Intergrowth

Intergrowth is a texture in which two or more crystalline phases occupy the same crystal grain or grow in a closely interwoven relationship within a single aggregate. The phases may be distinct minerals or end-members of a solid-solution system, and the interfaces between them can be coherent, semi-coherent, or incoherent.

Formation and textures: Intergrowths arise when multiple phases nucleate and grow together during crystallization from a melt or solution, or when a homogeneous solid solution separates into two end-members upon cooling or changing conditions (exsolution). The resulting textures are often lamellar or fibrous, but can also form interpenetrating networks or mosaic-like domains, depending on crystallographic relationships and diffusion rates.

Examples: A well-known example is perthite, where exsolution produces alternating lamellae of Na-rich and K-rich feldspar within a single crystal of alkali feldspar. Intergrowth textures are common in minerals that form solid solutions and subsequently exsolve or partition during cooling, including various feldspars, pyroxenes, and amphiboles, as well as other mineral pairs that crystallize together.

Analysis and significance: Intergrowth textures preserve information about crystallization histories, cooling rates, and post-crystallization processes. They can influence physical properties such as diffusion pathways, mechanical strength, and optical behavior. Study typically involves microscopy, electron diffraction, and microprobe analyses to determine phase identities, composition, and orientation relationships.

Distinctions: Intergrowth is distinct from simple inclusions or random polycrystalline mixing; exsolution is a common mechanism generating intergrowth textures. Proper interpretation requires consideration of crystallography, composition, and the thermodynamic history of the sample.