Optical second harmonic analysis has been conducted on samples of muscovite, phlogopite, biotite, margarite, ephesite, lepidolite, and zinnwaldite. Ephesite, margarites, and zinn-waldites gave appreciable second harmonic signals, indicating that these minerals are non-centrosymmetric. These results provide the first physical evidence for the absence of a center of symmetry in the minerals and support published refinements of margarite and zinnwaldite.
A very weak and questionable signal in a 2M2 lepidolite with a minor amount of admixed 1Mlepidolite suggests that the 1M form may be acentric. The absence of optical second harmonics for the remaining samples is fairly convincing evidence that they are all centrosymmetric. These results are important in providing constraints on possible cation ordering schemes, and they illustrate the usefulness of second harmonic generation in detecting small deviations from centric symmetry which are difficult to observe by other techniques.