Phlogopite

phyllosilicate

metamorphic

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
Environments:

Plutonic igneous environments
Carbonatites
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.

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.

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

At the Pyrites Mica mine, St Lawrence county, New York, USA, phlogopite often alters to clinochlore.

Alteration

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

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.

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

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.

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

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

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 schists, phlogopite may alter according to the above reaction.
The association of phlogopite and calcite is stable only in the absence of excess silica.

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

Common impurities: Mn,Ba,Cr,Na,Ti,Ni,Zn,Ca,Li,Rb,H2O

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