Amarantite

Amarantite
General
Category	Sulfate minerals
Formula (repeating unit)	${\displaystyle {\ce {Fe^{3+}2O(SO4)2.7H2O}}}$
Strunz classification	7.DB.30
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 8.9, b = 11.56 c = 6.64 [Å]; α = 95.55° β = 90.52°, γ = 97.42°; Z = 2
Identification
Color	Amaranth-red to brownish red, orange-red
Crystal habit	As radiating or matted aggregates of needles; may be columnar or bladed
Cleavage	Perfect on {010} and {100}
Tenacity	Brittle
Mohs scale hardness	2½
Luster	Vitreous
Streak	Lemon-yellow
Diaphaneity	Transparent
Specific gravity	2.189 - 2.286
Optical properties	Biaxial (-)
Refractive index	nα = 1.516 nβ = 1.598 nγ = 1.621
Birefringence	δ = 0.105
Pleochroism	X = colorless; Y = pale yellow; Z = reddish brown
2V angle	Measured: 30°
References

Amarantite is an amaranth-red to brownish mineral with the general formula of ${\displaystyle {\ce {Fe^{3+}2O(SO4)2.7H2O}}}$ or ${\displaystyle {\ce {Fe^{3+}(SO4)(OH).3H2O}}}$.

...
Wikipedia