Gneiss is a type of metamorphic rock. The minerals in gneiss may come from rocks which were originally either igneous or sedimentary. They were heated and squeezed, and the minerals recrystallized.
Orthogneiss is gneiss got from igneous rock (such as granite). Paragneiss is gneiss got from sedimentary rock (such as sandstone).
In gneisses, minerals tend to be foliated: layered and segregated into bands. Thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral.
Lewisian gneiss[change | change source]
The Lewisian complex or Lewisian Gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane. These rocks are of Archaean and Palaeoproterozoic age, ranging from 3.0–1.7 Ga.