What Minerals Are in Slate?

Clay Minerals

Clay minerals are a large component of slate rocks, and the mineral chlorite will often give the rock a green color. Clay minerals are hydrous aluminum phyllosilicates that form from the degradation of feldspars and micas during the natural weathering process. These minerals are very soft, listed as a 2.0 to 2.5 on the Mohs Hardness Scale of 1 (soft) to 10 (hard). Chlorite is a primary clay in green slate; other clay minerals that may be present either as primary or accessory minerals include illite, kaolinite and smectite. Clays occur in slate as small mineral flakes.

The minerals muscovite and biotite are the primary micas in slate. Muscovite is typically white, colorless, brown, rose or green while biotite is a darker brown. Micas are soft, phyllosilicate minerals that form thin crystal sheets stacked together like a book. The thin crystal "pages" are easy to flake with your fingernail. On the Mohs Hardness Scale, mica is a soft 2.5. Micas have a glassy luster, meaning that they have a shiny appearance.

Quartz is the most common mineral in the Earth's crust. In slate, quartz forms tiny, lens-shaped grains. Quartz is a hard, colorless silicate mineral with a shiny luster sometimes also known as rock crystal. The hardest mineral in slate, quartz is a 7.0 on the Mohs Hardness Scale. Quartz exists worldwide in all major rock types and has many uses in commercial and industrial processes.

Accessory minerals often occur in rocks but are not necessary for their formation. Common accessory minerals in slate include hematite, pyrite, cordierite and andalusite, among others. Hematite is a gray to black iron-oxide mineral with a reddish-brown streak color. Hematite can give slate a red to brown color because of its iron content. Pyrite, or "fool's gold," is an iron-sulfide mineral and is a common accessory in slate. Cordierite and andalusite are magnesium aluminum and aluminum silicates, respectively. They often occur together in low-temperature metamorphic rocks such as slate.