What defines a gneiss?
1 Answer
Multiple things, depending on the context of the question.
Explanation:
Personally, I define gneiss as a foliated metamorphic rock that most commonly is formed from shale (through a series of other transformations), as a result of regional metamorphism (formed under a lot of pressure and heat, over a fairly large area). Additionally, it usually has a banded appearance, which isn't unique among metamorphic rocks, but it does help it separate it from the non-banded rocks.
Most people can't legitimately say that gneiss can be defined by its composition, because it varies, but most gneiss samples will have bands of quarts (silicon dioxide) and feldspar (silica and aluminum containing). As a result, I don't primarily define gneiss from its chemistry.
Sometimes it's nice to see it compared to other rocks to help choose your own path to its definition. Below is an ID chart used to determine the rock type of some popular metamorphic rocks.
Here are some different gneiss rock samples, just to show you the variation:
