Schiefer

Schiefer, or slate, is a fine-grained metamorphic rock derived from clay- or mudstone beds that have been subjected to regional metamorphism at low to moderate temperatures. It is distinguished by slaty cleavage, a planar foliation that enables the rock to split readily into thin, durable sheets. This property has made slate a widely used material in roofing, flooring, and architectural finishes.

Formation and composition: Slate forms from pelitic sediments such as shale or mudstone that recrystallize under low-grade metamorphic conditions, typically greenschist facies. Metamorphic minerals such as micas (muscovite and illite) and chlorite align perpendicular to the compression direction, producing the characteristic foliation. The rock’s color varies with impurities and may range from gray to green, purple, or red-brown.

Properties and classification: Slate is relatively fine-grained and has a Mohs hardness around 3. Its defining feature is the ability to split into flat, durable sheets along slaty cleavage. It is generally dense and compact, with a weathering resistance that depends on mineral impurities and the presence of reactive inclusions. These qualities make it suitable for a range of exterior and interior applications.

Uses: Slate has long been used for roofing shingles and tiles, as well as flooring, wall cladding, and countertops. It is also used for cemetery stones and historically for billiard-table beds due to its flat, stable surface.

Distribution and production: Slate occurs in many regions worldwide. Historic and current production centers include Wales and Spain in Europe, parts of Germany and Italy, and in the United States (notably Vermont and Pennsylvania), among others.