Millosevichite is a rare sulfate mineral with the chemical formula Al₂(SO₄)₃.^[3] Aluminium is often substituted by iron. It forms finely crystalline and often porous masses.

Millosevichite
Porous yellow millosevichite. Dimensions: 14 mm × 10 mm × 8 mm. Locality: Kladno Mine (Nejedlý I Mine; Zdeněk Nejedlý Mine; Schoeller Mine; Schöller Mine), Libušin, Kladno, Central Bohemia Region, Bohemia (Böhmen; Boehmen), Czech Republic.
General
Category	Sulfate mineral
Formula (repeating unit)	Al2(SO4)3
IMA symbol	Msv[1]
Strunz classification	7.AB.05
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3
Unit cell	a = 8.05 Å, c = 21.19 Å; Z = 6
Identification
Color	Indigo, bright red, brick-red
Crystal habit	Granular aggregates of minute crystals; stalactitic porous masses
Mohs scale hardness	1.5
Luster	Vitreous
Diaphaneity	Semitransparent
Specific gravity	1.72 measured
Optical properties	Uniaxial (+)
Refractive index	nω = 1.500 nε = 1.515
Birefringence	δ = 0.015
Other characteristics	Hygroscopic
References	[2][3][4]

It was first described in 1913 for an occurrence in Grotta dell'Allume, Porto Levante, Vulcano Island, Lipari, Aeolian Islands, Sicily. It was named for Italian mineralogist Federico Millosevich (1875–1942) of the University of Rome.^[2]

The mineral is mainly known from burning coal dumps, acting as one of the main minerals forming sulfate crust. It can be also found in volcanic fumeroles (solfatara environments).^[2][5] It occurs with native sulfur, sal ammoniac, letovicite, alunogen and boussingaultite.^[2]