A NEW COMMENSURATE MODULATED STRUCTURE IN ORTHOCLASE

Transmission electron microscopy shows that there is an ordered modula ted structure in orthoclase (Or(84.6)Ab(13.1)An(2.3)) (Or = orthoclase , Ab = albite, An = anorthite) of an augite monzonite from Wulian, Sha ndong Province, Northern China. The modulated orthoclase is composed o f a series of triclinic (010) layer domains with C ($) over bar 1 symm etry. Each domain has a thickness of 4d(010) and the domains are perio dically arranged along the b axis. The modulation period along the b a xis is thus 104 Angstrom (= 8d(010)). The relationship between the ext ended unit-cell parameters of the modulated structure (supercell) and the triclinic subcell parameters is: a(sup) similar or equal to a(sub) ; b(sup) = 8d(010) similar or equal to 8b(sub); c(sup) similar or equa l to c(sub); beta(sup) similar or equal to beta(sub). The probable spa ce group of the commensurately modulated orthoclase is Pm. The modulat ed structure probably forms during the phase transition from sanidine (C2/m symmetry) to microcline (C ($) over bar 1 symmetry) through the segregation of Al atoms from T-2 sites into T-1(0) and T-1(m) sites, r espectively, in neighboring domains, which are in the albite-twin rela tionship. The ordering of Al atoms in the tetrahedral sites during the phase transition results in a sinusoidal deviation of the crystal str ucture from monoclinic symmetry. The Al-Si distribution near the domai n boundary positions is likely to be relatively disordered.