Hydrated magnesium sulfate in the form of white crystalline salt – epsom salts add magnesium to the soil.
Merriam-Webster Online Dictionary
Epsom salts (noun plural but singular or plural in construction)a bitter colorless or white crystalline salt MgSO·7HO that is a hydrated magnesium sulfate with cathartic properties - 4 2
Epsom Salts (Wikipedia)
Epsomite, Epsom salt, or magnesium sulfate heptahydrate, is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O.
| Epsomite | |
|---|---|
 | |
| General | |
| Category | Sulfate mineral |
| Formula | MgSO4·7H2O |
| IMA symbol | Esm |
| Strunz classification | 7.CB.40 |
| Dana classification | 29.6.11.1 |
| Crystal system | Orthorhombic |
| Crystal class | Disphenoidal (222) H-M symbol: (2 2 2) |
| Space group | P2₁2₁2₁ |
| Unit cell | a = 11.86, b = 11.99 c = 6.858 [Å]; Z = 4 |
| Identification | |
| Color | White, grey, colorless, or pink, greenish |
| Crystal habit | Acicular to fibrous encrustations |
| Twinning | Rarely observed on {110} |
| Cleavage | {010} perfect {101} distinct |
| Fracture | Conchoidal |
| Mohs scale hardness | 2 |
| Luster | Vitreous, silky when fibrous |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 1.67–1.68 |
| Optical properties | Biaxial (−) |
| Refractive index | nα = 1.433 nβ = 1.455 nγ = 1.461 |
| Birefringence | δ = 0.028 |
| 2V angle | Measured: 52° |
| Solubility | In water |
| Alters to | Dehydrates in dry air |
| References | |
Magnesium sulfate, MgSO4, bitter salts, bath salts, Epsomite