The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, further said: "And one should not perform tayammum with zarnikh (arsenic) because it is a mineral and not considered earth with soil on its surface." This is consistent with the earlier principle: if tayammum is obligatory with something called "soil," anything not referred to as soil cannot be used for tayammum. The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, also said: "If a person is in muddy terrain and needs to perform tayammum but cannot find soil, they should shake off dust from their clothes, the back of their animal, the saddle, or their luggage. If some dust is released, they should use it for tayammum. If no dust comes out, they should place their hands on the mud, then lift them, rub them together until no moisture remains, and use them to wipe their face and the backs of their hands."
Commentary #7