Original paper
Der Mineralbestand des Mittleren Muschelkalkes von Nagold, Württemberg
[Mineral composition of "Mittlerer Muschelkalk" from Nagold, Württemberg]
Lippmann, Friedrich; Pankau, Hans-Georg
Neues Jahrbuch für Mineralogie - Abhandlungen Band 158 Heft 3 (1988), p. 257 - 292
21 references
published: Feb 25, 1988
DOI: 10.1127/njma/158/1988/257
ArtNo. ESP154015803005, Price: 29.00 €
Abstract
A section of “Mittlerer Muschelkalk” was studied for its mineral composition on the basis of two bore-holes located near Nagold, Württemberg, Germany. The 24 samples studied contain more or less gypsum and/or anhydrite with corrensite and illite as the main clay minerals. However, (trioctahedral) chlorite occurs instead of corrensite in the lower part of the section where it may form up to 50 percent of the clay fraction. In the anhydrite rock of the lower part of bore-hole No. 9, a layer influenced by volcanic material was found to contain montmorillonite and rectorite in addition. Minor kaolinite was encountered only in one sample near the top of the section. The microscopic grain-size fractions consist essentially of quartz, dolomite, feldspars, micas, and polymineralic aggregates. A considerable part of the quartz and feldspar grains show authigenic overgrowths. Different from the overlying marine “Hauptmuschelkalk”, from which authigenic K-feldspars only have been reported, neoformation of albite occurs as well. An inquiry into the conditions of formation for the Mg-bearing clay minerals in terms of mineral stabilities shows that increased salinity due to evaporation was not the decisive factor leading to their formation as this is the case for gypsum and anhydrite. Rather, corrensite and chlorite are indicative of another aspect of a restricted basin, in that they attest to the influx of alkali-bearing brines derived from weathering on the surrounding continents.
Keywords
Evaporite • restricted basin • neoformation • trioctahedral clay minerals • authigenic feldspars