Biotite is a group of black color mica minerals uncovered in igneous and also metamorphic rocks.Article by: Hobart M. King, PhD, RPG

Biotite: Biotite from Bancroft, Ontario, Canada. Specimen is roughly 4 inch (10 centimeters) across.

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What is Biotite?

Biotite is a name provided for a large group of black mica minerals the are generally found in igneous and metamorphic rocks. These incorporate annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite, and many others. These micas differ in chemistry composition however are every sheet silicate minerals with very similar physical properties.

A generalized chemical composition because that the biotite team is:


The surname "biotite" is used in the field and also in entry-level courses since these minerals usually cannot be differentiated without optical, chemical, or x-ray analysis.

Biotite is a rock-forming mineral uncovered in a wide selection of crystalline igneous rocks such together granite, diorite, gabbro, peridotite, and pegmatite. It also forms under metamorphic problems when argillaceous rocks are exposed come heat and pressure to type schist and also gneiss. Back biotite is not really resistant to weathering and transforms into clay minerals, it is sometimes found in sediments and sandstones.

Physical nature of Biotite
Chemical Classification Dark mica
Color Black, dark green, dark brown
Streak White come gray, flakes often produced
Luster Vitreous
Diaphaneity Thin sheets room transparent come translucent, books are opaque.
Cleavage Basal, perfect
Mohs Hardness 2.5 to 3
Specific Gravity 2.7 to 3.4
Diagnostic Properties Dark color, perfect cleavage
Chemical Composition K(Mg,Fe)3(AlSi3O10)(F,OH)2
Crystal System Monoclinic
Uses Very small industrial use

Properties of Biotite

Biotite is an extremely easy come identify, and also with a little experience a human being will have the ability to recognize that on sight. The is a black color mica through perfect cleavage and also a vitreous luster ~ above the cleavage faces.

When biotite is separated right into thin sheets, the sheets are flexible but will break upon major bending. When organized up come the light, the sheets are transparent to translucent with a brown, gray, or greenish color. Skilled observers have the right to sometimes recognize phlogopite by its brown color.


Biotite angled view: Biotite from Bancroft, Ontario, Canada. Specimen is about 4 customs (10 centimeters) across.


Biotite side view: an edge check out of the biotite specimen indigenous the picture above. Specimen is approximately 3/8 customs (.95 centimeter) thick.

Biotite Minerals

As noted above, biotite is a name offered for a number of black mica minerals that have different chemical compositions however very similar physical properties. These minerals normally cannot be identified from one an additional without laboratory analysis. A small list the the biotite minerals is given listed below with your chemical compositions.

Mineral Chemical Composition
Annite KFe3(AlSi3)O10(OH)2
Phlogopite KMg3(AlSi3)O10(F,OH)2
Siderophyllite KFe2Al(Al2Si2)O10(F,OH)2
Eastonite KMg2Al(Al2Si3)O10(OH)2
Fluorannite KFe3(AlSi3)O10F2
Fluorophlogopite KMg3(AlSi3)O10F2

Uses the Biotite

Biotite has a small variety of commercial uses. Soil mica is provided as a filler and extender in paints, as an additive to drilling muds, as an inert filler and also mold-release certified dealer in rubber products, and as a non-stick surface coating on asphalt shingles and rolled roofing. The is likewise used in the potassium-argon and also argon-argon techniques of date igneous rocks.


Biotite in sandstone: main point samples the biotitic sandstone native the apologize Creek Formation, Copper Queen Mine, close to Salmon, Idaho. USGS image.


The best method to learn about minerals is to research with a arsenal of tiny specimens that you deserve to handle, examine, and observe your properties. Cheap mineral collections are easily accessible in the Store.

The other "Fool"s Gold"

Biotite and the brown mica known as phlogopite have been known to cause excitement in inexperienced gold panners. A few tiny flakes of this micas swishing in a gold pancan create bright bronze-colored reflect in the pan once struck by sunlight. These reflections have the right to fool the inexperienced panner right into thinking the he has found gold. If the panner regains his composure, clears one of these flakes indigenous the pan and also pokes it v a pin, it will break. First-time panners quickly learn to perform some testing before shouting "gold" - which most likely isn"t a great idea even when yellow is found because it can entice unwanted travellers to her panning spot.

Small flakes the biotite have additionally been well-known to reason excitement when they room observed in rocks. Your bronze-colored reflections deserve to fool the inexperienced observer right into thinking the tiny flakes of gold room present. Again, the pin test or a hand lens will commonly yield a fast answer regarding whether that is genuine gold or fool"s gold.

More Minerals
Mohs Hardness Scale
What Is Luster?
The Streak Test
Grape Agate
Hardness Picks

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