Purity: Mixtures in Tayammum Material

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 678:

If something like straw – with which performing tayammum is invalid – becomes mixed with soil or pebbles, one cannot perform tayammum with it. However, if the quantity of that thing [such as straw] is so little that it is considered to have disappeared in the soil or pebbles, then performing tayammum with the soil or pebbles is valid.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)