Each mineral species is a unique, naturally occurring combination of chemical composition and crystal system; e.g., graphite is hexagonal carbon and diamond is isometric carbon, and halite is isometric sodium chloride. (a) Thus, minerals may be classified according to their crystal system. (b) Minerals may be classified chemically according to Dana as (1) native elements and alloys; (2) sulfides, selenides, tellurides, arsenides, and antimonides; (3) sulfosalts, sulfarsenides, sulfantimonides, and sulfobismuthides; (4) halides; (5) oxides; (6) oxygen salts, carbonates, silicates, borates, etc.; (7) salts of organic acids; and (8) hydrocarbon compounds. Silicates are subdivided according to the structural arrangements of their (SiO₄)_4-tetrahedral groups and the number of corner oxygen ions shared between them (degree of polymerism). (c) Additionally, minerals may be classified into isostructural groups; e.g., spinel group, garnet group, mica group, pyroxene group, and zeolite group. (Structural classification is not entirely congruent with chemical classification, since some structural groups may contain more than one chemical group; e.g., the apatite group has mainly phosphates, but some arsenates, vanadates, and silicates have the apatite structure.) (d) Rutley classifies minerals according to group in accordance with the periodic table as regards dominant economic constituents. (e) Optically, minerals are classified as opaque (metallic luster) and nonopaque (transmit light in thin section). (f) Economically, minerals are classified as metallics if they are the source of metal from ores and nonmetallics if their products are not metals.