Emmonsite, also known as durdenite, is an iron tellurite mineral with the formula: Fe₂(TeO₃)₃·2(H₂O).^[2] Emmonsite forms triclinic crystals.^[2] It is of a yellowish-green color, with a vitreous luster,^[2] and a hardness of 5 on the Moh scale.^[3]

Emmonsite
Emmonsite. Locality: San Miguel Mine, Moctezuma, Sonora, Mexico (size: 6.3 x 4.1 x 1.1 cm)
General
Category	Tellurite mineral
Formula (repeating unit)	Fe2(TeO3)3·2(H2O)
IMA symbol	Ems[1]
Strunz classification	4.JM.10
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 7.90, b = 8.00 c = 7.62 [Å]; α = 96.73° β = 95°, γ = 84.47°; Z = 2
Identification
Color	Yellowish green
Crystal habit	Thin to hairlike crystals, occurring in rosettes and sprays; also fibrous globular aggregates and crusts
Twinning	Noted
Cleavage	Perfect on {010}; good on {100} and {001}
Mohs scale hardness	5
Luster	Vitreous
Diaphaneity	Opaque to translucent
Specific gravity	4.52–4.55
Optical properties	Biaxial (-)
Refractive index	nα = 1.962 nβ = 2.090 nγ = 2.100 - 2.120
Birefringence	δ = 0.138 - 0.158
Pleochroism	Weak
2V angle	Measured: 23°
References	[2][3][4]

Emmonsite was first described in 1885 for an occurrence in the Tombstone District, Cochise County, Arizona. It was named for the American geologist, Samuel Franklin Emmons, (1841–1911), of the United States Geological Survey.^[2][3]

Emmonsite is found, often with quartz or cerussite in the Tombstone, Arizona area.^[3] It is also associated with native tellurium, tellurite, native gold, pyrite, rodalquilarite, mackayite, sonoraite, cuzticite and eztlite.^[2]