Product Code Database
Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic Stratum. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume.
As a rule, greywackes do not contain , but organic remains may be common in the finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably indurated by recrystallization, such as the introduction of interstitial silica. In some districts, the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates.
Although the group is so diverse that it is difficult to characterize mineralogically, it has a well-established place in Petrology classifications because these peculiar composite arenite deposits are very frequent among Silurian and Cambrian rocks, and are less common in Mesozoic or Cenozoic strata. Their essential features are their gritty character and their complex composition. By increasing metamorphism, greywackes frequently pass into mica-, chloritic schists and sedimentary .
Greywackes are mostly grey, brown, yellow, or black, dull-colored sandy rocks that may occur in thick or thin beds along with and . Some varieties include feldspathic greywacke, rich in feldspar, and lithic greywacke, rich in other tiny rock fragments. They can contain a very great variety of , the principal ones being quartz, orthoclase and plagioclase feldspars, calcite, and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as felsite, chert, slate, gneiss, various , and quartzite. Among other minerals found in them are biotite, Chlorite group, tourmaline, epidote, apatite, garnet, hornblende, augite, sphene and . The cementing material may be siliceous or argillaceous and is sometimes calcareous.
They were an early object of geological study in Britain where the Geological Society was founded in 1807, and excited much public interest in geology. Greywacke was interesting because it was found in many places in Britain and its occurrence in particular places was evidence of the pattern of Stratum that had been laid down. (2025). 9781788163781, Profile Books. ISBN 9781788163781
Aside from its structural uses, greywacke stone (or molds taken from it) is valuable to practitioners of traditional motion picture miniature photography, because due to its unusually mixed nature, it remains looking natural when portraying a wide range of miniature scale ratios, from 1:1 to as high as 1:600.
File:Moehnesee wall 05.JPG|Wall of greywacke at Möhne Reservoir dam, Germany
|
|
