Forbach granite
The Forbach Granite , who also Raumünzach granite is called, is one in the northern Black Forest occurring granite . It is part of the northern Black Forest granite massif, which also includes Bühlertal granite and Wildbad granite . Forbach granite is mainly open in the Rastatt and Calw districts . The most important mining area for Forbach granite used to be in the middle Murgtal near Forbach . Today it is only broken near Raumünzach .
Origin and occurrence
The penetration of the molten rock, which later cooled to form Forbach granite, took place in the Asturian phase of the Variscan mountain formation . The Asturian phase is stratigraphically dated roughly to the border Westfal / Stefan des Oberkarbons . This means that Forbach granite is around 305 million years old. Its coarseness indicates a relatively slow cooling of the melt.
In the northern Black Forest granite massif, which is bordered to the west by the Upper Rhine Valley to Baden-Baden and Offenburg , to the east by the Enz valley near Enzklösterle and Bad Wildbad and to the south-east by Offenburg and Schönmünzach , the occurrence of Forbach granite is in the middle Murgtal in the area before Forbach . The granites occurring in the northern Black Forest granite massif differ in grain, color and texture, and the arrangement of the minerals. In the south, the Forbach granite changes into a weathered porphyry zone, which is characterized by numerous fissures in which pegmatite and aplite dikes are located. This is followed by the Wildbach valley around Raumünzach and Schönmünzach, where the Forbach granite shows large crystalline rock textures.
Rock description and mineral inventory
It is a light, gray to pink colored two-mica granite. Biotite gives this granite a clearly structured structure. It is a medium to coarse-grained rock in which the alkali feldspars are up to 5 cm in size. In addition to a quartz content of 26 percent, clearly recognizable as greasy, shiny deposits, the contained alkali feldspar, the orthoclase, is reddish in color and gives this natural stone its typical color. The proportion of alkali feldspar is 44 percent and the proportion of plagioclase 20 percent, biotite 5 and muscovite 5 percent. Opaque minerals and apatite contain less than 1 percent, monazite and xenotime rarely .
Economic importance and use
While the fine-grain Bühlertal granite was previously quarried in numerous quarries as stone for construction purposes, in 2008 Forbach granite is only mined in a quarry near Raumünzach, a district of Forbach, as Raumünzach granite, a local name for Forbach granite. Due to its high quartz content, this granite is highly wear-resistant and chemically resistant. This granite is used as floor and stair covering, as stone and for solid buildings, monuments and facades. It can be polished permanently.
Examples of use for the Forbach granite are the town hall in Bonn, the Federal Archives in Koblenz , the Schwarzenbachtalsperre in the Black Forest, the Swabian Center in Stuttgart and the Coca-Cola administration in Essen.
Natural monuments
In the distribution area, the Forbach granite forms some rock formations that are registered as nature reserves or natural monuments. The most famous are the Lautenfelsen and the Organ Rocks near Gernsbach or the Giersteine near Forbach-Bermersbach (both locations in the Rastatt district ) and the Murg Valley.
literature
- W. Dienemann and O. Burre: The usable rocks of Germany and their deposits with the exception of coal, ores and salts, Enke-Verlag, Stuttgart 1929, p. 2
- Otto Franz Geyer, Manfred P. Gwinner: Geology of Baden-Württemberg . 3rd completely revised edition. Schweizerbart, Stuttgart 1986, ISBN 3-510-65126-X .
- Wolf-Dieter Grimm: picture atlas of important monument rocks of the Federal Republic of Germany. Published by the Bavarian State Office for the Preservation of Monuments, Lipp-Verlag, Munich 1990, ISBN 3-87490-535-7 , rock no.011.
