ALUNITE
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).