Amarantite

Amarantite
General
Category Sulfate minerals
Formula
(repeating unit)		 Fe3+2O(SO4)2·7H2O
Strunz classification 07.DB.30
Crystal symmetry Triclinic pinacoidal
H-M symbol: (1)
Space group: P1
Unit cell a = 8.9 Å, b = 11.56 Å,
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
Crystal system Triclinic
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 [1][2][3]

Amarantite is an amaranth-red to brownish mineral with the general formula of Fe3+2O(SO4)2·7(H2O)[1][2] or Fe3+(SO4)(OH)·3(H2O).[3]

The name comes from the Greek word αμάραντος which means amaranth, an imaginary undying red flower, in allusion to its color.[3]

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.[4] Amarantite is anisotropic,[2] which means, the velocity of light varies with crystallographic direction, and there is more than one refractive index.[5]

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.[4]

References

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  1. ↑ 1.0 1.1 Amarantite in the Handbook of Mineralogy
  2. ↑ 2.0 2.1 2.2 Amarantite on Mindat.org
  3. ↑ 3.0 3.1 3.2 Amarantite data on Webmineral
  4. ↑ 4.0 4.1 Alpers, C.N., Jember, J.L. and, Nordstrom, D.K.(2000) Reviews in Mineralogy and Geochemistry (volume 40) Sulfate Minerals: Crystallography, Geochemisrty, and Environmental Significance
  5. ↑ Klein, C. and C. S. Hurlbut, Jr. (1985) Manual of Mineralogy, Wiley, 20th ed., p. 236 ISBN 0-471-80580-7