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Sedimentary rock composed of sand-sized grains in a fine-grained clay matrix. The sand-sized grains frequently include rock fragments of wide-ranging mineralogies (e.g., pyroxenes, amphiboles, feldspars, and quartz). The clay matrix may constitute up to 50% of the volume. Of the clay minerals, chlorite and biotite are most abundant. The matrix tends to bind the grains strongly and form a relatively hard rock.
Variants of GRAYWACKE
graywacke or dirty sandstone
This entry comes from Encyclopædia Britannica Concise. For the full entry on graywacke, visit Britannica.com.