phlogopite

[Greek phlogōpos, fiery-looking (phlox, phlog-, flame + ōps, ōp-, eye, face) + -ITE¹.]

A mineral with an ideal composition of KMg₃(AlSi₃)O₁₀(OH)₂. Phlogopite belongs to the mica mineral group. It has been occasionally called bronze mica. Phlogopite is a trioctahedral mica, where all three possible octahedral cation sites are occupied by magnesium (Mg). The magnesium octahedra, Mg(O,OH)₆, form a sheet by sharing edges. As in all micas, tetrahedra are located on either side of the octahedral sheet, which may be occupied by aluminum (Al) or silicon (Si). Adjacent tetrahedra share corners to form a two-dimensional network of sixfold rings, thus producing a tetrahedral sheet. Two opposing tetrahedral sheets and the included octahedral sheet form a 2:1 layer. Potassium (K) ions are located between adjacent tetrahedral sheets in the interlayer region.

Specific gravity is 2.86, hardness on the Mohs scale is 2.5–3.0, and luster is vitreous to pearly. Thin sheets are flexible. Color is yellow brown, reddish brown, or green, and thin sheets are transparent. Thermal stability varies greatly with composition, with iron or fluorine substitutions reducing or increasing stability, respectively. Weathering of phlogopite may produce vermiculite. See also Hardness scales; Vermiculite; Weathering processes.

Phlogopite occurs in marbles produced by the metamorphism of siliceous magnesium-rich limestones or dolomites and in ultrabasic rocks, such as peridotites and kimberlites. See also Dolomite; Limestone; Peridotite.

Phlogopite is used chiefly as an insulating material and for fireproofing. It has high dielectric properties and high thermal stability. See also Mica; Silicate minerals.

Environment

Usually a mineral of metamorphosed dolomite (dolostone), sometimes in serpentines and igneous rocks.

Good crystals not uncommon, generally embedded in crystalline dolomite. Often prismatic (elongated, for a mica); sometimes very large.

Light to dark brown. Luster often pearly or submetallic on cleavage face; hardness 2Ɖ-3; specific gravity 2.7; cleavage perfect basal, yielding thin, flexible, and elastic plates. Easily cleaved; translucent, especially marked through the prism faces; commonly asteriated (6- or 12-rayed star visible around a small and distant light source when viewed through a thin cleavage sheet). May be triboluminescent, glowing in the dark at the line of separation when sheets are pulled from a "book."

Alkaline potassium, magnesium, aluminum silicate (about 8% K ₂ O, 28% MgO, 16% Al ₂ O ₃ , 42% SiO ₂ , and 6% H ₂ O, F, and Fe).

Reacts much like muscovite but sometimes can be found to make a cloudy solution like biotite if boiled in strong sulfuric acid.

Best told from muscovite by association with crystalline marbles and its golden brown color (lighter than biotite). Usually less transparent than muscovite, with innumerable microscopic inclusions very apparent. The asterism is a useful guide, also a deepening change of color as the sheet is turned to view at a slant instead of directly through it. Sometimes twinned sheets of phlogopite show a distinct color break across their face as this color change is observed, one side showing the deeper hue.

Can occur in large sheets, and Canadian and Madagascar phlogopite is in good demand for electrical purposes. For some uses phlogopite mica is preferable to muscovite. Large sheets are obtained in the Burgess area, Ontario. Well-formed several-inch (decimeter-sized) crystals of phlogopite have been found in abundance at Franklin, New Jersey, and in St. Lawrence Co., New York.

The noun has one meaning:

Meaning #1: a brown form of mica consisting of hydrous silicate of potassium and magnesium and aluminum

Phlogopite
Phlogopite, Monte Somma, Italy
General
Category	Mica, phyllosilicates
Chemical formula	K(Mg,Fe,Mn)3Si3AlO10(F,OH)2
Identification
Color	Brownish red, dark brown, yellowish brown, green, white
Crystal habit	Tabular, scaly masses, rarely perfect phenocryst tablets
Crystal system	Monoclinic (2/m) Space Group: C 2/m
Twinning	Composition twinning
Cleavage	Perfect basal [001]
Tenacity	Tough, flexible thin laminae
Mohs scale hardness	2–2.5
Luster	Pearly, sometimes slightly metallic on cleavage surfaces
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.78 - 2.85
Optical properties	Biaxial (-), 2V=12
Refractive index	nα = 1.530 - 1.573 nβ = 1.557 - 1.617 nγ = 1.558 - 1.618
Birefringence	δ =0.0280-0.0450
2V angle	16 - 20°
Other characteristics	Fluorescent
References	[1][2][3]

Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.

Phlogopite is the magnesium endmember of the biotite solid solution 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 identification, it shares most of the characteristic properties of biotite.

Contents 1 Paragenesis 1.1 Basaltic association 1.2 Ultrapotassic association 1.3 Ultramafic rocks 2 Miscellaneous 3 References

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 aureoles of intrusive igneous rocks with magnesian country rocks.

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 basalts, ultrapotassic igneous rocks, and ultramafic rocks.

Basaltic association

The basaltic occurrence of phlogopite is in association with picrite basalts and high-alumina basalts. Phlogopite is stable in basaltic compositions at high pressures and is often present as partially-resorbed phenocrysts 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, kimberlite, lamproite, 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.

Phlogopite bearing peridotite from Finero, Italy. Coin of 1 Swiss franc (diameter 23 mm) for scale. The phlogopites are the glittering minerals surrounded by the green groundmass of olivine.

Ultramafic rocks

Phlogopite is often found in association with ultramafic intrusions as a secondary alteration phase within metasomatic margins of large layered intrusions. In some cases the phlogopite is considered to be produced by autogenic alteration during cooling. In other instances, metasomatism has resulted in phlogopite formation within large volumes, as in the ultramafic massif at Finero, Italy, within the Ivrea zone. Trace phlogopite, again considered the result of metasomatism, is common within coarse-grained peridotite xenoliths carried up by kimberlite, and so phlogopite appears to be a common trace mineral in the uppermost part of the Earth's mantle. Phlogopite is encountered as a primary igneous phenocryst within lamproites and lamprophyres, the result of highly fluid-rich melt compositions within the deep mantle.

Miscellaneous

The largest documented single crystal of phlogopite was found in Lacey mine, Ontario, Canada; it measured 10x4.3x4.3 m³ and weighed ~333 tons.^[4] Similar-sized crystals were also found in Karelia, Russia.^[5]

References

1. ^ Phlogopite WebMineral
2. ^ http://rruff.geo.arizona.edu/doclib/hom/phlogopite.pdf Handbook of Mineralogy
3. ^ http://www.mindat.org/min-3193.html Mindat
4. ^ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885-907. http://www.minsocam.org/ammin/AM66/AM66_885.pdf. 
5. ^ "The giant crystal project site". http://giantcrystals.strahlen.org/europe/kovdor.htm. Retrieved 2009-06-06. 

• Deer, W.A., R.A. Howie, and J. Zussman, (1963) Rock-forming minerals, v. 3, "sheet silicates", p. 42–54

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