A new commensurate modulated structure in orthoclase

Abstract

Transmission electron microscopy shows that there is an ordered modulated structure in orthoclase (Or84.6Ab13.1An2.3) (Or = orthoclase, Ab = albite, An = anorthite) of an augite monzonite from Wulian, Shandong Province, Northern China. The modulated orthoclase is composed of a series of triclinic (010) layer domains with C\overline 1 symmetry. Each domain has a thickness of 4d010 and the domains are periodically arranged along the b axis. The modulation period along the b axis is thus 104 A (= 8d010). The relationship between the extended unit-cell parameters of the modulated structure (supercell) and the triclinic subcell parameters is: asup \simeqasub; bsup = 8d010 \simeq 8bsub; Csup \simeq Csub; βsup \simeq βsub. The probable space group of the commensurately modulated orthoclase is Pm. The modulated structure probably forms during the phase transition from sanidine (C2/m symmetry) to microcline (C\overline 1 symmetry) through the segregation of Al atoms from T2 sites into T1(0) and T1(m) sites, respectively, in neighboring domains, which are in the albite-twin relationship. The ordering of Al atoms in the tetrahedral sites during the phase transition results in a sinusoidal deviation of the crystal structure from monoclinic symmetry. The Al–Si distribution near the domain boundary positions is likely to be relatively disordered.