Final answer:
Fissile materials, specifically isotopes like 233U, 235U, and 239Pu, can sustain a nuclear fission chain reaction. The term 'rock' is out of context here, as rocks do not exhibit fissile properties.
Step-by-step explanation:
Material capable of sustaining a nuclear fission chain reaction is described as fissile or fissionable. In a technical sense, fissile material can experience fission with neutrons of any energy, whereas fissionable material necessitates high-energy neutrons. The phenomenon becomes self-sustaining when produced neutrons balance or exceed the absorbed and escaped neutrons. Three specific isotopes, 233U, 235U, and 239Pu, are traditionally recognized as the accessible nuclides that are fissile with slow (thermal) neutrons.
The term 'rock' is mentioned in the question but seems out of context as rocks do not possess fissile properties. Fissile materials are typically not associated with geological rocks but with specific isotopes relevant to nuclear chemistry. However, igneous rocks, mentioned as being composed of feldspar and quartz among other minerals, unlike fissile materials, are characterized by their composition and formation rather than by their fissionable characteristics.