Abstract
Nyaung iron is structurally a medium octahedrite (IIIAB) with several large inclusions of troilite-daubreelite aggregates and rare small inclusions of rhabdites. Primary cooling related textures include Widmanstätten structures on mesoscopic scale and large variety of plessites (net, comb and black variety); the latter formed under low temperature through martensitic transformations. Neumann lines (<130 kb) and widely spread matte kamacites (>600 kb) refer to shock- induced secondary changes. Post- shock annealing event is well documented with a large number of microtextures which include disappearance of older Neumann lines and development of decorated Neumann lines, sub-grain boundaries associated with triple point bearing polygonised kamacite, polygonised plessite and spherodised taenite particles; local recrystallisation coupled with degeneration of ε - kamacite, plastic flowage of taenite lamellae and moving grain boundary pinned with taenite beads. Mean cooling rate for Nyaung octahedrite is estimated ~385°C/ Ma with cooling rate variation by a factor of 3.2. Rapid cooling rate is explained in terms of undisturbed fractional crystallisation of small molten core insulated within a very thin silicate mantle.