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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
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.
Localities
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).
Alteration
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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