Phyllite

Physical Properties

Hardness: 1-2 (can be scratched by a penny if soft), Color: Silver-grey to greenish-grey, Luster: Silky or pearly sheen, Crystal Structure: Microscopic micas, Cleavage: Well-developed slaty cleavage, Specific Gravity: 2.72-2.73

Formation & Geological History

Formed through regional metamorphism of pelitic (shale-rich) sedimentary rocks. It represents a metamorphic grade between slate and schist. This process typically occurs in tectonic mountain-building zones.

Uses & Applications

Used primarily as decorative stone, roofing tiles (though less durable than slate), architectural stone, and sometimes as an aggregate in construction.

Geological Facts

The characteristic 'phyllitic luster' comes from the alignment of tiny mica crystals (sericite and chlorite) that have grown large enough to reflect light but are too small to see individually with the naked eye.

Field Identification & Locations

Identify in the field by its distinct wavy or crinkled foliation and its silky reflective sheen on the surface. It is often found in the Appalachian Mountains of the US or the Scottish Highlands.

Identified on: 4/14/2026

Mode: Standard