Annite
A variety of Minerals
What is Annite?
Annite is a phyllosilicate mineral in the mica family. It has a chemical formula of KFe3AlSi3O10(OH)2. Annite is the iron end member of the biotite mica group, the iron rich analogue of magnesium rich phlogopite. Annite is monoclinic and contains tabular crystals and cleavage fragments with pseudohexagonal outlines. There are contact twins with composition surface {001} and twin axis {310}. Annite was first described in 1868 for the first noted occurrence in Cape Ann, Rockport, Essex County, Massachusetts, US. It also occurs on Pikes Peak, El Paso County, Colorado. It occurs in igneous and metamorphic rocks that are deficient in magnesium and is associated with fluorite and zircon in the type locality.
Uses & Applications
Annite is a member of the mica group and has very similar properties as other micas such as muscovite and biotite. More importantly, annite is interesting to geologists because it can be used for potassium-argon dating. Because annite contains large amounts of potassium, it can be used to find the absolute age of articles older than 1000 years. This type of dating also preserves a record of the direction and intensity of the local magnetic field, giving field geologists better knowledge of their surroundings.
Market Value Factors
Pricing varies for every rock and mineral, so use these universal factors to gauge Annite before comparing listings or appraisals.
Size & Weight
Larger, intact specimens usually command higher prices.
Rarity & Demand
Scarce material or popular varieties sell at a premium.
Condition & Finish
Chips, repairs, and heavy wear lower value; clean prep helps.
Treatment & Provenance
Untreated specimens with documented locality are prized.
Annite Localities Map
See where Annite is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.
Key Characteristics
Characteristics of Annite
The relief of a mineral refers to the way a mineral may stand out in plane polarized light. A mineral may be referred to as having a low or high relief. Minerals with a high relief, such as annite, have sharp grain boundaries and display good fracture and cleavage. When viewed under a microscope, this mineral may appear to stick out of the other minerals in the thin section. Relief primarily depends on the index of refraction of the mineral. The index of refraction of a mineral is a measure of the speed of light in the mineral. It is expressed as a ratio of the speed of light in vacuum relative to that in the given mineral. Annite has three indices of refraction known to be nα = 1.625 - 1.631 nβ = 1.690 nγ = 1.691 - 1.697. It is also an anisotropic mineral, meaning under the cross polars of a microscope the mineral will become extinct every 90°. However, in plane polarized light, annite appears as a brown or green platy form and is pleochroic, meaning the mineral changes colors under a microscope without the polars being crossed.
Quick Facts
Physical Properties
- Color
- Black, brown
- Hardness (Mohs)
- 2.5 - 3
- Density
- 3.36 g/cm³
Chemical Properties
- Chemical Formula
- KFe2+3(AlSi3O10)(OH)2
- Elements
- Al, Fe, H, K, O, Si
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Annite FAQs
How do I identify Annite?
Annite can be identified by its hardness of 2.5 - 3 on the Mohs scale, Black color. Look for these key characteristics when examining specimens.
What color is Annite?
Annite typically appears in Black, brown. Color can vary depending on impurities and formation conditions.
How hard is Annite?
Annite has a hardness of 2.5 - 3 on the Mohs scale. This makes it a soft mineral that can be scratched easily.
What is the characteristics of annite of Annite?
The relief of a mineral refers to the way a mineral may stand out in plane polarized light. A mineral may be referred to as having a low or high relief. Minerals with a high relief, such as annite, have sharp grain boundaries and display good fracture and cleavage. When viewed under a microscope, this mineral may appear to stick out of the other minerals in the thin section. Relief primarily depends on the index of refraction of the mineral. The index of refraction of a mineral is a measure of the speed of light in the mineral. It is expressed as a ratio of the speed of light in vacuum relative to that in the given mineral. Annite has three indices of refraction known to be nα = 1.625 - 1.631 nβ = 1.690 nγ = 1.691 - 1.697. It is also an anisotropic mineral, meaning under the cross polars of a microscope the mineral will become extinct every 90°. However, in plane polarized light, annite appears as a brown or green platy form and is pleochroic, meaning the mineral changes colors under a microscope without the polars being crossed.
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