Alumstone (alunite rock) from the Tertiary of Utah, USA. (5.7 cm across at its widest)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The sulfate minerals all contain one or more sulfate anions (SO4-2).

Alunite is a potassium aluminum hydroxy-sulfate 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 above). 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).

The alumstone sample shown here formed as a replacement of previously existing material by volcanic H2S-rich steam and hydrothermal fluids during the Early Miocene, at 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).

Locality: Marysvale area, Sevier River Valley, southern Utah, USA

Photo gallery of alunite: