A PHASE is a physically separable part of a thermodynamic system. (A SYSTEM is any portion of the universe that we choose to study: a beaker of water + NaCl; a quartz crystal; a granitic magma, the gas in a piston, an assemblage of minerals + a vapor phase at a given P & T in the Earth's crust, etc. The "rest" of the universe is called the SURROUNDINGS). A PHASE is physically and chemically homogeneous. In the system beaker - water (saturated with NaCl) and solid NaCl, the beaker is a phase (glass), the aqueous solution is a phase, and the crystalline NaCl is a phase. One may also consider the air (a gaseous solution) to be part of the system, and in that case air is also a phase in the system. In a metamorphic environment, the genesis of a biotite-garnet-Kspar-quartz-muscovite rock (a metapelite) may be defined as a system containing the stated mineral phases plus or minus a vapor phase (the 5 minerals + the vapor comprising the six phases of the system).

The term PHASE should not be confused with, or used synonymously with, the term 'STATE OF MATTER', which refers to the state of aggregation of the phase: solid, liquid, or gas (a plasma, a partially ionized gas, is sometimes referred to as a fourth state of matter; alternatively, it can simply be referred to as an ionized gas - I prefer the latter).

A crystal or multiple crystals in a system can constitute a single phase IF the crystal or crystals are chemically homogeneous - i.e., have the same chemical composition and crystal structure (within the level of precision assume for the problem at hand). For example, the olivine in a basalt (black lava) with olivine and plagioclase phenocrysts may be described as Fo 90 (Mg/Mg+Fe = 0.9 by mole) ± 1 (one sigma), and at that level of uncertainty, the million olivine phenocrysts per cubic meter of basalt may be considered as a PHASE in the magmatic system. However, the zoned crystals of plagioclase feldspar would not be a PHASE, because the ratio of Na-feldspar to Ca feldspar varies within any single crystal. When discussing chemical equilibrium involving plagioclase in a magmatic system, one must specify the composition of the phase.