Edenite

Edenite
Edenite crystal from Bancroft District, Ontario
General
Category	Amphibole mineral
Formula (repeating unit)	NaCa2Mg5(Si7Al)O22(OH)2
IMA symbol	Ed[1]
Strunz classification	9.DE.15
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Unit cell	a = 9.83, b = 17.95 c = 5.3 [Å], β = 105.18°; Z = 2
Identification
Formula mass	834.25 g/mol
Color	White to gray, pale to dark green, also brown and pale pinkish-brown
Crystal habit	Prismatic crystals, fibrous, as reaction rims on pyroxenes
Twinning	Simple or multiple parallel to {010}
Cleavage	Good on (110)
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Streak	White
Diaphaneity	Semitransparent
Specific gravity	3.05–3.37
Optical properties	Biaxial (+)
Refractive index	nα = 1.606 – 1.649, nβ = 1.617 – 1.660, nγ = 1.631 – 1.672
Birefringence	δ = 0.025
Pleochroism	Distinct, greens, blue-greens, and yellow-browns
2V angle	50–82°
References	[2][3][4][5]

Edenite or edenitic hornblende is a double chain silicate mineral of the amphibole group with the general chemical composition NaCa₂Mg₅(Si₇Al)O₂₂(OH)₂. Edenite is named for the locality of Edenville, Orange County, New York, where it was first described.^[4]

Edenite has been found primarily in metamorphic rocks, occurring in pods of other magnesium-rich minerals within a marble formation or with garnet-rich lherzolites from deep within the Earth's crust.^[6][7] Thus, finding edenite in the field can indicate high-temperature regional metamorphism of the surrounding rocks.

While edenite is not important for commercial or industrial applications, it is often studied because of its unique chemical substitution properties. Results from research performed on amphiboles have shown that edenite is particularly suited for fitting chloride anions into its chemical framework.^[8] This makes edenite a good candidate for use in chlorine isotope fractionation in amphibole-bearing rocks. Many synthetic variations of edenite are also used in geochemical research to produce a boron analogue of fluoroedenite.^[9]

Edenite is a member of the monoclinic crystal system and is in the crystal class 2/m (space group C2/m).^[6] This means crystalline edenite is symmetrical around a two-fold rotation axis that is then reflected across a mirror plane perpendicular to the long axis of the mineral.^[10]

Edenite is a biaxial positive mineral. When viewed in thin section with a petrographic microscope, it is white-gray with pale green pleochroism in plane-polarized light. Under crossed polars, its interference colors range from first order gray to first order blue.^[3]