One broad-brush approach to systematizing minerals is to divide them into two categories: (1) native elements and their alloys (e.g., Au, Ag, Cu, C, S, Ni3Fe, Fe5C2) and (2) compounds (e.g., sulfides, sulfates, oxides, halides, silicates, carbides, carbonates, phosphides, phosphates, tungstates, nitrides). Compounds are grouped together based on their monatomic or polyatomic anions. This is a natural classification system (first adopted by Jacob Berzelius) because the chemical properties of minerals within these groups tend to be similar. For example, the properties of anhydrite (CaSO4) more closely resemble those of other sulfates such as barite (BaSO4) and celestine (SrSO4) than those of other Ca-rich minerals, e.g., calcite (CaCO3), oldhamite (CaS), fluorite (CaF2), antarcticite (CaCl2·6(H2O)), lime (CaO), apatite (Ca5(PO4)3(F,Cl,OH)), scheelite (CaWO4), portlandite (Ca(OH)2), powellite (CaMoO4), and wollastonite (CaSiO3).