Despite its name, the mineral chlorite contains absolutely no chlorine at all.

Nature · Published 3 October 2026 Swipe more like this →

In short: Named for the Greek word for green, this common mineral group is a staple of metamorphic and igneous rocks. While it often resembles mica, its unique atomic structure—a sandwich of tetrahedral and octahedral layers—sets it apart, making it a vital indicator for geologists studying the Earth's history.

The chlorite group is a collection of phyllosilicate minerals frequently found in low-grade metamorphic rocks and altered igneous formations. Its name is derived from the Greek word chloros, meaning green, which accurately describes the color it imparts to many slates, schists, and igneous rocks. Despite the linguistic similarity, chlorite contains no chlorine; it is a nonswelling clay mineral with a relatively low capacity for cation exchange, meaning it does not readily absorb water into its interlayer spaces.

Structurally, chlorite is defined by a TOT-O arrangement. This consists of alternating TOT layers—often called talc layers—and positively charged O layers, sometimes referred to as brucite layers. This differs from mica, which also features negatively charged TOT layers but relies on individual cations like potassium or sodium to bond them together. Because magnesium, iron, aluminium, and silicon can substitute for one another within this framework, chlorite exhibits a wide range of physical and optical properties, allowing it to remain stable across diverse temperature and pressure conditions.

In petrology, chlorite serves as a diagnostic marker for the greenschist facies, typically occurring at temperatures near 450 °C and pressures around 5 kbar. G.M. Barrow, a pioneer in the study of metamorphic facies, famously identified the chlorite zone as the region of mildest metamorphism in the Scottish Highlands. As temperatures rise, chlorite eventually breaks down, reacting with minerals like potassium feldspar or phengite to form biotite, muscovite, and quartz. While it has little direct industrial utility, its presence in clay and its role as a secondary mineral in hydrothermal systems make it a ubiquitous feature of the geological landscape, from the ocean floor to the Earth's mantle.

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