NATROALUNITE

    Class : Sulfates, chromates, molybdates
    Subclass : Anhydrous sulfates
    Crystal system : Trigonal
    Chemistry : NaAl3(SO4)2(OH)6
    Rarity : Rare to uncommon

Natroalunite is an uncommon sulfate, the sodium equivalent of alunite. It is much less common than the latter, it is found in quite varied environments : solfataras, sometimes metamorphic rocks or hydrothermal veins. Its name comes from its chemical composition, from the Latin natrium (sodium) and its chemical analogy with alunite. Natroalunite is a white, sometimes grayish to yellowish or pale brownish mineral that usually occurs in grainy masses, rarely in pseudocubic rhombohedral crystals.

Main photo : Natroalunite from San Pedro Mine, Spain © Gascón Cuello Fernando

Natroalunite in the World

Natroalunite is known in different American, Australian and Chilean localities. The best samples come from National Belle in Colorado.

Right photo : Natroalunite from Cerro do Algaré Mine, Portugal © Pedro Alves - SEM picture

Natroalunite in France

Natroalunite is reported at Montagne Pelée (Martinique).

Twinning

No twins known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 3.5 to 4
Density : 2.6 to 2.9
Fracture : Irregular to conchoidal
Streak : White

TP : Translucent to transparent
RI : 1.568 to 1.590
Birefringence : 0.022
Optical character : Uniaxial +
Pleochroism : None
Fluorescence : None

Solubility : Sulfuric acid

Magnetism : NoneRadioactivity : None