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Phlogopite


Called "brown mica" after its color, "magnesia mica" after its composition.


Article by: , Ph.D., RPG


Phlogopite

Barrel-Shaped Phlogopite Crystal and small phlogopite flakes in white marble. Cleavage steps can be seen on the top of the crystal. This specimen was collected from Franklin, New Jersey and measures 8.6 x 7.1 x 6.4 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

What Is Phlogopite?

Phlogopite is an aluminosilicate mineral of potassium and magnesium, and a member of the mica group. Phlogopite usually ranges in color from yellow to brown to reddish brown. Its color can be helpful in distinguishing it from other mica minerals. Phlogopite and muscovite are the only two mica minerals that are used commercially.


Geologic Occurrence

Phlogopite is most commonly found in metamorphic rocks. An ideal condition for phlogopite formation is when a dolomitic limestone or a magnesium-rich limestone, with some clay content, is subjected to hydrothermal metamorphism. The result is typically one of the following: scattered flakes and small crystals of phlogopite throughout the rock; concentrations of phlogopite in parts of the limestone where clay was abundant; or, concentrations of phlogopite in metamorphosed shale along the margins of the limestone. Phlogopite schist can form through this same process.

Phlogopite is also found in igneous rocks. These include ultramafic rocks such as peridotite, kimberlite, lamproite, and serpentinite. Phlogopite is also found in high-alumina basalts. Almost all sheet and block mica is found in pegmatites.

Physical Properties of Phlogopite

Chemical Classification Silicate, phyllosilicate
Color Usually yellowish, yellowish brown, brown, reddish brown. Rarely green, colorless or almost black.
Streak White, often sheds tiny flakes
Luster Pearly to vitreous. Reflections from cleavage faces can appear silver, gold or copper metallic.
Diaphaneity Transparent to translucent in small pieces. Large pieces appear opaque until a small piece is cleaved off.
Cleavage Perfect
Mohs Hardness 2 to 2.5
Specific Gravity 2.75 to 2.9
Diagnostic Properties Color, cleavage, transparency, luster.
Chemical Composition KMg3(AlSi3O10)(F,OH)2
Crystal System Monoclinic
Uses Compared to muscovite, phlogopite has fewer industrial uses because of its color and is not used when the color of the particles are important. Used in manufacturing plastics, rubber, asphalt roofing, drilling mud.



Physical Properties of Phlogopite

Phlogopite has a few physical properties that can help you identify it. The first is a yellow to brown to reddish brown color. Next, as a mica, phlogopite easily splits into thin sheets that are transparent, flexible, and tough.

Phlogopite crystals can be tabular with a pseudohexagonal shape, or they can be barrel-shaped prisms with a pseudohexagonal cross-section. Although phlogopite is a monoclinic mineral, the c-axis is so gently inclined that it would be easy to think that phlogopite is hexagonal.

Phlogopite has many properties that make it valuable in manufacturing. It can be cleaved into thin sheets that can serve as electronics boards. These are stiff but flexible, and they can easily be cut to shape, punched, or drilled. Phlogopite is heat resistant, does not transmit electricity, and is a poor conductor of heat.

Phlogopite Information
[1] Mica by Kenneth C. Curry, 2016 Minerals Yearbook, an annual publication by the United States Geological Survey, 8 pages, published February 2019.

Uses of Phlogopite

Phlogopite and muscovite are the only two mica minerals that are used commercially. Phlogopite is used less often than muscovite because it is less available and because its brown color is undesirable for some uses. Much phlogopite is used in plastic composite body parts for automobiles. Phlogopite increases the stiffness of the plastic, provides greater dimensional stability, and reduces distortion upon temperature change.

Ground phlogopite is used as a substitute for asbestos in automobile brake linings and clutch plates. It is added to industrial coatings used to increase strength, increase stiffness, and to improve resistance to heat, chemicals and ultraviolet rays. It is an additive that increases the strength of epoxies, nylons, and polyesters.

Sheet mica is used in the electronics industry to make stiff, heat-resistant, non-conducting boards for electronic components. Sheet mica boards can be cut, punched, stamped, and machined to close tolerances. Sheet mica is the most valuable mica product because each piece must be recovered by hand. Sheet mica was selling for $148 per pound in 2014. [1]

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