Autocrysts

Autocrysts are crystals that have formed within a rock, which has subsequently undergone some form of deformation. This deformation can affect the autocryst's original shape, leading to a variety of textural features. The term "autocryst" itself implies that the crystal grew in situ, as opposed to being an inclusion of pre-existing material.

The key characteristic of autocrysts is their relationship with the surrounding rock matrix. If the rock is deformed after the autocryst has formed, the autocryst may be fractured, rotated, or have a strained internal structure. The matrix surrounding the autocryst will often show evidence of flow or recrystallization around the more rigid autocryst. This can result in a preferred orientation of the matrix minerals, which bends or wraps around the autocryst.

Common examples of rocks containing autocrysts include metamorphic rocks such as schists and gneisses. Minerals like garnet, staurolite, and porphyroblasts of feldspar or mica are frequently observed as autocrysts in these settings. The interpretation of autocrysts can provide valuable information about the pressure, temperature, and deformation history of a rock. Their presence and characteristics are crucial for understanding the geological processes that have affected the rock.