Albite ordering under hydrothermal conditions
Vestnik Otdelenia nauk o Zemle RAN
Structural state of albite is a good indicator of mineral genesis conditions. Earlier it has been shown [McKenzie, 1957; Taylor, 1967] that aluminum distribution between unequivalent positions in albite structure depends on the temperature. Quite disordering "high" albite contains ¼ aluminum atoms in every alumosilicic tetrahedron (Al,Si)O 4 . But in quite ordering "low" albite all aluminum places in tetrahedron Т 1 (0) [Taylor, 1967; Deer, et al., 1966] . Cell parameters of albite change
... albite change depending on structural condition [Kroll&Ribbe, 1980]. It has been shown [Bambauer, et al., 1967a, b] the connection between albite structural state and distance between 131 and 1-31 peaks which is usual to mark as Δ131. For high albite Δ131 is 2.01 (Cu-K α ) and for low albite Δ131 is 1.06(Cu-K α ). The dependence of Δ131 parameter of synthetic albite on temperature was studied in works of different authors [McKenzie, 1967; Mason, 1979; Martin, 1969; Senderov, et al., 1971] . It has to mark the works of Martin and Senderov in which the high alkalinity of solution is shown to essentially influence to the structural condition of albite: the increase of solution pH promoted the forming of more ordering albites. The experimental data connected the Δ131 parameter and temperature have been obtained in temperature interval 350-1000 o C. But because of some causes estimated by kinetics of ordering processes the experimental data obtained at the temperature >400 o C are in better agreement. The purpose of our work was to study the processes of ordering albite synthesis at low temperature (200-500 o C) and to obtain the dependence of Δ131 on temperature at T<500 o C. Albite synthesis was carried out under hydrothermal conditions at temperature 200÷500 o C and pressure 0.5÷1.0 kbar. Gel mixtures of albite composition with addition of 10 wt% amorphous SiO 2 were used as initial mixtures. Synthesis was carried out in NaOH and Na 2 SiO 3 *9H 2 O solutions with the concentration of 2÷10 wt%. The alkaline solutions activate the synthesis of ordering feldspars as it has been previously shown [Martin, 1969; Senderov, et al., 1971] . Experimental duration was up to 65 days.The natural low albite was added to initial mixture (1÷2 wt% of charge mass). Low albite and quartz (and sometimes sodium silicates) were detected in run products. Microprobe analysis has shown a good correspondence of synthetic albites to NaAlSi 3 O 8 formula. Cell parameters of albites were calculated by X-ray study data. The data obtained are presented in tabl. 1 and fig. 1 . These high degree ordering albites with Δ131=1.087 (for Cu-K α ) were synthesized first at 200 o C and 0.5 kbar. The analytical dependence of albtite ordering degree on temperature has been calculated. This dependence may be used as geothermometer for albite-bearing mineral paragenesis: t,(ºC)= [-1255.2+1356.6*(Δ131)] ±20ºC. This equation is worked adequately in temperature interval 100÷500 o C.