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[SCG46-03] Extremely slow grain growth of albitic feldspar
Keywords:Albite, Grain Growth, Crustal Rheology
Feldspar is one of the main constituent minerals of the Earth’s crust. The grain growth and the mechanical behavior of plastic deformation of feldspar have well been studied for anorthite (CaAl2Si2O8), because it is considered that this mineral controls the strength of the lower crust. However, the grain growth of albite (NaAlSi3O8) has not been studied previously. Grain growth follows the grain growth law and the rate constant was determined in the experiments.
The starting materials were prepared as follows. Albite powders for industrial use were pulverized using a HERZOG HP-MS60 automatic pulverizer at the GSJ Laboratory (GSJ-Lab.) of Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ, AIST), and fine-grained fractions less than a few hundred nm were separated by decantation. The fine-grained powders were hot pressed using a multi-purpose high temperature furnace (Fuji Dempa High Multi 10000 at AIST Chubu) at temperature of 1080 ºC and pressure of 120 MPa for 16 hours. Then the sample were cut into small pieces to perform the grain growth experiments at different conditions. Surfaces of samples were mirror polished using 1 mm diamond paste before the experiments to observe the grain boundaries by thermal or chemical etching. The grain growth experiments were performed by annealing the sample in a tube furnace (Carbolite 14/450 at AIST Tsukuba Central 7) at the atmospheric pressure and temperatures of 1030, 1050, 1065 ºC. The grain size was determined by linear intercept method after chemical etching for the initial grain size. The sample surfaces were observed in the HITACHI SU-3500 scanning electron microscope (SEM) installed at the GSJ-Lab. of GSJ, AIST.
The initial grain size is 172.9 nm. The grains grow to the grain sizes of about 282 nm after 156 hours, about 378 nm after 190 hours and about 329 nm after 64 hours at the temperatures of 1030 ºC, 1050 ºC and 1065 ºC, respectively. When the growth exponent is assumed to be 2.6, the regressions of the results yield the rate constants of 1.26×10-23 m2.6s-1, 2.58×10-23 m2.6s-1 and 5.07×10-23 m2.6s-1 at 1030 ºC, 1050 ºC and 1065 ºC, respectively. The Arrhenius plot for the results of the annealing experiments of albite yield the activation energy of Q=580 kJ/mol and the pre-exponent of the rate constant k0=1.83 m2.6s-1.
The extrapolation of the grain growth curves to geological time scale indicates that it takes almost 1 century for grains to grow from 1 μm to 10 μm even at the temperature of 1050 ºC, the grain growth is extremely slow. The experiments in this study can be considered to have been under anhydrous conditions and may be different from the natural hydrous conditions. An experiment of reaction rim of albite that were formed from nepheline (NaAlSiO4) and quartz (SiO2) suggests that grain growth rate under hydrous and anhydrous conditions are not significantly different at the temperature of 1100 ºC.
The results in this study and above discussion suggest that once fine-grained feldspar with composition close to albite is formed, these feldspar grains will never grow and affect the mechanical behavior for long time. This process enables the grain-size sensitive creep of plagioclase to be important in the wide range of crustal conditions.
The starting materials were prepared as follows. Albite powders for industrial use were pulverized using a HERZOG HP-MS60 automatic pulverizer at the GSJ Laboratory (GSJ-Lab.) of Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ, AIST), and fine-grained fractions less than a few hundred nm were separated by decantation. The fine-grained powders were hot pressed using a multi-purpose high temperature furnace (Fuji Dempa High Multi 10000 at AIST Chubu) at temperature of 1080 ºC and pressure of 120 MPa for 16 hours. Then the sample were cut into small pieces to perform the grain growth experiments at different conditions. Surfaces of samples were mirror polished using 1 mm diamond paste before the experiments to observe the grain boundaries by thermal or chemical etching. The grain growth experiments were performed by annealing the sample in a tube furnace (Carbolite 14/450 at AIST Tsukuba Central 7) at the atmospheric pressure and temperatures of 1030, 1050, 1065 ºC. The grain size was determined by linear intercept method after chemical etching for the initial grain size. The sample surfaces were observed in the HITACHI SU-3500 scanning electron microscope (SEM) installed at the GSJ-Lab. of GSJ, AIST.
The initial grain size is 172.9 nm. The grains grow to the grain sizes of about 282 nm after 156 hours, about 378 nm after 190 hours and about 329 nm after 64 hours at the temperatures of 1030 ºC, 1050 ºC and 1065 ºC, respectively. When the growth exponent is assumed to be 2.6, the regressions of the results yield the rate constants of 1.26×10-23 m2.6s-1, 2.58×10-23 m2.6s-1 and 5.07×10-23 m2.6s-1 at 1030 ºC, 1050 ºC and 1065 ºC, respectively. The Arrhenius plot for the results of the annealing experiments of albite yield the activation energy of Q=580 kJ/mol and the pre-exponent of the rate constant k0=1.83 m2.6s-1.
The extrapolation of the grain growth curves to geological time scale indicates that it takes almost 1 century for grains to grow from 1 μm to 10 μm even at the temperature of 1050 ºC, the grain growth is extremely slow. The experiments in this study can be considered to have been under anhydrous conditions and may be different from the natural hydrous conditions. An experiment of reaction rim of albite that were formed from nepheline (NaAlSiO4) and quartz (SiO2) suggests that grain growth rate under hydrous and anhydrous conditions are not significantly different at the temperature of 1100 ºC.
The results in this study and above discussion suggest that once fine-grained feldspar with composition close to albite is formed, these feldspar grains will never grow and affect the mechanical behavior for long time. This process enables the grain-size sensitive creep of plagioclase to be important in the wide range of crustal conditions.