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It often occurs in amorphous or fine-grained mineraloids such as flint, opal or obsidian, but may also occur in crystalline minerals such as quartz. Subconchoidal fracture is similar to conchoidal fracture, but with less significant curvature. Note that obsidian is an igneous rock, not a mineral, but it does illustrate conchoidal fracture well.
A conchoidal fracture is a break or fracture of a brittle material that does not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically slightly concave, showing concentric undulations resembling the lines of growth of a shell". [ 1 ]
The classic example is conchoidal fracture, like that of quartz; rounded surfaces are created, which are marked by smooth curved lines. This type of fracture occurs only in very homogeneous minerals. Other types of fracture are fibrous, splintery, and hackly.
It is the emission of light from the fracture (rather than rubbing) of a crystal, but fracturing often occurs with rubbing. Depending upon the atomic and molecular composition of the crystal, when the crystal fractures, a charge separation can occur, making one side of the fractured crystal positively charged and the other side negatively charged.
Planar fractures are parallel sets of multiple planar cracks or cleavages in quartz grains; they develop at the lowest pressures characteristic of shock waves (~5–8 GPa) and a common feature of quartz grains found associated with impact structures. Although the occurrence of planar fractures is relatively common in other deformed rocks, the ...
Spodumene is an example where the planes meet at a 90 degree angles. Hornblende is an example where the planes intersect at 56° and 124°. Cubic cleavage occurs when there are three cleavage planes intersecting at 90 degrees. Halite (or salt) has cubic cleavage, and therefore, when halite crystals are broken, they will form more cubes.
The shear fracture envelope is approximated by a pair of lines that are symmetric across the σ n axis. As soon as the Mohr circle touches the lines of the fracture envelope that represent a critical state of stress, a fracture will be generated. The point of the circle that first touches the envelope represents the plane along which a fracture ...
For quartz and feldspar-rich rocks in continental crust, the transition zone occurs at an approximate depth of 20 km, [2] at temperatures of 250–400 °C. At this depth, rock becomes less likely to fracture , and more likely to deform ductilely by creep because the brittle strength of a material increases with confining pressure , while its ...