Formula: KMg3(AlSi3O10)(OH)2
Phyllosilicate (sheet silicate) mica group
Crystal System: Monoclinic
Specific gravity: 2.78 to 2.85 measured 2.79 calculated
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 lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is located in the Grenville Geological Province, which consists mostly of marble, gneiss, and quartzite. Syenite - migmatite was also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the fact that perfectly formed euhedral crystals of different minerals can often be found floating in calcite with no points of contact with the walls. Sometimes these crystals have inclusions of calcite, irregular or rounded in shape. It has been argued that at least some of the vein-dikes were formed as a result of melting of Grenville marble.
Phlogopite forms brown-yellow flaky aggregates on spinel. It is often partially or fully replaced by vermiculite (R&M 97.6.556-564).

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).

At the Dafoe property, Pierrepont, St. Lawrence county, New York, USA, phlogopite is present but uncommon. It occurs as black hexagonal crystals to 5 cm (R&M 97.3.248).

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 (R&M 96.6.552). Phlogopite occurring as primary crystals in marble shows a moderate cream-white fluorescence. When it is an overgrowth coating on meionite or diopside, it fluoresces a strong yellow-orange under short wave UV (R&M 97.5.443).
Phlogopite pseudomorphs after meionite are rare. The appearance of the intact surface suggests that the phlogopite is an overgrowth of existing, sharp, glassy, dark gray meionite crystals, rather than an alteration from the original meionite to phlogopite. These pseudomorphs were found in a small region of the locality enriched in phlogopite that also included phlogopite pseudomorphs after diopside and well-crystallized phlogopite, whereas phlogopite itself is rarely encountered elsewhere at the Mound. Phlogopite pseudomorphs after diopside are also rare here. They were found in the same small region as the phlogopite pseudomorphs after meionite and clearly show evidence of epitactic overgrowth rather than alteration (R&M 97.5.434-444).

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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