Alunite forms from the action of sulfuric acids upon potassium rich feldspars in a process called "alunitization". The sulfuric acids accompany hydrothermal solutions, usually rich in certain ore metals. These solutions can result in large bodies of alunite, making alunite a rock forming mineral. As such, alunite can easily be mistaken for massive rock forming dolomite or limestone (calcite). An acid test should prove adequate in identification as alunite does not bubble even when powdered. Alunite also forms at volcanic fumaroles.
The symmetry of alunite is the same as the members of the Tourmaline Group. Crystals of alunite however do not form prismatic crystals like those of the typical tourmaline mineral. Alunite's crystals are more flattened and resemble nearly cubic rhombohedrons. The "rhombohedrons" are actually a combination of two trigonal pyramids.
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