Abstract
Many minerals contain structural modulations that arise from cation site ordering, from slight positional displacements, or from a combination of both. Such fluctuations can be produced during crystal growth, cooling, spinodal decomposition, chemical reaction, or from other sorts of transformations. The effects are subtle and are commonly difficult to observe with many of the traditional mineralogical tools; this is especially true if some structural disorder also occurs. Related effects occur in minerals that contain intergrowths of different structure types on a unit cell scale.
Transmission electron microscopy, because of its high spatial resolution and its ability to image non-periodic features, is well adapted for the study of modulated crystals and intergrowth structures. High-resolution imaging permits direct observation of structural fluctuations, whereas electron diffraction provides a highly sensitive probe of larger regions as well as distinguishing between modulations resulting from displacive vs. substitutional effects. Microanalysis using the electron beam as a primary excitation source holds potential for detecting and measuring compositional differences across small regions, in some cases having dimensions comparable to modulation wavelengths. Electron beam instruments clearly are a powerful way for investigating the long-period structural complexities that occur in many minerals.
