Alunite is a potassium hydroxy-aluminosilicate mineral (KAl3(SO4)2(OH)6). It's fairly nondescript, in terms of its physical properties. It has a nonmetallic luster, a white streak, and varies in color from whites to grays to pinks to reds. One of the more distinctive physical properties is its hardness (H=4).
Sometimes, alunite is a rock-forming mineral (see below). A ~monomineralic alunite rock is called alumstone. Most alunite forms as sulfuric acid (H2SO4) breaks potassium-bearing rocks in near-surface environments. Alunite (alumstone) has been economically exploited in the past, particularly in the Sevier River Valley in the vicinity of Marysvale, southern Utah, USA. Alunite was used as a source of "potash" (K), aluminum (Al), and sulfur (S).
Alunite (alumstone) (5.7 cm across) from the Marysvale area, Sevier River Valley, southern Utah, USA. This alunite formed as a replacement of previously existing material by volcanic H2S-rich steam and hydrothermal fluids during the Early Miocene (21 million years ago). Near the surface, the H2S gas oxidized to H2SO4 (sulfuric acid), which altered volcanic rocks of intermediate chemistry to form alunite rocks (alumstone).