Amarantite

Amarantite
General
Category	Sulfate minerals
Formula (repeating unit)	Fe3+2O(SO4)2·7H2O
IMA symbol	Ama[1]
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	2+1⁄2
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	[2][3][4]

Amarantite is an amaranth-red to brownish mineral with the general formula of Fe³⁺₂O(SO₄)₂·7H₂O^[2][3] or Fe³⁺SO₄(OH)·3H₂O.^[4]

The name comes from the Greek word αμάραντος which means amaranth, an imaginary undying red flower, in allusion to its color.^[4]

Amarantite is triclinic, which means crystallographically, it has only one symmetry fold. It must be rotated 360 degrees to be exactly the same. Due to it being triclinic it falls into the biaxial optical class, the axis degrees do not equal 90 degrees and the sides of each axis are not the same length.^[5] Amarantite is anisotropic,^[3] which means, the velocity of light varies with crystallographic direction, and there is more than one refractive index.^[6]

Amarantite is a very rare mineral and can only be found in a couple of places such as Carocoles, Chile. Although it is a source of iron, there is not enough amarantite to be mined for iron. However, when found in crystal form its red orange color gives it value as a collectors item.^[5]