Alunite
Mineral KAl3(SO4)2(OH)6
Also called Alumstone
About it
Usually a fine-grained white to pink mass in volcanic rock altered by acid sulfate fluids, and once a source of alum. Hard to tell from kaolin by eye; known from Tolfa in Italy and Marysvale in Utah.
- Type
- Mineral
- Group
- Sulfate, alunite group
- Formula
- KAl3(SO4)2(OH)6
- Hardness (Mohs)
- 3.5–4
- Specific gravity
- 2.6–2.9
- Streak
- white
- Lustre
- vitreous (glassy), pearly, earthy
- Crystal system
- Trigonal
- Colours
- white, grey, yellowish, reddish, pink
- Transparency
- transparent, translucent, opaque
- Cleavage and fracture
- good basal; conchoidal fracture, usually fine-grained masses
Typical values: real specimens vary. For more, see mindat.org.