Definition: composed primarily of aluminum-rich minerals with such as garnet, kyanite, andalusite, or sillimanite.

The composition of sediment in intermontane areas or alluvial plains may be classified as an aluminous metamorphic rock. This is not to imply the landform is rock but that it is primarily composed of sediments of that composition and may have rock fragments of other compositions and/or have windblown sediments from undetermined origins as well.

Types of Occurrences
Hornfels: Typically formed by heating a rock or sediment layer by igneous activity (contact metamorphism), usually from below. The resulting rock is composed of fine grains close to the same size with no preferred orientation.

Schist: Metamorphic rock with coarse minerals in parrallel alignment, schistosity. Metamorphic minerals given in the name are often used in the field in order to make stratigraphic correlations i.e. a garnet-sillimanite schist has both garnet and sillimanite (Yardley, 1991).

Gneiss: Typically formed in mountain building processes (regional metamorphism). The rock is composed of layers of granular (visible to the eye without help) mineral grains with narrow layers of smaller platey minerals that may be darker in color.

Undifferentiated: The type of metamorphism that formed the rock is unknown or the rock shows evidence of both types of metamorphism.

Outcrop Photograph

(kyanite schist, 

100K quadrangle)

Practical Implications

Building Material: Mica and chlorite schists have been used as building stones and greenstone has been used as crushed stone (Dietrich & Skinner, 1979). Some schists contain asbestos minerals.

Economic: Talc schists are a commercial source of talc. Kyanite, andalusite, and sillimanite schists are used in ceramic, chemical, electrical and metallurical industries (Dietrich & Skinner, 1979).

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(last modified 10/16/98)