Annite is a phyllosilicate mineral in the mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

 family. It has a chemical formula of KFe₃²⁺AlSi₃O₁₀(OH,F)₂. Annite is the iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 end member of the biotite

Biotite

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...

 mica group, the iron rich analogue of magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 rich phlogopite

Phlogopite

Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O102. Iron substitutes for magnesium in variable amounts...

. 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

Cape Ann

Cape Ann is a rocky cape in northeastern Massachusetts on the Atlantic Ocean. The cape is located approximately 30 miles northeast of Boston and forms the northern edge of Massachusetts Bay. Cape Ann includes the city of Gloucester, and the towns of Essex, Manchester-by-the-Sea, and...

, Rockport

Rockport, Massachusetts

Rockport is a town in Essex County, Massachusetts, United States. The population was 6,952 at the 2010 census. Rockport is located approximately 25 miles northeast of Boston at the tip of the Cape Ann peninsula...

, Essex County

Essex County, Massachusetts

-National protected areas:* Parker River National Wildlife Refuge* Salem Maritime National Historic Site* Saugus Iron Works National Historic Site* Thacher Island National Wildlife Refuge-Demographics:...

, Massachusetts, US. It also occurs on Pikes Peak

Pikes Peak

Pikes Peak is a mountain in the Front Range of the Rocky Mountains, west of Colorado Springs, Colorado, in El Paso County in the United States of America....

, El Paso County

El Paso County, Colorado

El Paso County is the most populous of the 64 counties of the state of Colorado of the United States, now more populous than Denver County. The United States Census Bureau concluded that the county population was 622,371 in 2010. In recent years, the population had come closer to that of Denver...

, Colorado. It occurs in igneous

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

 and metamorphic rocks that are deficient in magnesium. It occurs associated with fluorite

Fluorite

Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...

 and zircon

Zircon

Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

 in the type locality

Type locality (geology)

Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

.

Properties

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 n_α = 1.625 - 1.631 n_β = 1.690 n_γ = 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.

Uses

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.