Mineral description : Phlogopite

Phlogopite crystal in volcanic rock from Alnö.

Mineral class Silicates : Phyllosilicates : Mica group.
Chemical formula KMg3(F,OH)3(AlSi3O10)
Crystal system Monoclinic.
Habitus Crystals as hexagonal mica books. Also as irregular mica books.
Cleavage Basally perfect, producing thin flakes.
Hardness 2.5-3
Density 2.9
Color Pale greenish, yellowish brown to brown, transparent to translucent. (Darker with increasing iron contents.)
Streak White.
Luster Vitreous to pearly.
Description A little rarer member than the other micas but not uncommon. It's been mined for it's good heat and electrical insulating properties.
Occurance Common in dolomites and metamorphic limestones. Also in some pegmatites.
Associates Dolomitic marbles, Hornblende, Garnets, Schorl.
Notes Thin flakes shows an asterism, or six rayed star when a light source is wiewed through the crystal. This is due to inclusions of other minerals.
Phloghopite makes up a substitution serie with biotite, where phlogopite is rich in magnesium and biotite is rich in iron.
Locations A not so common mineral in Sweden.
  • Liäng limestone quarry, Forsmark, Uppland. As green hexagonal crystals.
  • Mansjöberg eulysite area, Los, Hälsingland.
  • Alnö area, Sundsvall, Medelpad.
  • Långban, Värmland. As hexagonal crystals in calcite veins in dolomite, together with clinohumite.

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Last changed : 2000-06-11