What is Biotite?
Biotite is a name used for a large group of black mica minerals that are commonly found in igneous and metamorphic rocks.
These include: annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite and many others. These micas vary in chemical composition but are all
sheet silicate minerals with very similar physical properties.
A generalized chemical composition for the biotite group is:
K(Mg,Fe)2-3Al1-2Si2-3O10(OH,F)2
The name "biotite" is used in the field and in entry-level
geology courses because these minerals generally can not be distinguished without optical, chemical or x-ray analysis.
Biotite is a primary mineral found in a wide range of crystalline igneous rocks such as granite, diorite, gabbro, peridotite and pegmatite.
It also forms under metamorphic conditions when argillaceous rocks are exposed to heat and pressure to form schist and
gneiss. Although biotite is not very resistant to weathering and transforms into clay minerals, it is sometimes found in sediments and sandstones.
Properties of Biotite
Biotite is very easy to identify and with a little experience a person will be able to recognize it on sight.
It is a black mica with perfect cleavage and a vitreous luster on the cleavage faces. When biotite is separated into thin sheets,
the sheets are flexible but will break upon severe bending. When held up to the light the sheets are transparent to translucent with a
brown, gray or greenish color. Experienced observers can sometimes recognize phlogopite by its brown color.
Physical Properties of Biotite |
| Color |
black, dark green, dark brown |
| Streak |
white to gray, flakes often produced |
| Luster |
vitreous |
| Diaphaneity |
thin sheets are transparent to translucent, books are opaque |
| Cleavage |
basal, perfect |
| Mohs Hardness |
2.5 - 3 |
| Specific Gravity |
2.7 - 3.4 |
| Diagnostic Properties |
dark color, perfect cleavage |
| Chemical Composition |
K(Mg,Fe)2-3Al1-2Si2-3O10(OH,F)2 |
| Crystal System |
monoclinic |
Biotite Minerals
As noted above, biotite is a name used for a number of black mica minerals that have different chemical compositions but very similar physical properties. These minerals generally can not be distinguished
from one another without laboratory analysis. A small list of the biotite minerals is given below with their chemical compositions.
| Mineral |
Chemical Composition |
| Annite |
KFe3(AlSi3)O10(OH)2 |
| Phlogopite |
KMg3(AlSi3)O10(OH)2 |
| Siderophyllite |
KFe2Al(Al2Si2)O10(F,OH)2 |
| Eastonite |
KMg2Al(Al2Si3)O10(OH)2 |
| Fluorannite |
KFe3(AlSi3)O10F2 |
| Fluorophlogopite |
KMg3(AlSi3)O10F2 |
Uses of Biotite
Biotite has a small number of commercial uses. Ground mica is used as a filler and extender in paints, as an additive to drilling muds, as an inert filler and mold-release agent in rubber products, and as a non-stick surface coating on asphalt shingles and rolled roofing. It is also used in the potassium-argon and argon-argon methods
of dating igneous rocks.
The Other "Fool's Gold"
Biotite has been known to cause excitement in inexperienced gold panners. A few tiny flakes of biotite swishing in a gold pan
can produce bright bronze-colored reflections in the pan when struck by sunlight. These reflections can fool the inexperience
panner into thinking that he has found gold. If the panner regains his composure, removes one of these flakes from the pan
and pokes it with a pin it will break. First-time panners quickly learn to do some testing before shouting "gold" - which probably isn't a good idea even when gold is found
because it can attract unwanted visitors to your panning spot.
Small flakes of biotite have also been known to cause excitement when they are observed in rocks.
Their bronze-colored reflections can fool the inexperienced observer into thinking that tiny flakes of gold are present. Again
the pin test or a hand lens will usually yield a quick answer.
Contributor: Hobart King
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| Biotite from Bancroft, Ontario, Canada. Specimen is approximately 4 inches (10 centimeters) across. |
| Biotite from Bancroft, Ontario, Canada. Specimen is approximately 4 inches (10 centimeters) across. |
| An edge view of the biotite specimen from the photo above. Specimen is approximately 3/8 inch (.95 centimeter) thick. |
| Core samples of biotitic sandstone from the Apple Creek Formation, Copper Queen Mine, near Salmon, Idaho. USGS image. |
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