Explanation:

Gneiss is a metamorphic rock form characterized by banding caused by segregation of different types of rock, typically light and dark silicates. Rather than an indication of specific mineral composition, the term is an indication of texture. The "gneissic texture" refers to the segregation of light and dark minerals. It is indicative of high-grade metamorphism where the temperature is high enough, say 600-700 °C, so that enough ion migration occurs to segregate the minerals.