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.
This is the case with many fine-grained aphanitic rocks. Obsidian is famous for its conchoidal fracture surface. 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.
Width of sample is 11 cm. 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. If the force is applied correctly, a flake of obsidian is peeled away leaving obsidian with a smoothly curving fracture surface and sharp edges. It was the way our ancestors made sharp cutting tools. Why is the fracture surface smoothly curving?
Because we apply a force to only one point. This is where the brittle deformation starts. The energy of the blow spreads in the material like seismic waves travels through the Earth. So the curving lines are like the fronts of seismic energy recorded on the fracture surface. The fracture can occur only if the blow is energetic enough to peel off a flake. 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.
Another piece of obsidian with nicely curving conchoidal fracture surfaces. The existence of water speeds up this process of obsidian. Although the newly formed obsidian has a low water content, usually less than 1 percent by weight, it is slowly hydrated to form perlite when exposed to groundwater. How can you identify obsidian? The lack of a crystalline structure indicates that obsidian is not a true mineral and causes extreme sharpness of the fracture surfaces.
Since prehistoric times, obsidian has been used in cutting tools and is still used today in surgical scalpels. Explore obsidian where cooling is rapid in the margins of lava flows. Glass Buttes in central Oregon is one of the best places to find obsidian in the U. Pieces of fist size can be found on the surface in abundance here.
It has a distinctive appearance of smooth glass. Obsidian is a frozen liquid that contains small amounts of mineral impurities.
See the color Because pure obsidian is usually dark, on rare occasions it may also be almost white. Consider the effect of impurities on the obsidian color For examples, iron and magnesium may make obsidian dark green. Hematite or limonite add a red or brown color to the obsidian. A lot of microscopic rock and mineral particles usually cause the jet black color most closely associated with obsidian.
It can cause the obsidian to have a gold or silver shine if the bubbles were spread almost flat. Obsidian has a strong conchoidal fracture and luster. It means that the top of the fracture is curving smoothly like a seashell. Obsidian appears to be black.
Minute inclusions and tiny crystals in the glass create it hue. Due to the near absence of mineral crystals in the glass, this smooth, curved form of fracture surface occurs.
It is thanks to minerals and mineraloids like the quartz family and obsidian and their tendency to fracture in a conchoidal pattern that our ancestors were able to survive and thrive. The undulating shell-like concavities bowl-like depressions and convexities rounded, domed elevations that result when a piece of obsidian or flint is struck by a stone, stick, or bone produces serrated razor-sharp edges that enabled stone age cultures to produce arrowheads, spear points, knives, scrapers, drills, and ax blades in processes known as knapping and flaking.
Knapping is a fun lapidary art form that you, too, can pursue with proper care and precautions to produce stone age tools from natural rocks and minerals. Thursday, November 11, JBT Minerals.
0コメント