The term dolomite includes a spectrum of mineral compositions and structures. In contrast to the two dolomites commonly recognized, we suggest here that there are three types of dolomite in sediments and sedimentary rocks. Group A dolomite, typical of dolomitic limestones and some dolomicrites, is calcian (> 53% Ca) and has a low manganese partitioning ratio (MnPR < 25), a small crystal size (< 0.05 mm), stable-isotope values of +1 per thousand to +3 per thousand delta 13 C and -4 per thousand to +2 per thousand delta 18 O, and an expanded unit cell ( c axis 16.04-16.12 A). This is also the typical dolomite of Holocene sediments, wherein it differs from dolomite in lithified units in being more calcian, less well ordered, commonly with a very broad Mn ESR signal, and with an MnPR < 5. In contrast, group B dolomite is nearly stoichiometric (< 52% Ca), has a high MnPR (> 50), large crystal size (0.1-0.5 mm), stable-isotope values of -3 per thousand to +4 per thousand delta 13 C and -9 per thousand to -4 per thousand delta 18 O, and a compact unit cell ( c axis 16.01-16.04 A). Group O dolomite, common in evaporite-related dolomicrites, is stoichiometric (49-52% Ca), and has a low MnPR (10-25) and a compact unit cell ( c axis 16.01-16.04 A). Two other dolomites exist and are peripherally considered here. Protodolomite (poorly ordered, Ca;t3:;t3Mg near 60:40, broad Mn (super 2+) ESR spectra, MnPR near 1 when determinable, crystal size 1.0-5.0 mu m, spherular to rhombic grains) is found in some modern sediments and hydrothermal bomb products. Metamorphic dolomite is similar to group B but has a temperature-dependent range of MnPRs (5-250), a larger crystal size (0.2-2.0 mm), and a very negative delta 18 O. The composition of dolomite in sedimentary rocks reflects its environment of formation. Once the crystal lattice is established the Ca:Mg ratio and MnPR do not change with time, unless exposure to elevated temperatures drives the dolomite toward a Mg:Ca ratio of 50:50, MnPR > 50, and very negative delta 18 O.