MAXIMUM EXTENSION ALONG B0 OF K-WHITE MICAS WITH INCREASING PRESSURE TO HP/UHP – RETRIEVE BY MODEL CALCULATION

b₀ cell-dimension geobarometer is based on statistic of those b₀s of K-white micas from well investigated metamorphic terrains from low to high pressure types. A general regulation of this geobarometer is that the higher the pressure, the longer the b₀ cell-dimension. It is evident that with increasing pressure to HP/UHP, K-white micas series will change their components from muscovite (Ms) to aluminousceladonite (Al-Cd) to celadonite (Cd). Recent works reveal that the pressure plays an important role in affecting the composition of mica series mainly on the IV and VI sites substitution, and also the interlayer site. This work tries to find out those extensions along b₀ cell-dimension in K-white micas series induced by those substitutions, from which the relationship between pressures and b₀ cell-dimension could be possibly found.

It is summarized for those substitutions that Si⁴⁺↔Al³⁺ (↔ stands for substitution) refers to the substitution in IV site, Fe²⁺Mg↔Al³⁺ and Fe³⁺↔Al³⁺ describe the VI site substitution, and Na¹⁺↔K¹⁺ renders the mainly interlayer site substitution. From the compositions of Ms, Al-Cd and Cd, it is clear that all sites of interlayer of Ms, Al-Cd and Cd are occupied by K¹⁺ and therefore the influence on the space of interlayer by interlayer site substitution is eliminable. Si⁴⁺↔Al³⁺ produces 25% linear reduction and from the formulae (apfu) of Ms, Al-Cd and Cd, the sites of IV are filled with 6Si (Ms) and 8Si (Al-Cd and Cd), this means the maximum substitution of Si⁴⁺↔Al³⁺ is 1/4 and therefore the 25.23% linear reduction is 6.3% effectively happened in tetrahedrons; Fe²⁺Mg↔Al³⁺ induces 24% linear extension and Fe³⁺↔Al³⁺ induces 2.8% linear reduction. Thus, for the VI site substitution it produces 21.6% linear extension within octahedrons. These data indicate that the substitutions of Fe²⁺Mg↔Al³⁺ and Fe³⁺↔Al³⁺ within octahedrons plays a more important role than the Si⁴⁺↔Al³⁺ substitution in tetrahedrons if increasing pressure, or in other word, it is totally about 15.3% extension if increasing pressure. It is expected that the 15.3% total extension would induce the variation along a₀ and b₀ and, by the way, if Fe²⁺Mg↔Al³⁺ happen in the M-trans site (M site is also called the VI site) the extension could contribute mostly to b₀ direction. This agrees with the statistic results of K-white mica b₀ geobarometer.