On May 27th I joined the first of two field trips this semester about industrial minerals in Bavaria. We saw several active open pits. The first location is an active clay pit (Fig. 1). Here relatively pure kaolinite and lignite that are closely inter-calated are being mined to produce chamotte. The clays and lignite are of pliocene age and were deposited in the glacial out-wash valley of the ancient Naab river. The mine is host to a large pond of acid mine drainage in the centre that is pumped out and neutralised with burned lime. Contrary to many acid mine drainage problems this is caused by the high organic content of the clay and lignite. The humic acids are also respondsible for the colors.
Fig. 1: Clay pit with acid mine drainage in center. Dark layers are lignite within kaolinite.
One of the pecularities of this clay pit are siderite concretions (Fig. 2). Peculiar as such as these clays are already very iron poor by nature. Siderite concretions are fairly common and can be the size of a football. What strikes in the field is that these concrections almost always have a small piece of coal or wood at their core. Not visible to the naked eye is that the clays are strongly iron depleted in the vicinity of the concretions. Sulphure isotope data points to the source of formation. Bacterial activity is to blame!
Fig. 2: Yellowish-brownish siderite concretion around wood chips in kaolinite.
The coal is not boring either. Below (Fig. 3) you can see a well preserved fossil leaf about 3 to 4 cm long. I have no idea what exactly this is but the pliocene coals of the region are known for their well preserved and rich plant fossils.
Fig. 3: Leaf on pliocene coal.
And finally a brief look (Fig. 4) into the oven! The sieved and prepaired clay is calcined at ca. 1200°C for a period of roughly 6 hours. The lignite supplies about 1/3 of the required energy, the remaining 2/3 supplied by natural gas. Fine-spread lignite within the kaolinite clay generates a high-porosity chamotte. Clay with few or no coal burns to a dense, low-porosity chamotte.
Fig. 4: Readily calcined and still glowing chamotte exiting the oven.
Thanks to my PhD supervisor for leading this interesting field trip and to
Rohstoffgesellschaft mbH Ponholz for showing us their mine and plant!