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When observed at room temperature using a transmitted light microscope, most fluid inclusions have a rather sharp outer boundary marking the edge of the inclusion cavity (Fig. 2.1). This is because of a significant difference in refractive index between inclusion fluids and their mineral hosts: most aqueous fluids have refractive indices between 1.33 and 1.45 whereas the minerals in which they are included have refractive indices from 1.43 to as high as 3.22. Hydrocarbon liquids, however, have refractive indices that may be similar to their mineral hosts (Burruss, 1981), and thus, are not all easily visible. The inclusion cavity generally contains a large amount of bright, clear liquid (Fig. 2.1A, D, E) and some may contain a small dark bubble of vapor or gas (although any liquid-to-vapor ratio is possible) that is dark because of internal reflection (Fig. 2.1D). However, as shown in Figure 2.1E, bubbles in flat inclusions may not appear that dark. Though most liquids appear colorless, some hydrocarbon liquids may have colors ranging from reddish-brown to yellow.

Inclusions smaller than 1 µm currently are not possible to study because of microscope optical limitations. The sizes of most inclusions readily studied in diagenetic phases are about 2 to 7 µm in longest dimension. For the most part, coarsely crystalline diagenetic minerals contain more workable-sized inclusions than fine-grained minerals, and smaller inclusions are typically much more abundant than larger inclusions in diagenetic phases. Because of the small size of the inclusions, petrographic study requires a good microscope properly adjusted,

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