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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]
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.
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.
Quartz is, therefore, classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar. [10] Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation ...
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 instance, water and quartz can react to form a substitution of OH molecules for the O molecules in the quartz mineral lattice near the fracture tip. Since the OH bond is much lower than that with O, it effectively reduces the necessary tensile stress required to extend the fracture.
They originate as tension fractures that are parallel to the major stress orientation, σ 1, in a shear zone. They are subsequently filled by precipitation of a mineral, typically quartz or calcite. As soon as they form, they begin to rotate in the shear zone. Subsequent growth of the fracture therefore causes the vein to take on a sigmoidal shape.
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 ...