R. N. Sobolev, V. V. Maltsev, E. A. Volkova.
ABSTRACT
Melting process of single mineral grain as well as polymineral rock sample has been analyzed. It was demonstrated that melting is a lengthy, multi-stage process that occurs in the temperature range and starts in the surface layer of a grain. Experimental research of the melting process of oligoclase, alkali feldspar and quartz monomineral fractions (powder and grains) have been carried out. Granitoids and gneisses, from which they were extracted, have also been investigated. It is shown that upon heating rock forming minerals melt under different temperatures. Grains of alkali feldspars melt in the temperature range T = 1000–1160°С. Subsequently, alkali feldspar melt interacts with plagioclase grains, and macro-homogeneous feldspar melt appears at 1160–1350°С. As the temperature rises above 1350°С it starts to react with quartz grains. Thus melt of each mineral is disordered to a variable degree under the same temperature. Complete melt of polymineral rock samples occurs at the temperature sufficiently higher than those for fine dispersed mixture of the same constitution. Two melt formation approaches are considered. For a single grain melt formation can be described as the first order transition: surface melting → volume melting. When heating a polymineral sample, the low-temperature phase melts first. As temperature increases this melt reacts with the surface layer of high-temperature phases. As a result melts with intermediate chemical composition are generated. Thus, three trends for a sequence of melt forming can be highlighted: (1) trend in mineral grain: outer → intermediate → inner parts of a grain; (2) trend for the sequence of gradual minerals transition into the melt: alkali feldspar → plagioclase → quartz; (3) trend in the rock sample volume: outer → intermediate → inner parts of a sample.
KEYWORDS
cluster, crystal lattice, non-equilibrium state, melting process, phase transition, local thermodynamic equilibrium
DOI: 10.31857/S023501062003010X