Plagioclase is a mineral series within the feldspar group that is exceptionally common, making up a large portion of the Earth’s crust. It is a fundamental component of various rock types, which makes it a crucial subject in the study of geology and mineralogy. Its complex nature stems from its variable chemical makeup, yet its physical properties allow for reliable identification in the field and the laboratory.
Defining Plagioclase as a Mineral
Plagioclase is definitively a mineral because it satisfies all classification criteria: it is naturally occurring, inorganic, solid, possesses a definite chemical composition, and has an ordered internal atomic structure. Plagioclase meets these requirements as a naturally formed crystalline solid found globally.
Its highly ordered internal structure places it within the tectosilicate class, or framework silicates, characterized by a three-dimensional network of silica and alumina tetrahedra. Plagioclase is a member of the feldspar family, the most abundant group of minerals in the Earth’s crust. Unlike minerals with a fixed formula, plagioclase represents a series where the chemical composition varies.
The Plagioclase Solid Solution Series
Plagioclase is a continuous solid solution series rather than a single mineral species because its chemical composition changes along a seamless gradient. This variability results from a phenomenon called isomorphism, where sodium (Na) and calcium (Ca) substitute for one another within the crystal lattice.
The series is defined by two pure end-members: Albite (sodium-rich) and Anorthite (calcium-rich). The chemical formula ranges between \(\text{NaAlSi}_{3}\text{O}_{8}\) (Albite) and \(\text{CaAl}_{2}\text{Si}_{2}\text{O}_{8}\) (Anorthite). A sample’s composition is typically expressed as the percentage of the Anorthite component (\(\text{An}\%\)).
The plagioclase series is divided into six categories based on their Anorthite content. The intermediate members, in order of increasing calcium content, are:
- Oligoclase (10–30% An)
- Andesine (30–50% An)
- Labradorite (50–70% An)
- Bytownite (70–90% An)
The substitution of calcium for sodium requires a coupled substitution of aluminum for silicon to maintain electrical neutrality in the crystal structure.
Identifying Physical Characteristics
Identifying plagioclase relies on distinctive physical characteristics that differentiate it from other common minerals like quartz or potassium feldspar. The most diagnostic feature is polysynthetic twinning, which appears as fine, parallel striations on cleavage surfaces. These striations are caused by the repeated twinning of the crystal structure and are rarely seen in other feldspars.
Plagioclase exhibits cleavage in two directions that meet at an angle slightly deviating from 90 degrees, typically 93 to 94 degrees. This slightly oblique cleavage is the source of the mineral’s name, derived from the Greek words for “oblique” and “fracture.” The mineral possesses a Mohs hardness ranging from 6 to 6.5, making it harder than glass.
Plagioclase commonly shows a vitreous, or glassy, luster and is often white or gray. Colors can vary to include pink, yellow, or iridescent blue-green in some Labradorite specimens. The specific gravity increases smoothly with calcium content, ranging from 2.62 for pure Albite to 2.76 for pure Anorthite.
Geological Importance and Occurrence
Plagioclase is one of the most abundant rock-forming minerals, making up an estimated 29% of the Earth’s crust. Its wide range of chemical compositions allows it to crystallize in nearly all types of igneous, metamorphic, and some sedimentary rocks. It is a major constituent in igneous rocks, including intrusive types like granite and gabbro, and extrusive rocks such as basalt and rhyolite.
The composition of plagioclase is directly linked to the rock type. Sodium-rich varieties (like Albite and Oligoclase) dominate felsic (silica-rich) rocks such as granite. Conversely, calcium-rich plagioclase (like Labradorite and Bytownite) is prevalent in mafic (magnesium and iron-rich) rocks such as basalt and gabbro. This predictable relationship helps geologists classify rocks and understand their formation temperature and origin.
Plagioclase minerals are common in metamorphic rocks like gneiss and schist, where they either persist from the original rock or form anew during metamorphism. In sedimentary rocks, plagioclase fragments are found as detrital grains, although they weather more easily than quartz. The presence and composition of plagioclase are fundamental to understanding the overall geology of a region.