Class F - Geometallic
Age: 1-3 billion years
Diameter: 10,000 - 15,000 km
Surface: Volcanic eruptions due to molten core
Atmosphere: Hydrogen compounds
Evolution: Cools to become Class-G
Life-Forms: Silicon-based (Horta)
Examples: Janus VI
Glass F planets are generally located in a star's habitable zone. Similar in size to Earth, they rage from 10,000 to 15,000 kilometers in diameter. Class F planets are former Class E planets that have cooled, but retain a molten core, and an atmosphere containing hydrogen compounds. Class F planets will eventually cool to become class G.
Although they are not well suited for humanoid life forms, they support silicon-based life quite well. Janus VI, the mineral rich home world of the Horta in "Devil in the Dark" (TOS), is one such world.