Purity: Disapproved and Invalid Soils for Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 688:
It is disapproved to perform tayammum with the earth of a pit, the soil of a road, and the ground of a salt marsh on which salt has not settled; and if salt has settled on it, the tayammum is invalid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)