Final answer:
A mineral at equilibrium maintains a stable composition and crystal structure, indicative of a state called chemical equilibrium. This happens when the reaction quotient equals the equilibrium constant, ensuring no net change under stable conditions. Minerals like olivine can have variable compositions but still retain equilibrium.
Step-by-step explanation:
When a mineral is at equilibrium with its environment, it implies that there is no tendency for a change in its composition or crystal structure. This state is known as chemical equilibrium, where the conditions such as temperature, pressure, and chemical composition are stable, and the rate of a forward reaction equals the rate of the reverse reaction, leading to no net change.
A mineral is considered an inorganic solid with a definite chemical composition and a characteristic crystalline structure. Since minerals are the building blocks of rocks, their stability is crucial to geological processes.
The law of constant composition states that in any given chemical compound, the elements always combine in the same proportion. When the reaction quotient (Q) equals the equilibrium constant (K), a mineral system is said to be in equilibrium, meaning no further change occurs unless external conditions are altered.
For instance, the mineral olivine can have varying compositions of magnesium and iron within its chemical formula (Mg,Fe)2SiO4, yet it can still reside in equilibrium given the right conditions. This demonstrates that some minerals have a range of compositions but still maintain their equilibrium state as long as the system remains closed and undisturbed.