The Mineralogy Journey: Unraveling The Bowen Reaction Series

The Bowen Reaction Series, developed by Canadian geologist N.L. Bowen in the early 20th century, is a fundamental concept in the field of geology, specifically in the study of igneous rocks. It provides crucial insights into the formation of igneous rocks, their mineral composition, and the sequence in which minerals crystallize as molten rock (magma) cools and solidifies. The series is divided into two branches: the continuous (felsic minerals: feldspars) and the discontinuous (mafic minerals). The discontinuous series includes minerals such as olivine, pyroxene, amphibole, and biotite, while the continuous series includes feldspar, alkali feldspar, and quartz. These series illustrate the different crystallization temperatures of minerals, with those forming at higher temperatures found at the top and those forming at lower temperatures at the bottom. This temperature gradient helps geologists understand the cooling history of igneous rocks and has practical applications in various fields, including mineral exploration, petrology, and volcanology.

Continuous Series (Felsic minerals)

The Continuous Series (Felsic minerals) describes the sequence of minerals that form as the temperature decreases in a more gradual and continuous manner. It does not involve discrete phases like the Discontinuous Series but represents a gradual transition.

The Continuous Series begins with the crystallisation of calcium-rich plagioclase feldspar (anorthite) at higher temperatures. As the temperature decreases, the plagioclase feldspar compositions change to more sodium-rich varieties: bytownite, labradorite, andesine, oligoclase, and albite. As the temperature continues to fall, these two series merge, and more minerals crystallise in the following order: Alkali feldspar, muscovite, and quartz.

At lower temperatures, the branches of the Continuous and Discontinuous Series merge, and we obtain the minerals common to the felsic rocks: muscovite mica, orthoclase feldspar, and quartz. Felsic rocks are usually light in colour and have specific gravities of less than 3. The most common felsic rock is granite.

The sequence of crystallisation in the Continuous Series is based on idealised conditions and can vary depending on factors such as magma composition, pressure, and cooling rate. Additionally, not all minerals in the Continuous Series are present in every igneous rock; their presence depends on the specific conditions of magma crystallisation.

Discontinuous Series (Mafic minerals)

The Discontinuous Series, also known as the Discontinuous Branch, is one of the two main branches of Bowen's Reaction Series, the other being the Continuous Series. The Discontinuous Series refers to the crystallization of specific minerals at distinct temperature intervals.

In the early 1900s, Canadian geologist Norman L. Bowen conducted experiments on igneous rocks, melting and heating them to various temperatures and then allowing them to cool. Through these experiments, he discovered the sequence in which minerals crystallize from a cooling magma, which became known as the Discontinuous Series.

The Discontinuous Series starts with the crystallization of olivine, which occurs at temperatures just below 1300°C. As the temperature continues to drop, olivine becomes unstable and reacts with the silica in the remaining liquid magma, converting into pyroxene. This process continues as long as there is still silica left in the liquid. With decreasing temperatures, the silica tetrahedrons in olivine first manage to join together in chains (forming pyroxene), then in ribbons (amphiboles), and then sheets (micas). As the magma further cools, the pyroxene reacts with the liquid and amphibole crystals replace it, and then biotite replaces amphibole.

The Discontinuous Series is a sequence of mafic minerals, which are silicate minerals or igneous rocks rich in magnesium and iron. Most mafic minerals are dark in colour, and common mafic rocks include basalt, diabase, and gabbro. The Discontinuous Series explains why mafic igneous rocks crystallize at higher temperatures than felsic igneous rocks.

Plagioclase (Calcium-rich)

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. It is a continuous solid solution series, more properly known as the plagioclase feldspar series. The series ranges from albite to anorthite endmembers, where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure.

Plagioclase is a major constituent mineral in Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. It is also a major constituent of rock in the highlands of the Moon and is the most abundant mineral in the crust of Mars.

Plagioclase is a common clast produced during the weathering of igneous and metamorphic rocks. It is a dominant mineral in most igneous rocks of the Earth’s crust and is a major constituent in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt.

Plagioclase feldspars are important components of many building stones. Labradorite, spectrolite, sunstone, and moonstone are gem-quality plagioclase feldspars that are popular because of their optical phenomena.

Plagioclase in Bowen's Reaction Series

Plagioclase feldspar is a key mineral in Bowen's Reaction Series, a graphical representation of the sequence in which minerals crystallise from a cooling magma. Bowen's Reaction Series outlines two main branches – the discontinuous branch and the continuous branch.

The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich. The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica.

