Potassium Alum
Alum-(K) (also known as Potassium Alum) is a sulfate mineral that is very rarely available as a faceted gem. The chemical formula of Alum-(K) is KAl(SO4)2·12(H2O) (Hydrated Potassium Aluminum Sulfate) and its molecular weight is 474.39 gm. It is a very unusual mineral in that it is both barely radioactive and water soluble. Alum-(K) has a specific gravity of 1.757 (g/cm3) and the refractive index of 1.456 Isotropic. It was originally named Potassium Alum but was renamed to Alum-(K) by Mineralogical Record, v39 (2008), p131 to indicate that it is the K-dominant member of the Alum Group. Alum-(K) is commonly used in water purification, leather tanning, and fireproofing textiles. It also has cosmetic uses as an ingredient in deodorant and as an aftershave treatment due to its astringent qualities.
There are very few localities to find gem quality crystals of Alum-(K). A few of these are the El Desierto mine, San Pablo de Napa, Daniel Campos Province, Potosí Department, Bolivia; Alaunhöhle, near Dresden, Saxony, Germany; Alum mine, Alum District, Esmeralda County, Nevada, USA; Alum King Mine, Marysvale District (Marysvale Uranium area), Piute County, Utah, USA.
| Chemical Formula: | KAl(SO4)2·12(H2O) |
| Hydrated Potassium Aluminum Sulfate | |
| Molecular Weight: | 474.39 gm |
| Composition: | Potassium | 8.24 % | K | 9.93 % | K2O |
| Aluminum | 5.69 % | Al | 10.75 % | Al2O3 | |
| Hydrogen | 5.10 % | H | 45.57 % | H2O | |
| Sulfur | 13.52 % | S | 33.75 % | SO3 | |
| Oxygen | 67.45 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE | |||
| Crystallography: | Isometric – Diploidal |
| Crystal Habit: | Rarely as tiny octahedral crystals, if precipitated from pure water solution; cubic from alkaline solutions. Commonly stalactitic, columnar, granular, or massive efflorescences. |
| Twinning: | Very rare; on {111} |
| Cleavage: | {111} Poor/Indistinct, Interrupted |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Moh’s Hardness: | 2.0 – 2.5 |
| Density: | 1.757 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Barely Detectable; GRapi = 124.32 (Gamma Ray American Petroleum Institute Units) |
| Other: | Soluble in water, taste sweetish and astringent; deliquesces at 91º C |
| Color: | Colorless, White; Colorless in transmitted light |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous |
| Refractive Index: | 1.456 Isotropic |
| Birefringence: | None (Isotropic); may be weakly anomalously birefringent and sectored |
| Dispersion: | n/a |
| Pleochroism: | None |
