Efremovite

Efremovite is a rare ammonium sulfate mineral with the chemical formula: (NH₄)₂Mg₂(SO₄)₃. It is a white to gray cubic mineral.^[4] This anhydrous sulfate occurs as constituent in sulfate crusts of burning coal dumps. It is hygroscopic and when exposed to humid air it slowly converts to the hydrate form, boussingaultite.^[5][6]

Efremovite
General
Category	Sulfate minerals
Formula	(NH4)2Mg2(SO4)3
IMA symbol	Efr[1]
Strunz classification	7.AC.10
Crystal system	Cubic
Crystal class	Tetartoidal (23) H-M symbol: (23)
Unit cell	a = 9.99 Å; Z = 2
Identification
Color	White to gray
Crystal habit	Equant grains and crusts
Cleavage	None
Fracture	Uneven
Mohs scale hardness	2
Luster	Vitreous
Diaphaneity	Transparent to nearly opaque
Specific gravity	2.52 (calculated)
Optical properties	Isotropic
Refractive index	n = 1.550
Alters to	Readily hygroscopic
References	[2][3][4]

It was first described in 1989 for an occurrence in the Chelyabinsk coal basin, Southern Urals, Russia. It was named for Russian geologist Ivan Antonovich Yefremov (1907–1972). It has also been reported from several coal mining areas across Europe.^[3] It occurs in association with native sulfur, kladnoite [it], mascagnite, and boussingaultite.^[4]