Formula: KMg3(AlSi3O10)(OH)2 phyllosilicate (sheet silicate) mica group
Specific gravity: 2.78 to 2.85
Hardness: 2 to 3
Streak: White
Colour: Brown, gray, green, yellow, or reddish brown
Solubility: Slightly soluble in sulphuric acid
Common impurities: Mn,Ba,Cr,Na,Ti,Ni,Zn,Ca,Li,Rb,H2O

Plutonic igneous environments
Metamorphic environments

Phlogopite is found in metamorphosed Mg-rich limestone, dolostone and ultramafic rocks.

It is an essential constituent of kimberlite.
It also may be found in peridotite, dolostone and skarn.

At Carlingford, County Louth, Eire, phlogopite has been found in thermally metamorphosed and metasomatised limestone associated with monticellite, spinel, apatite, vesuvianite and cuspidine (DHZ 3 p50).

At the Mansjöberg Limestone Quarries, Hälsingland, Sweden, phlogopite enclosing remnants of calcite occurs in a chondrodite-spinel limestone associated with pargasite, scapolite, vesuvianite, prehnite, apatite and titanite (DHZ 3 p50).

At Iron Hill, Colorado, USA, phlogopite occurs in hydrothermal contact metamorphosed marble associated with aegirine and sodium-rich tremolite (DHZ 3 p50).

Amity, Town of Warwick, Orange county, New York, USA, is an area of granite intrusions into marble and associated gneiss. The marble is mostly composed of white crystalline calcite that often has small flakes or spheres of graphite and phlogopite. Phlogopite is the most common mica in the Franklin Marble and occurs at Amity in large tabular crystals that are brownish-red, light bronze or silver (R&M 96.5.438).

The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in marble. The development of veins of large crystals probably occurred as a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during this primary event, with additional species resulting from the subsequent alteration of scapolite. There seems to be little, if any, secondary, late-stage mineralisation present.
Phlogopite occurs as honey-brown, transparent crystals to 4.5 cm in massive calcite, associated with purple diopside and pink spinel. It also forms pseudomorphs after both scapolite and diopside. The phlogopite fluoresces yellow under shortwave ultraviolet illumination (R&M 96.6.552).

At the Pyrites Mica mine, St Lawrence county, New York, USA, phlogopite often alters to clinochlore (R&M 93.4.341).


almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
(JVW p 179). This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks (KB p129).

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies (DHZ 2A p631).

antigorite and muscovite to phlogopite, amesite, SiO2 and H2O
5Mg3Si2O5(OH)4 + 3KAl2(AlSi3O10)(OH)2 → 3KMg3(AlSi3O10)(OH)2 + 3Mg2Al(AlSiO5)(OH)4 + 7SiO2 + 4H2O
(DHZ 3 p23).

dolomite, K-feldspar and H2O to phlogopite, calcite and CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O = KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of phlogopite according to the above equation (DHZ 5B p213).

dolomite and muscovite to phlogopite, calcite, CO2 and Al2O3
3CaMg(CO3)2 + KAl2(AlSi3O10)(OH)2 → KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 3CO2 + Al2O3
The excess alumina may be used to form spinel (DHZ 3 p51).

Al-rich hornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
(DHZ 2A 631)

phlogopite, calcite and silica to diopside, K-feldspar, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc - mica schist, phlogopite may alter according to the above reaction (DHZ 2A p272).
The association of phlogopite and calcite is stable only in the absence of excess silica (DHZ 3 p51).

phlogopite, muscovite and SiO2 to orthoclase, pyrope and H2O
KMg3(AlSi3O10)(OH)2 + KAl2(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2K(AlSi3O8) + Mg3Al2(SiO4)3 + 2H2O
(DHZ 3 p72).

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