Phlogopite
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Phlogopite is named from the Greek phlogopos meaning resembling fire in allusion to its common red tint.

Discovered in 1841; IMA status: Valid (IMA Approved 1988)

Chemistry

Chemical Formula:

KMg₃AlSi₃O₁₀(OH,F)₂

Potassium Magnesium Aluminum Silicate Hydroxide Fluoride

Molecular Weight:

419.25 gm

Composition:

Potassium

9.33 %

K

11.23 %

K₂O

Magnesium

17.39 %

Mg

28.84 %

MgO

Aluminum

6.44 %

Al

12.16 %

Al₂O₃

Silicon

20.10 %

Si

42.99 %

SiO₂

Hydrogen

0.24 %

H

2.15 %

H₂O

Oxygen

41.98 %

O

Fluorine

4.53 %

F

4.53 %

F

-  %

F

-1.91 %

-O=F₂

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/H.11-80

Nickel-Strunz 10th Ed. ID:

9.EC.20

9 : SILICATES (Germanates)
E : Phyllosilicates
C : Phyllosilicates with mica sheets, composed of tetrahedral and octahedral nets

Related to:

Mica Group. Biotite-Phlogopite Series. The hydroxyl analogue of Fluorophlogopite and the magnesium analogue of Hendricksite. Also, generally the micas on the Phlogopite end (magnesium end) of the Biotite-Phlogopite Series, including Phlogopite, Fluorophlogopite, Eastonite, and Hendricksite.

Members of Group:

Mica Group: Aluminoceladonite, Anandite, Annite, Annite-Phlogopite Series, Aspidolite, Balestraite, Biotite, Bityite, Boromuscovite, Brammallite, Celadonite, Chernykhite, Chromceladonite, Chromphyllite, Clintonite, Eastonite, Ephesite, Ferroaluminoceladonite, Ferroceladonite, Ferrokinoshitalite, Fluorannite, Fluorphlogopite, Ganterite, Glauconite, Hendricksite, Illite, Kinoshitalite, Lepidolite, Luanshiweiite, Margarite, Masutomilite, Montdorite, Muscovite, Muscovite-Celadonite Series, Nanpingite, Natro-glauconite, Norrishite, Oxykinoshitalite, Oxyphlogopite, Paragonite, Phengite, Phlogopite, Polylithionite, Preiswerkite, Roscoelite, Shirokshinite, Shirozulite, Siderophyllite, Sokolovaite, Suhailite, Tainiolite, Tetraferriannite, Tetraferriphlogopite, Tobelite, Trilithionite, Voloshinite, Wonesite, Yangzhumingite, Zinnwaldite

Varieties:

Barium-Phlogopite, Barytbiotite, Chrome Phlogopite, Chromian Phlogopite, Ferroan Phlogopite, Fluorian Phlogopite, Manganoan Phlogopite

Synonyms:

Bronze Mica, Brown Mica, Hydroxyl-Phlogopite, Magnesia Mica, Rhombic Mica

Prototypes:

Phlogopite-1M, Phlogopite-2M1, Phlogopite-3T

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Crystals six-sided, thick tabular to prismatic, commonly tapered, to 10 m and 270 t. Also coarse-grained, platy, and as small scales.

Twinning:

Composition plane {001}, twin axis [310].

Physical Properties

Cleavage:

Perfect on {001}

Fracture:

Irregular/Uneven

Tenacity:

Tough; thin laminae flexible and elastic

Moh's Hardness:

2.0 - 3.0

Density:

2.78 - 2.85 (g/cm³)

Luminescence:

Fluorescent; straw yellow to lemon yellow under SW UV, weak blue-white to blue-gray under LW UV

Radioactivity:

Barely Detectable; GRapi = 133.53 (Gamma Ray American Petroleum Institute Units)

Optical Properties

Color:

Brownish red, dark brown, yellowish brown, green, white; colorless, pale yellow, or green in thin section.

Transparency:

Transparent to Translucent

Luster:

Pearly to Submetallic on cleavages

Refractive Index:

1.530 - 1.618  Biaxial ( - ) 

Birefringence:

0.028 - 0.045

Dispersion:

Distinct; r < v

Pleochroism:

Visible; X = yellow; Y = Z = brownish red, green, yellow.

Occurances

Geological Setting:

In metamorphosed dolostones and magnesium-rich limestones; in ultramafic rocks as kimberlites, peridotites, lamproites, and serpentinites.

Common Associations:

Dolomite, Calcite, Diopside, Tremolite, Scapolite, Vesuvianite, Apatite, Titanite, Epidote, Olivine, Augite, Magnetite

Common Impurities:

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

Type Locality:

St. Lawrence County, at Edwards, New York, USA

Year Discovered:

1841; Redefined as the OH end-member of the Mica Group by the IMA Mica Subcommittee in 1998.

View mineral photos:

Phlogopite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Phlogopite is a rarer member of the Mica Group of minerals and is not well known even by mineral collectors. It has been mined however for its heat and electrical insulating properties which are considered superior to other Micas. The typical light brown color of Phlogopite is characteristic although it is difficult to distinguish brown Biotite from dark brown Phlogopite. The two are end members of a series that is dependent on the percentage of iron. Phlogopite is iron poor and Biotite is iron rich. The darker color and density increase with an increase in the iron content. Phlogopite, like other Micas, has a layered structure which produces its perfect cleavage. Phlogopite is rarely considered a valuable mineral specimen and is rarely available as a cabochon gem.

Distribution: Some localities for well-crystallized material include: in the USA, from Antwerp and Natural Bridge, Jefferson County, and Edwards and Pierrepont, St. Lawrence County, New York; from Franklin, Sussex County, New Jersey. In Canada, large crystals from the Lacey mine, Frontenac County, and in North and South Burgess Townships, Ontario; from near Perkin's Mills, and elsewhere in Gatineau County, Quebec. In the Slyudyanka region, near Lake Baikal, Siberia, Russia. At Ødegården, near Feset, Norway. From Campolungo, near St. Gotthard, Ticino, Switzerland. In the Val di Fassa, Trentino-Alto Adige, and on Monte Braccio, Val Malenco, Lombardy, Italy. From Saharakara and Ampandrandava, Madagascar. At Anxiety Point, Nancy Sound, New Zealand.
 

  
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