Alunite crystals morphologically are rhombohedra with interfacial angles of 90° 50´, causing them to resemble cubes. Crystal symmetry is trigonal. Minute glistening crystals have also been found loose in cavities in altered rhyolite. Alunite varies in color from white to yellow gray. The hardness on the Mohs scale is 4 and the specific gravity is between 2.6 and 2.8. It is insoluble in water or weak acids, but soluble in sulfuric acid.
Alunite from Marysvale, UtahSodium can substitute for potassium in the mineral, and when the sodium content is high, is called natroalunite.
Crystal structure of aluniteAlunite is an analog of jarosite, where aluminium replaces Fe3+. Alunite occurs as a secondary mineral on iron sulfate ores.
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