The plagioclase feldspars are a set of minerals that are in a series from a sodium-rich end member, albite, to a potassium-rich end member, anorthite. The intermediate members of the series are given arbitrary boundaries based on their percentage of sodium or calcium.

The specific minerals that form depend on the magma’s composition and cooling rate. The sequence depends on the composition of the magma.

Biotite (Black Mica)

Biotite, also known as black mica, is a common group of phyllosilicate minerals within the mica group. It is a mixture composed of variable proportions of four basic aluminosilicates of potassium, iron, magnesium, or aluminium: annite, siderophyllite, phlogopite, and eastonite. Biotite is regarded as a mineral group, rather than a single species, by the International Mineralogical Association (IMA).

Biotite is a rock-forming mineral, present in at least some percentage in most igneous and metamorphic rocks. It is found in pegmatites, granites, rhyolite, and other intrusive igneous rocks. Biotite is also an essential phenocryst in some varieties of lamprophyre. It is occasionally found in large cleavable crystals, especially in pegmatite veins in the USA, Canada, and Norway.

Biotite is used extensively to determine the age of rocks, through potassium-argon or argon-argon dating. It is also useful in assessing the temperature history of metamorphic rocks, as the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature. Biotite is also used in electronics as an insulator, in paint as an extender, and in rubber products as an inert filler.

Biotite was named in honour of the French physicist Jean-Baptiste Biot, who in 1816 researched the optical properties of mica. Biotite has a chemical formula of K(Mg,Fe)3AlSi3O10(F,OH)2. It has a monoclinic crystal system, with tabular to prismatic crystals and an obvious pinacoid termination. It has four prism faces and two pinacoid faces, forming a pseudohexagonal crystal. It has a hardness of 2.5-3 on the Mohs scale.

Orthoclase (K-feldspar)

Orthoclase, or orthoclase feldspar, is a tectosilicate mineral that forms igneous rock. It is a type of potassium feldspar, also known as K-feldspar. It is one of the most abundant rock-forming minerals of the continental crust. Orthoclase is most widely known as the pink feldspar found in many granites and as the mineral assigned a hardness of "6" on the Mohs scale.

Formation and subtypes

Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. The pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember of plagioclase. While slowly cooling within the earth, sodium-rich albite lamellae form by exsolution, enriching the remaining orthoclase with potassium. The resulting intergrowth of the two feldspars is called perthite.

The higher-temperature polymorph of orthoclase is sanidine, which is common in rapidly cooled volcanic rocks such as obsidian and felsic pyroclastic rocks. The lower-temperature polymorph is microcline. Adularia is a low-temperature form of either microcline or orthoclase, originally reported from the low-temperature hydrothermal deposits in the Adula Alps of Switzerland.

Uses

Orthoclase has several commercial uses. It is a raw material used in the production of glass, ceramic tile, porcelain, dinnerware, bathroom fixtures, and other ceramics. It is used as an abrasive in scouring powders and polishing compounds. It is also cut as a gemstone. An adularescent gem material known as moonstone is an intergrowth of orthoclase and albite.

Occurrence

Most orthoclase forms during the crystallization of a magma into intrusive igneous rocks such as granite, granodiorite, diorite, and syenite. Significant amounts of orthoclase are also found in extrusive igneous rocks such as rhyolite, dacite, and andesite.

Large crystals of orthoclase are found in igneous rocks known as pegmatite. They are normally no more than a few inches in length, but the largest reported orthoclase crystal was over 30 feet in length and weighed about 100 tons. It was found in a pegmatite in the Ural Mountains of Russia.

Orthoclase is also a significant constituent of the metamorphic rocks known as gneiss and schist. These rocks most often form during regional metamorphism when granitic rocks are subjected to heat and pressure at convergent plate boundaries involving continental crust. The orthoclase in these metamorphic rocks is inherited from their igneous protoliths.

Identification

Orthoclase is similar in appearance to quartz, but can be distinguished by its cleavage and twinning. Orthoclase has two good cleavage directions that intersect at 90 degrees, whereas quartz lacks cleavage. Orthoclase often exhibits distinctive twinning, which can be plaid (cross-hatched) or simple twins that divide grains into two domains.

Orthoclase is also similar to plagioclase, but can be distinguished by its lack of striations across its cleavage plane. Cleavage faces of orthoclase appear smooth, whereas plagioclase has fine parallel lines (striations) across its cleavage plane. Orthoclase typically has a colour range between white and pink, whereas plagioclase is usually white to grey.

Frequently asked questions