Their shells have similar growth ripples. The sample above is sandstone with abundant phosphatic brachiopod ( Lingulata) shells from the Ordovician of Estonia. “Conchoidal fracture” is named so because the curved lines on the fracture surface resemble the rippling growth lines on the shells of clams or conchs. The sample from Kazakhstan is 14 cm in width. Red color is given by hematite impurities. There is a red variety of chalcedony which is known as carnelian on the picture above. Width of sample 11 cm.Ĭhalcedony is a very fine-grained rock type that is compositionally close to quartz and chert. Quartz is not the only mineral without cleavage, but it is the best known and most widespread of them. Hence, it has no preferred planar surfaces along which to break. Quartz is a mineral with crystalline structure, but there are no planes of weakness inside the crystal. It was also widely used as a tool-making material by our ancestors. Width of the sample is 12 cm.Ĭhert is another rock type that commonly exhibits a conchoidal fracture. However, the fracture surface is not as smooth and shiny because microscopically it is pretty rough. The sample from Mexico is 11 cm in width.Įven crystalline rocks like basalt may display conchoidal fracture if they are fine-grained enough.
It has no crystalline structure and it is brittle at room temperature when force is applied rapidly. This rock sample looks very much like obsidian but it is actually an amorphous fine-grained asphalt known as gilsonite. We need to apply more force if we want to shatter a large piece of material and much less if we just want to peel off a small flake from the edge.Īnother piece of obsidian with nicely curving conchoidal fracture surfaces.
The fracture can occur only if the blow is energetic enough to peel off a flake. So the curving lines are like the fronts of seismic energy recorded on the fracture surface. The energy of the blow spreads in the material like seismic waves travels through the Earth. This is where the brittle deformation starts. Why is the fracture surface smoothly curving? Because we apply a force to only one point. It was the way our ancestors made sharp cutting tools. If the force is applied correctly, a flake of obsidian is peeled away leaving obsidian with a smoothly curving fracture surface and sharp edges. Smoothly curving fracture surface develops when force is rapidly applied to brittle objects like hitting a piece of obsidian (volcanic glass) with a hard pointy object. This rock type was highly valued during the Stone Age because it makes a fine cutting blade if treated (fractured by precise and forceful blows) correctly. Obsidian is famous for its conchoidal fracture surface. This is the case with many fine-grained (aphanitic) rocks. However, conchoidal fracture is common in crystalline materials also if they have no cleavage (like mineral quartz), or if they are composed of very small mineral grains so that the fracture surface which is actually zigzagging between the grains appears smooth to our eyes. Such a curving fracture surface is characteristic of glass and other brittle materials with no crystal structure. Conchoidal fracture is a smoothly curving fracture surface of fine-grained materials which have no planar surfaces of internal weakness or planes of separation (no cleavage).
0 kommentar(er)
