After Captain Kirk and Spock had an encounter with the Mother Horta on Janus VI in 2267, resulting in their phasering off a piece of the creature, Kirk observed that what they removed was not animal tissue. Upon further observation, Spock's closest approximation of what the material was, was that of fibrous asbestos, supporting his theory that the Horta was a silicon-based lifeform. (TOS: "The Devil in the Dark" )