Study On The Microscopic Characteristics And Growth Mechanism Of Pyroxene In Melt

Pyroxene is a common silicate rock-forming mineral with a single-chain structure.It is mainly distributed in basic-ultrabasic magmatic rocks and metamorphic rocks.It is the mineral with the highest content except olivine in the upper mantle.Through the study of the microscopic characteristics of pyroxene and its cry stal growth process,we can understand and master its crystal growth law,which is helpful to better understand the growth mechanism of pyroxene crystal in melt,and at the same time,it provides a theoretical basis for the related magmatic process.Pyroxene crystals are easy to precipitate in the melt with calcium or magnesium as the main basic component,and can be widely used in the preparation of pyroxene based glass-ceramics,ceramic materials,fiber materials and other directions.Predecessors mainly studied the growth process of crystals under equilibrium and near equilibrium conditions and related influencing factors,and paid less attention to the growth of crystals under conditions far from equilibrium.The growth mechanism of pyroxene in melt was established by investigating the morphology of pyroxene dendrites at different melt components and melting temperatures.It not only provides experimental mineralogical theory for pyroxene crystal growth in igneous rocks,but also provides theoretical reference for mineral resources utilization and artificial mineral materials preparation.According to the mineral composition of pyroxene and the formation environment in the natural geological body,30 sets of orthogonal experiments were carried out with the reaction system components and temperature as variables.The phase composition,micromorphology and element occurrence state of the samples were investigated by adjusting the Si-Al ratio and other related components.By changing the reaction melting temperature,the change of crystal decomposition,melting,reconstruction and the mechanism of crystal growth in the melting process were studied.In order to investigate the microscopic characteristics and growth process of pyroxene,the phase,composition and morphology of raw materials and synthetic pyroxene crystals were analyzed by means of differential thermal analyzer,X-ray diffractometer(XRD),field emission electron probe(EPMA)and scanning electron microscope(SEM).The experimental results show that:(1)The mineral composition of the selected raw materials is mainly quartz,calcite,garnet,and contains a small amount of dolomite,mica,etc.The chemical composition is mainly SiO2,CaO,Fe2O3,Al2O3,MgO.(2)Samples with main crystal phases of diopside,calcium-ferroxene and common pyroxene were prepared by melting method.The effects of melt composition and melting temperature on the crystallization growth of pyroxene were analyzed and discussed.The crystallization of pyroxene is inhibited by the increase of Si-Al ratio in the melt components.The increase of Fe2O3 component promotes the formation of pyroxene in the matrix and increases the crystallization tendency of pyroxene in the system.With the increase of CaO fraction,pyroxene precipitates in the melt change from diopside to calcium-ferroxene.(3)When the melting temperature does not reach the condition of complete melting,the pyroxene crystals in the sample mainly grow along the nucleus with the residual material as the core.When the melting temperature rises to complete melting,pyroxene has a high degree of idiomorphism,and columnar crystals with well developed crystal surfaces appear.As the melting temperature continues to rise,the viscosity of the melt increases rapidly when the melt cools rapidly in the cooling process.At this time,the resistance to crystallization of the melt increases,and the crystals are too late to nucleate and grow up,resulting in the formation of a large number of glass phase,and only a small amount of pyroxene crystals precipitate out.(4)The crystal growth rule of pyroxene was discussed by using the crystal growth theory,combining with experimental conditions and product characteristics.It is considered that pyroxene crystals grow under the joint action of two dimensional nucleation and diffusion control mechanism in melt.(5)The different dendritic morphologies of pyroxene in the melt were studied systematically,and the morphologic evolution process of pyroxene crystal was obtained,which provided a theoretical basis for the crystallization process of other similar silicate minerals.The pyroxene crystal growth was realized under laboratory conditions,and the specific parameters of the physical and chemical conditions for the growth of pyroxene crystal were obtained.In conclusion,the study on the morphology and variation of pyroxene group minerals in melt is of great significance in the study of dendrite growth model,growth conditions,crystal growth mechanism and other related studies.