the procedure of Al leaching in a very acid medium, in deposited organic materials (lignite) with clay. The organic materials can accelerate the solubilization and transportation of Al ions with intervention of organic complex. Thiry (1984) proposed the following scenarios for this dissolved Al. Dissolved Al can be transported with complex to a less acidic medium. 1- if there is any quartz in this medium, it can react, and we obtain well-ordered kaolinite minerals 2- In absence of quartz, Al will precipitate as a hydroxide mineral: gibbsite. This theory alone cannot explain what is observed in-situ in the some samples of the “BD” deposit, where gibbsite was found in sandy layers containing quartz.
Ref: Koneshloo M.,Viches M.,Rolley J.P. (2005), "Modelling of sedimentary deposits of kaolin clays, in continental environment: application to the Charentes deposits, France,20th World Mining Congres.