Muscovite- and phengite-3T; crystal structure and conditions of formation

The crystal structures of two 3T phengitic muscovites from schists in North Kazakhstan (6 << P << 9 kbar, T approximately 600 degrees C) and from the pyrope-bearing whiteschists of the Dora-Maira massif, Italy, (P approximately 30 kbar, T approximately 700 degrees C) have been refined in space group P3 (sub 1) 12. Their respective formulae and cell parameters are (K (sub 0.93) Na (sub 0.03) )(Al (sub 1.54) Fe (sub 0.25) Mg (sub 0.21) Ti (sub 0.04) )- (Si (sub 3.34) Al (sub 0.66) )O (sub 10) (OH) (sub 2) , a 5.212, c 29.804 Aa; and (K (sub 0.92) Na (sub 0.01) )- (Al (sub 1.41) Mg (sub 0.60) Ti (sub 0.02) )(Si (sub 3.54) Al (sub 0.46) )O (sub 10) (OH) (sub 2) , a 5.215, c 29.755 Aa. Although the two independent tetrahedral sites are similar in both specimens, the two independent octahedral sites appear to differ geometrically. Assuming that high P favours less distorted structures and that the presence of two independent octahedral filled sites in 3T allows for higher flexibility, the structural features of 3T micas are consistent with a greater abundance of phengite-3T in high-P metamorphic rocks. This is explained in terms of easier fitting between T and O sheets in phengite-3T compared to muscovite-3T, because of composition, and monoclinic phengite, which has only one independent filled octahedral site.