Why do certain minerals appear together in rocks while others remain forever apart?

Nature · Published 3 October 2026 Swipe more like this →

In short: The secret lies in the cooling of magma. As molten rock loses heat, minerals crystallize in a specific, predictable sequence known as Bowen's reaction series. This framework explains how early, high-temperature crystals react with the remaining liquid to transform into entirely new minerals, dictating the final composition of igneous rock.

Developed by petrologist Norman L. Bowen in the early 1900s, this series provides an idealized roadmap for how silicate minerals form from cooling basaltic magma. Through experiments with powdered rock, Bowen observed that as temperature drops, minerals do not simply solidify in isolation. Instead, they undergo complex chemical reactions with the surrounding liquid. This process, known as fractional crystallization, allows geologists to look at a rock today and infer the specific thermal conditions under which it originally formed.

The series splits into two distinct branches. The continuous branch involves plagioclase feldspar, which gradually shifts its chemical makeup from calcium-rich to sodium-rich as cooling progresses. Simultaneously, the discontinuous branch features mafic minerals like olivine, orthopyroxene, clinopyroxene, amphibole, and biotite. In this branch, minerals change composition abruptly; for instance, if olivine remains in contact with the cooling magma, it will eventually recrystallize into pyroxene. These reactions are highly sensitive to variables like cooling rates, the loss of volatile gases, and whether early crystals are physically separated from the melt.

Beyond formation, the series serves as a guide to mineral stability. Because minerals are most stable under conditions similar to their birth, those that crystallize at the highest temperatures—like olivine—are often the first to weather when exposed to the Earth's cooler surface. Conversely, minerals that form at lower temperatures are generally more resistant to environmental breakdown. By mapping this progression, Bowen’s work explains why specific mineral assemblages are common in nature, while others are geologically incompatible, providing a fundamental tool for interpreting the history of the Earth's crust.

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