Gneiss (pronounced “nice”) is a widely distributed, high-grade metamorphic rock formed deep within the Earth’s crust under extreme heat and pressure. Its most recognizable characteristic is its distinct, striped appearance, known as gneissic banding. This banding, a form of foliation, serves as the primary identifier for classifying the rock.
The Primary Mineral Ingredients
The mineral composition of gneiss is highly variable, but it commonly shares a chemical similarity with granite, making quartz and feldspar its most abundant minerals. These light-colored minerals, often referred to as felsic minerals, are responsible for forming the bright bands within the rock’s structure. These components often include potassium feldspar, such as orthoclase, and sodium-rich plagioclase feldspar.
The remaining dark bands are composed of mafic minerals, rich in magnesium and iron. The primary dark constituents are typically mica minerals, specifically biotite (black mica) and, less commonly, muscovite. Amphibole minerals, particularly hornblende, also contribute significantly to the dark coloration and density of these layers.
The specific proportions of these light and dark minerals will determine the exact color and overall composition of the gneiss. For example, a gneiss derived from a silica-rich parent rock will contain a higher percentage of quartz and feldspar, while one formed from a more iron-rich rock might have a greater concentration of biotite and hornblende. This mineralogical makeup provides the raw material that the metamorphic process then organizes into the rock’s defining striped pattern.
Defining Gneiss Texture
The texture of gneiss is defined by its coarse-grained structure and its unique foliation—the parallel alignment of its mineral grains. This alignment is a highly organized physical arrangement resulting from intense directional pressure deep underground. The specific texture is gneissic banding, characterized by layers often several millimeters to centimeters thick.
Banding occurs because the minerals recrystallize and segregate into distinct, alternating layers of light and dark material. During metamorphism, the lighter quartz and feldspar separate from the darker micas and amphiboles under non-hydrostatic stress. This process creates a texture where the individual mineral grains are large enough to be clearly visible.
The separation of minerals into these bands distinguishes gneiss from lower-grade metamorphic rocks like schist, which feature a finer, more planar foliation dominated by platy minerals. The result is a durable, crystalline structure where the foliation is pronounced but does not necessarily cause the rock to split easily along the band planes.
The Origin of the Material
Gneiss is the product of high-grade regional metamorphism, occurring over vast areas, typically in the deep roots of mountain ranges or within ancient continental cores. Formation requires high temperatures, often exceeding 600°C, and pressures ranging from 2 to 15 kilobars. These extreme conditions allow the original minerals to completely recrystallize without the rock fully melting, establishing the gneissic texture.
The original rock that undergoes this transformation is known as the protolith, and its identity determines the two main classifications of gneiss. Protoliths that were originally igneous rocks, such as granite or diorite, produce a final rock called orthogneiss. The chemical composition of the original magma dictates the mineral content of the resulting orthogneiss.
Conversely, paragneiss forms when the protolith was a sedimentary rock, such as shale, mudstone, or sandstone. The presence of certain minerals, like sillimanite or kyanite, can often hint at a sedimentary origin, as these minerals are aluminum-rich and commonly found in metamorphosed clay-rich sediments. The material that makes up gneiss, therefore, can be traced back to either volcanic or depositional origins.