THE MINERAL NATROJAROSITE

Chemistry: NaFe3(SO4)2(OH)6, Sodium Iron Sulfate Hydroxide.
Class: Sulfates
Group: Alunite
Uses: Only as mineral specimens.
Specimens

Natrojarosite is a somewhat rare sulfate mineral. It is closely related to the mineral jarosite. 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 natrojarosite is the same as the members of the Tourmaline Group. Crystals of natrojarosite however do not form prismatic crystals like those of the typical tourmaline mineral. Natrojarosite's crystals are more flattened and resemble nearly cubic rhombohedrons. The "rhombohedrons" are actually a combination of two trigonal pyramids. Unfortunately crystals of natrojarosite are rather scarce.

PHYSICAL CHARACTERISTICS:

Color is yellow, orange or brown.
Luster is vitreous.
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 scarce, 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 3 - 4.
Specific Gravity is approximately 3.1 - 3.3 (slightly heavy for translucent minerals, but hard to obtain from crusts)
Streak is yellow.
Associated Minerals are barite, goethite, limonite, hematite and other iron minerals.
Notable Occurrences include Black Hills, South Dakota and Soda Springs Valley, Nevada USA; Chili; Russia; Mexico and Greece.
Best Field Indicators are crystal habit, associations, color and hardness.