Phyllite

Physical Properties

Hardness: 1-2 (Mohs); Color: Grey, black, or greenish; Luster: Silky, pearly, or metallic sheen; Crystal structure: Microcrystalline (foliated phyllitic texture); Cleavage: Perfect slaty-to-phyllitic cleavage.

Formation & Geological History

Formed through regional metamorphism of pelitic (clay-rich) sedimentary rocks like shale. It represents a metamorphic grade between slate and schist. Formed under low-to-medium temperature and pressure environments.

Uses & Applications

Primarily used as decorative stone in landscaping, flooring, and exterior cladding. Historically used for roofing slates and as an aggregate in construction.

Geological Facts

Phyllite is distinguished from slate by its characteristic 'phyllitic sheen,' which is caused by the alignment of tiny mica flakes (sericite or chlorite) that are just large enough to reflect light but too small to be seen as individual grains.

Field Identification & Locations

Identify in the field by its crinkled or wavy foliation and characteristic shiny, silky luster. It is common in mountain belts and metamorphic terrains like the Appalachians or the Scottish Highlands.

Identified on: 4/30/2026

Mode: Standard