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THE MINERAL JAROSITE

  • Chemistry: KFe3(SO4)2(OH)6, Potassium Iron Sulfate Hydroxide.
  • Class: Sulfates
  • Group: Alunite
  • Uses: Only as mineral specimens.
  • Specimens

Jarosite is not a common mineral. It is closely related to the mineral natrojarosite . Jarosite is isostructural with natrojarosite which means that they have the same crystal structure but different chemistries. In this case, jarosite contains potassium instead of natrojarosite's sodium (natro is derived from the Latin for sodium, natrium , from where sodium gets its symbol, Na ). The two minerals are difficult to distinguish without a chemical test.

Both minerals are isostructural with alunite with a formula of KAl3(SO4)2(OH)6, who lends its name to the Alunite Group of which all three minerals belong.

The symmetry of jarosite is the same as the members of the Tourmaline Group. Crystals of jarosite however do not form prismatic crystals like those of the typical tourmaline mineral. Jarosite's crystals are more flattened and resemble nearly cubic rhombohedrons. The "rhombohedrons" are actually a combination of two trigonal pyramids.

PHYSICAL CHARACTERISTICS:

  • Color is an amber yellow or brown.
  • Luster is vitreous to resinous.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is trigonal; 3 m
  • Crystal Habits include tabular to flattened rhombohedral looking crystals. The "rhombohedrons" are actually a combination of two trigonal pyramids. Crystals are somewhat scarce and small, more commonly as earthy masses, films or crusts, botryoidal and granular.
  • Cleavage is good in one direction but only seen in the larger crystals.
  • Fracture is uneven.
  • Hardness is 2.5 - 3.5.
  • Specific Gravity is approximately 2.9 - 3.3 (average to slightly heavy for translucent minerals, but hard to obtain from crusts)
  • Streak is a pale yellow.
  • Associated Minerals are barite, turquoise, galena, goethite, limonite, hematite and other iron minerals.
  • Notable Occurrences include Jaroso ravine, Sierra Almagrera, Spain and Iron Arrow Mine, Colorado; Maricopa Co., Arizona; Idaho and California, USA.
  • Best Field Indicators are crystal habit, associations, color and hardness.

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