Amesite is a

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

with general formula of Mg₂Al₂SiO₅(OH)₄. Amesite crystallizes in the

triclinic crystal system 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...

.Roberts, W.L. (1974) Encyclopedia of Minerals, p. 18. Van Nostrand Reinhold Company, New York, New York. It contains three axes of unequal length, not at right angles. It was first described in 1876 for an occurrence in the Chester Emery Mines, Chester,

Hampden County, Massachusetts Hampden County is a non-governmental county located in the Pioneer Valley of the state of Massachusetts, in the United States. As of the 2020 census, Hampden County's population was 465,825. Its traditional county seat is Springfield, the Co ...

. It was named for mine owner James Ames. It occurs in an environment of low-grade metamorphism affecting rocks with high aluminium and magnesium content. It occurs associated with

vesuvianite Vesuvianite, also known as idocrase, is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism. It was first discovered within ...

, chlorite,

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at visib ...

, diaspore,

grossular Grossular is a calcium-aluminium species of the garnet group of minerals. It has the chemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from t ...

, calcite,

diopside Diopside is a monoclinic pyroxene mineral with composition . It forms complete solid solution series with hedenbergite () and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull ...

and

clinozoisite Clinozoisite is a complex calcium aluminium sorosilicate mineral with formula: Ca2Al3(Si2O7)(SiO4)O(OH). It forms a continuous solid solution series with epidote by substitution of iron(III) in the aluminium (m3 site) and is also called ''alumi ...

in various locations. Amesite is an uncommon

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

which has been reported from a variety of locations worldwide. Amesite has the first reported natural occurrence of the 6R

polytype In materials science, polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon. Allotropy refers to polymorphism ...

for a trioctahedral 1:1 layer silicate.

References

Aluminium minerals Magnesium minerals Serpentine group Triclinic minerals Minerals in space group 1 {{silicate-mineral-stub