What is "pseudotachylite"

n. (label en mineralogy) A dark, glassy rock generated by frictional heat within faults

Pseudotachylite or Pseudotachylyte (original spelling) is a cohesive glassy or very fine-grained rock that occurs as veins and often contains inclusions of wall-rock fragments. Pseudotachylite is typically dark in color; and is glassy in appearance. It was named after its appearance resembling the basaltic glass, tachylyte. Typically, the glass is completely devitrified into very fine-grained material with radial and concentric clusters of crystals. The glass may also contain crystals with quench textures that formed via crystallization from the melt. Chemical composition of pseudotachylyte generally reflects the local bulk chemistry. Pseudotachylyte may form via frictional melting of faults, in large-scale landslides, and by impact processes. Many researchers often define the rock as one formed via the melting (c.f. fault rock). However, the original description/definition by Shand did not include interpretation about its generation, and it is suggested that there are pseudotachylytes formed via comminution without melting (e.g. Wenk, 1978).