Amesite

Amesite is a mineral with general formula of Mg₂Al₂SiO₅(OH)₄.[2]

Amesite
General
Category	Phyllosilicates Kaolinite-serpentine group
Formula (repeating unit)	Mg2Al2SiO5(OH)4
Strunz classification	9.ED.15
Crystal system	Triclinic
Crystal class	Pedial (1) (same H-M symbol)
Space group	C1
Unit cell	a = 5.307(1), b = 9.195(2) c = 14.068(3) [Å] α = 90.09(2)° β = 90.25(2)°, γ = 89.96(2)°; Z = 4
Identification
Color	White, colorless, pink to lilac, pale green
Crystal habit	Crystals form as platy to tapering elongated pseudohexagonal prisms
Twinning	Common as six-fold sector twins on {001} and polysynthetic twins parallel to {010}
Cleavage	Perfect on {001}
Tenacity	Brittle
Mohs scale hardness	2.5-3
Luster	Pearly
Streak	White with pale green tint
Diaphaneity	Transparent to translucent
Specific gravity	2.77
Optical properties	Biaxial (+)
Refractive index	nα = 1.597 nβ = 1.599 nγ = 1.615
Birefringence	δ = 0.018
Other characteristics	Kaolinite-Serpentine group
References	[1][2][3]

Amesite crystallizes in the triclinic crystal system.[4] 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. It was named for mine owner James Ames.[3][2] It occurs in an environment of low-grade metamorphism affecting rocks with high aluminium and magnesium content. It occurs associated with vesuvianite, chlorite, magnetite, rutile, diaspore, grossular, calcite, diopside and clinozoisite in various locations.[1]

Amesite is an uncommon silicate mineral which has been reported from a variety of locations worldwide.[2] Amesite has the first reported natural occurrence of the 6R polytype for a trioctahedral 1:1 layer silicate.[5]

References

1. Handbook of Mineralogy
2. Mindat.org
3. Webmineral data
4. Roberts, W.L. (1974) Encyclopedia of Minerals, p. 18. Van Nostrand Reinhold Company, New York, New York.
5. http://www.minsocam.org/ammin/AM61/AM61_497.pdf