Abstract: In the prevailing igneous theory, the term magma is acquiescently considered synonymous to natural melt. Consequently, all mineral crystals in an igneous rock are crystallized from melt and can be named as melt crystals. However, many observations imply that crystals crystallized from supercritical fluid phases also occur in igneous rocks and can be classified as supercritical fluid crystals or simply fluid crystals. In this paper, based on the analysis of three typical instances, we proposed that fluid crystals not only can form directly in a supercritical fluid phase, but also can form in the melt phase generated by the supercritical fluid through concentration. By all accounts, the precondition for fluid crystals is that the magmatic fluid attains to the supersaturation state. In order to satisfy this fundamental condition, the transmagmatic fluid process and/or rapid ascent of the magma are required. Based on previous investigations of melt-fluid equilibrium condition, and considering the dependencies of melt viscosity on volatile content and of magma ascent rate on magma viscosity, we found a coupling relationship between transmagmatic fluid process and magma ascent. This relation can be used to explain the nonlinear behavior of a magma system. Thus, it is very important to study fluid crystals as (1) it provides a new approach to reconstruct the fluid condition of a magma system; (2) it reveals the deviation level of a magma system from the ideal state; (3) it adds a new constraint to the inversion of dynamic processes of a magmatic system; (4) it provides new mineralogical indicators for recognizing ore-causal intrusions and assessing ore producing potentials; and (5) it provides solutions to some long-standing ambiguous issues in the prevailing igneous theory, such as solubility of magmatic volatile components, reactivation of frozen magma and specificity of magmatic metallogenesis.

Key words: fluid crystal, transmagmatic fluid, volatile, decompressing discharge, solubility, melt