Slate

Explore slate, a fine-grained foliated metamorphic rock with perfect cleavage.

Overview

Slate is a fine-grained foliated metamorphic rock that forms from the low-grade metamorphism of shale. It is characterized by perfect cleavage, allowing it to split into thin, flat sheets. Slate has been used for roofing, flooring, and writing surfaces for centuries.

What Is Slate?

Definition: Slate is a fine-grained foliated metamorphic rock formed from the low-grade metamorphism of shale or mudstone. It has perfect cleavage, splitting into thin, flat sheets. Composition: Primarily: - Clay minerals (recrystallized) - Quartz (fine-grained) - Mica (very fine-grained, gives the cleavage) - Sometimes chlorite or other minerals Color: Usually gray, black, green, or red. Can be mottled or variegated. Texture: Very fine-grained, feels smooth. Perfect cleavage into thin sheets.

Formation and Occurrence

How It Forms: Slate forms from the low-grade metamorphism of shale: - Low temperature (200-300°C) - Low to moderate pressure - Directed pressure creates the cleavage The fine-grained texture of the original shale is preserved, but minerals are recrystallized and aligned. Where It's Found: - Areas of low-grade regional metamorphism - Often in sequences with other low-grade metamorphic rocks - Famous quarries in Wales, Vermont, Pennsylvania Geological Settings: - Shallow levels of mountain ranges - Areas of past low-grade metamorphism

Physical Properties

Hardness: 3-4 on Mohs scale (relatively soft) Cleavage: Perfect, splits into thin, flat sheets. This is the key characteristic. Durability: Very durable when used properly. Resistant to weathering. Uses: - Roofing tiles (traditional use) - Flooring - Billiard tables - Chalkboards (traditional) - Decorative stone - Writing surfaces (historical)

Varieties and Types

Gray Slate: Most common variety. Black Slate: Contains carbon or other dark minerals. Green Slate: Contains chlorite. Red Slate: Contains iron oxides. Variegated Slate: Multiple colors, mottled appearance. Phyllite: Slightly higher grade than slate, has a sheen.

Rockhounding Value

Collecting Considerations: - Interesting cleavage patterns - Colorful varieties - Historical significance (used for roofing and writing) - Sometimes contains fossils (if low-grade metamorphism) Where to Look: - Old quarries - Road cuts - Areas with slate outcrops - Historical slate-producing regions What to Collect: - Specimens showing perfect cleavage - Colorful varieties - Large, flat pieces - Specimens from famous localities