The halide group consists of minerals containing a halogen element, generally chlorine or fluorine, as an essential anion. Although many halides exist, only halite and sylvite are common in sedimentary rocks; they are quite rare in rocks of other sorts. Halite is typically found as rock salt in massive salt beds, often occurring with other evaporite minerals such as gypsum or anhydrite, and sometimes with sulfur. Sylvite is much less common than halite. When found, however, it is usually associated with halite. Fluorite, most commonly, is a hydrothermal mineral found with lead, zinc, and other metal ore minerals. Less commonly, it is found in vugs or fractures in limestone or dolostone. Other halides are rare and have more complex chemistries.
The photos seen here show crystals of halite (Figure 7.52) and fluorite (Figure 7.53). Sylvite crystals commonly look the same as the halite crystals here. When euhedral, both minerals form cubic crystals, a reflection of the internal order of their atoms. The most common fluorite is purple, but just about any color is possible for this mineral.
Figures 7.54 and 7.55 show atomic arrangements in halite and fluorite. In halide minerals, anions and cations alternate and bonds are nearly entirely ionic. In halite (or sylvite), Na+ (or K+) and Cl– alternate in a cubic three-dimensional arrangement. In fluorite, the arrangement of atoms is also cubic, but two F– are present for every Ca2+.