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The Sulfates are an important mineral class and include some very interesting and attractive specimens. Although many minerals belong to this class, only barite, gypsum, and anhydrite can be considered common. The basic chemical unit is the (AO4) complex anion with a charge of negative two (-2). The sulfites, selenites and tellurites (notice the spelling) have a basic unit of (AO3)The A can be either sulfur (S), chromium (Cr), tungsten (W),selenium (Se), tellurium (Te) and/or molybdenium (Mo). The principle anion group never shares oxygens with other principle anion groups and this limits the structural possibilities. The A atom at the center of the AO4 anion has a positive six charge (+6) and the oxygens have their obligatory negative two charge (-2). The AO4 anions form symmetrical tetrahedrons when A is either sulfur or chromium and flattened tetrahedrons when A is either molybdenium, selenium or tungsten. The flattened tetrahedrons form a square outline and help produce (in most of those minerals) a tetragonal (four fold) symmetry, which is an uncommon symmetry in minerals. The typical Sulfate Class mineral is vitreous, average to above average in density, average in hardness and are originally formed in veins, oxidation zones, contact metamorphic zones and in evaporite deposits. Some Sulfate Class minerals are soluble and several are fluorescent. All other properties are variable.