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Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica or amber mica.


Chemical formula: KMg3(OH, F)2AlSi3O10 - Potassium magnesium aluminum silicate hydroxide
Crystal system - monolitic
Mohs scale hardness - 2,5 -3,0
Cleavage - perfect basal
Streak - white
Fracture - uneven, foliate
Specific gravity - 2,6 do 3,2 g/cm3
Color - brownish red, dark brown, yellowish brown, green, white
Luster - pearly, sometimes slightly metallic on cleavage surfaces

  • mineral with outstanding properties as collector items
  • is used in electronics,
  • excellent electrical insulator
  • after pulverization used as a non-stick surface coating on asphalt shingles and rolled roofing
  • used as a filler in the chemical plant protection
Creates - often large in size - plates or lamellar crystals, macroscopically very similar to biotite (usually somewhat lighter). Often has pseudohexagonal character. It also occurs in clusters of granular, lamellar and squamosals.

Phlogopite, also called Magnesium Mica, is typically light to dark brown in color and the dark brown variety may be difficult to distinguish from Biotite. Phlogopite is essentially a complex magnesium, aluminum, iron silicate. Which may show an asterism or six rayed star when a light source is viewed through a thin crystal. Caused, as in garnet, by iron inclusions.

Phlogopite is common in dolomites and metamorphic limestones (magnesium rich marbles). Also in some pegmatites. Associated minerals are dolomitic marbles, hornblende, garnets and schorl. 

Crystal habits include tabular to prismatic crystals with a prominant pinacoid termination. Phlogopite's four prism faces and two pinacoid faces can form a pseudo-hexagonal crystal which is also called a Book. It can also occur as lamellar or granular rock forming masses.

Calcination temperature of phlogopite mica is reached at about 800 C. Generally speaking, phlogopite mica is softer than muscovite mica and this for instance makes it more suitable for the manufacture of commutator micanite used in commutators of flush running designs. 

Phlogopite mica is commonly called amber mica and varies in color from light silver to dark brown. Phlogopite mica has poor electrical properties and chemical resistance in comparison to muscovite mica


Basic properties of phlogopite mica

Characteristic Unit Value
Color    brownish red, dark brown, yellowish brown, green, white
Density g/cm³ 2.6 - 3.2
Hardness Mohs 2.5 - 3.0
Tensile Strength kg/cm2 ~ 1 000
Shear Strength kg/cm2 1 000 - 1 300
Compression Strength kg/cm2 -
Modulus of Elasticity kg/cm2 1.4 - 2.1
Coefficient of expansion per C *)   30x10-6 - 60 x10-6
Calcining Temperature °C 900 - 1 000
Maximum Operating Temperature °C 800 - 900
Thermal Conductivity *) W/(m · °C) ~0.419
Water of Constitution % 3.0
Moisture Absorption   very low
Apparent Electric Strength 
(@ 25 - 75 µm thick)
kV/mm 115 - 140
Permittivity (@ 15 °C)   5 - 6
Power Factor (loss Tangent) @ 15 °C   1 - 5 x 10-3
Volume Resistivity 25 °C  Ω·cm 1x1012 - 1x1014
Acid Reaction   Affected by H2SO4


*) Perpendicular to cleavage plan


Chemical Composition
Silicon dioxide SiO2 42.99%
Alumina Al2O3 12.16%
Potassium oxide K2O 11.23%
Magnesium oxide MgO 28.84%
Loss on ignition H2O 2.15%


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