Phlogopite is a yellow, greenish, or reddish-brown member of the

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

family of

phyllosilicates Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

. It is also known as

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

mica. Phlogopite is the magnesium

endmember An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...

of the

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more ...

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

series, with the chemical formula KMg₃AlSi₃O₁₀(F,OH)₂. Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content. For physical and

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

identification, it shares most of the characteristic properties of biotite.

Paragenesis

Phlogopite is an important and relatively common end-member composition of biotite. Phlogopite micas are found primarily in igneous rocks, although it is also common in contact

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

aureoles of intrusive

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

s with magnesian country rocks and in marble formed from impure dolomite (dolomite with some siliclastic sediment). The occurrence of phlogopite mica within igneous rocks is difficult to constrain precisely because the primary control is rock composition as expected, but phlogopite is also controlled by conditions of crystallisation such as temperature, pressure, and vapor content of the igneous rock. Several igneous associations are noted: high-alumina

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

s, ultrapotassic igneous rocks, and

ultramafic rock Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are compose ...

Basaltic association

The basaltic occurrence of phlogopite is in association with

picrite basalt Picrite basalt or picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts (20-50%) and black to dark brown pyroxene, mostly augite. The olivine-rich ...

s and high-alumina basalts. Phlogopite is stable in basaltic compositions at high pressures and is often present as partially resorbed

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s or an accessory phase in basalts generated at depth.

Ultrapotassic association

Phlogopite mica is a commonly known phenocryst and groundmass phase within ultrapotassic igneous rocks such as

lamprophyre Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magnesium ...

kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond called the Star of S ...

lamproite Lamproite is an ultrapotassic mantle-derived volcanic or subvolcanic rock. It has low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements. Lamproites are geographically widespread ...

, and other deeply sourced ultramafic or high-magnesian melts. In this association phlogopite can form well preserved megacrystic plates to 10 cm, and is present as the primary groundmass mineral, or in association with pargasite amphibole, olivine, and pyroxene. Phlogopite in this association is a primary igneous mineral present because of the depth of melting and high vapor pressures.

Ultramafic rocks

Phlogopite is often found in association with ultramafic intrusions as a secondary alteration phase within metasomatic margins of large

layered intrusion A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around to over and several hundred metres to ...

s. In some cases the phlogopite is considered to be produced by autogenic alteration during cooling. In other instances,

metasomatism Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

has resulted in phlogopite formation within large volumes, as in the ultramafic massif at Finero, Italy, within the

Ivrea zone The Ivrea zone is a tectonic terrane in the Italian Alps, that consists of a steeply dipping piece of the Earth’s lower crust of the Apulian plate. The zone is named after the Italian city of Ivrea. Geologically the Ivrea zone is consi ...

. Trace phlogopite, again considered the result of metasomatism, is common within coarse-grained

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

xenolith A xenolith ("foreign rock") is a rock fragment ( country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ig ...

s carried up by

, and so phlogopite appears to be a common trace mineral in the uppermost part of the

Earth's mantle Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly so ...

. Phlogopite is encountered as a primary igneous

within

s and

lamprophyres Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magnesium o ...

, the result of highly fluid-rich melt compositions within the deep mantle.

Uses

Miscellaneous

The largest documented single crystal of phlogopite was found in Lacey mine,

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central Ca ...

, Canada; it measured 10x4.3x4.3 m³ and weighed about 330 tonnes. Similar-sized crystals were also found in

Karelia Karelia ( Karelian and fi, Karjala, ; rus, Каре́лия, links=y, r=Karélija, p=kɐˈrʲelʲɪjə, historically ''Korjela''; sv, Karelen), the land of the Karelian people, is an area in Northern Europe of historical significance fo ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

References

*Deer, W.A., R.A. Howie, and J. Zussman, (1963) ''Rock-forming minerals'', v. 3, "sheet silicates", p. 42–54 * {{Phyllosilicates Phyllosilicates Potassium minerals Magnesium minerals Iron(II) minerals Manganese minerals Monoclinic minerals Minerals in space group 12 Mica group Luminescent minerals