Purity: Hierarchy of Materials for Tayammum (When Primary Items are Unavailable)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 675:
If a person cannot find soil, pebbles, a clod of earth, or stone, he must perform tayammum with mud; and if one cannot find any mud, he must perform tayammum on top of a carpet, clothing, or similar thing that has gathered dust or on which dust has settled but not to the extent that it can commonly be considered soil. If none of these can be found, the recommended precaution is that one should perform prayers without performing tayammum; however, it is obligatory for him to make them up afterwards.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)