|Crystal class||Rhombohedral (3)
H-M symbol: (3)
|Unit cell||a = 8.05 Å, c = 21.19 Å; Z = 6|
|Color||Indigo, bright red, brick-red|
|Crystal habit||Granular aggregates of minute crystals; stalactitic porous masses|
|Mohs scale hardness||1.5|
|Specific gravity||1.72 measured|
|Optical properties||Uniaxial (+)|
|Refractive index||nω = 1.500 nε = 1.515|
|Birefringence||δ = 0.015|
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.
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 solfatara environments. It occurs with native sulfur, sal ammoniac, letovicite, alunogen and boussingaultite.
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