Complex sentence: Steatite is relatively soft (because of the high talc content from which it was originally comprised has a definitional value of 1 on the Mohs hardness scale), and may feel soapy when touched, hence the name.
Split into simpler sentences: Steatite is relatively soft (because of the high talc content from which it was originally comprised has a definitional value of 1 on the Mohs hardness scale). Softer grades may feel soapy when touched, hence the name.