Liesegang bands are poorly understood chemical structures often seen in rocks, especially sandstones. They were discovered more than a hundred years ago by the German chemist Raphael E. Liesegang, when he accidentally dropped a drop of silver nitrate solution on a layer of gel containing potassium dichromate, and concentric rings of silver dichromate started to form.
In sedimentary rocks, Liesegang bands appear well after the sediment has become a rock (that is, it got compacted and cemented). Stratification and lamination within the sansdtone are typically cross-cut by the Liesegang bands; fractures usually have a more obvious effect on the distribution and orientation of these.
The rocks shown here are turbidites of the Permian Skoorstenberg Formation, in the Karoo desert of South Africa. This Liesegang banding developed in the neighborhood of a small thrust and consists of brown bands of iron oxide that entirely 'ignore' the original lamination of the sandstone (not visible in the photos), but clearly like to precipitate along some of the fractures in the rock.
Very interesting patterns. Would make nice wonder stones, or polished slabs.
ReplyDeleteWe have a lovely red-purple banded sandstone here in Central NYS from Potsdam, NY. I find it in glacial outwash deposits near my home - 150 miles south of Potsdam. The banding is cross deposited in some specimens creating a mystery as to the orientation of the bedrock when the iron oxide ground water left its evidence.
ReplyDeleteCould the liesegang effect be responsible for this phenomena ?
Dave Wade