ABSTRACT
Minerals which are not silicates have been grouped together in this chapter for the description of their properties. However, unlike the silicates, the crystal structures and chemical variation of members of the group are not easily related to mineralogical properties and the mode of occurrence. Even subdivision of the group into transparent and opaque minerals is impractical, since closely related minerals, and even compositional varieties of the same mineral, may vary in opacity. For example, sphalerite is transparent when it is pure zinc sulphide, but it becomes progressively more opaque with increasing iron substitution of zinc (see Plate 4a).
