Poikiloblastic

Poikiloblastic is a term used in geology, specifically in the study of metamorphic rocks, to describe a texture characterized by the presence of large, often irregular, mineral grains that have grown within a finer-grained matrix of other minerals. These large grains are referred to as poikiloblasts. The poikiloblastic texture arises when a mineral grows rapidly or at a slower rate than the surrounding minerals during metamorphism, enclosing smaller crystals of the matrix as it does so. The enclosed mineral grains, or inclusions, are typically smaller and more numerous than the poikiloblasts themselves.

The size and shape of poikiloblasts can vary significantly depending on the specific minerals involved and the conditions of metamorphism. The inclusions within the poikiloblasts often retain their original shape and orientation, providing clues about the pre-metamorphic rock or the deformation history. Common minerals that form poikiloblasts include garnet, staurolite, cordierite, and kyanite. The matrix minerals are typically quartz, feldspar, or micas.

The poikiloblastic texture is indicative of a specific sequence of mineral growth during metamorphism. It suggests that the poikiloblastic mineral was stable and growing while the surrounding matrix minerals were also present. The texture can be observed in a wide range of metamorphic rocks, from low-grade greenschists to high-grade granulites. Understanding poikiloblastic textures helps geologists interpret the thermal and pressure history of rocks.