Gneiss (rock)

The term “gneiss” is used loosely to encompass many different mineral combinations and a variety of structures. It includes a great many rocks of uncertain origins.

Definition

In the narrowest meaning of “gneiss” (pronounced “nice”), it is defined as a coarse-grained, feldspar-rich, metamorphicrock with a parallel structure (foliation) that assumes the form of streaks and bands.

Gneiss is primarily identified by its structure rather than by its composition. It is a medium- to coarse-grained banded or coarsely foliated crystalline rock. The rock is characterized by a preferred orientation of platy grains such as biotite, muscovite, or hornblende, or the segregation of minerals into bands or stripes. Unlike schist, gneiss is more often characterized by granular minerals than by platy minerals. Most gneisses are light to dark gray, pink, or red because of the high feldspar content.

Overview

Gneiss is exposed in regions of uplift where erosion has stripped away surficial rocks (sediments and lower grade metamorphic rocks) to expose rocks that have been altered at depth. In North America, gneiss may be found in New England, in the central Atlantic states, the Rockies, the Cascades, and much of Canada.

Gneiss, with mineralogy similar to that of granite, has similar uses except that it is generally restricted by the presence of a higher percentage of ferromagnesium minerals and micas, which weather rapidly to weaken and discolor the finished stone. The major use is as riprap, aggregate, and dimension stone. Wavy foliation in polished slabs results in an especially decorative stone for monuments.

The most common gneisses are similar to granite in composition and resemble granite except for the foliation. The predominant minerals are equidimensional grains of quartz and potassium feldspar, usually microcline. Sodium plagioclase may also be present. Biotite, muscovite, and hornblende, alone or in combination, are the most common minerals that define the foliation. Other minerals, almost exclusively metamorphic in origin, that may be present in minor quantities include almandine garnet, andalusite, staurolite, and sillimanite.

True gneiss is a high-grade metamorphic rock formed by recrystallization and chemical reaction within existing rocks in response to high temperature and pressure at great depths in the Earth’s crust. Often the precursor rock is a feldspar-rich sandstone, a clay-rich sediment such as shale, or granite. Gneissic fabric may be produced in some igneous rocks by flowage within a magma. Some gneisses are formed by intrusion of thin layers of granitic melt into adjacent schists, which produces lit-par-lit structure or injection gneiss.

The rock name is often modified by the addition of a term to indicate overall composition, unique mineral, or structure. Thus, granitic gneiss or gabbroic gneiss may distinguish between gneisses composed predominantly of quartz and feldspars and those composed of calcium-rich feldspar and ferromagnesian minerals such as pyroxene. In like manner, garnet gneiss or sillimanite gneiss may be used to flag the appearance of an important metamorphic mineral. The term “augen gneiss” (Augen being the German word for “eyes”) is used to describe those rocks which have prominent almond-shaped lenses of feldspar or feldspar and quartz, which are produced by shearing during the formation of the rock.