Alum-(K)
Mineral KAl(SO4)2·12H2O
Also called Potassium alum and Potash alum
Take care.
- Damaged by water: Keep it dry: don't wash it, and keep it out of damp places.
- Breaks down in damp air: Keep it dry and in steady humidity; some crumble or rot as they take on moisture.
About it
Natural potassium alum, usually as crusts and efflorescences on weathered shale and around volcanic vents. Large clear octahedrons sold as alum are nearly always lab-grown.
- Type
- Mineral
- Group
- Sulfate, alum group
- Formula
- KAl(SO4)2·12H2O
- Hardness (Mohs)
- 2–2.5
- Specific gravity
- 1.75–1.76
- Streak
- white
- Lustre
- vitreous (glassy)
- Crystal system
- Isometric (cubic)
- Colours
- colourless, white
- Transparency
- transparent, translucent
- Cleavage and fracture
- indistinct octahedral; conchoidal fracture
Typical values: real specimens vary. For more, see mindat.org.