Physicochemical environments for the formation of quartz-free manganese oxide ores from the Hotazel Formation of the Kalahari manganese field, northern Cape Province, South Africa, are discussed quantitatively on the basis of the phase relations in the system Mn-Fe-Si-O and qualitatively in the system Ca-Mn-Fe-Si-O. The phase relations involve major oxide minerals of the ores, namely, braunite, braunite II, hausmannite, bixbyite, jacobsite, and hematite, and were constructed under quartz-deficient conditions, using existing and estimated thermody-namic data and mineral assemblages from the ores. Standard enthalpy delta HM 0 (sub f.298) and Gibbs free energy delta G 0 (sub f.298) values of braunite are estimated to be -1,011.9 + or - 5 and -937.3 + or - 5 kcal/ mole, respectively, on the basis of published experimental data and predicted entropy and heat capacity. The low-grade (diagenetic to very low grade metamorphic) Mamatwan-type ores, consisting mainly of braunite, hausmannite, jacobsite, hematite, kutnohorite, and calcite, are altered by hydrothermal solutions into the high-grade, high-temperature Wessels-type ores consisting of braunite II, bixbyite, hausmannite, hematite, calcite, and andradite with trace amounts of braunite. The peak temperature of formation of the high-grade ores appears not to have exceeded about 300 degrees to 400 degrees C. The ores are oxidized from oxygen fugacity levels defined by the assemblage hausmannite-jacobsite-hematite to that defined by the assemblage bixbyite-hausmannite, with increasing temperature. Under hydrothermal quartz-deficient conditions, the stability of braunite is enlarged to higher temperatures with increasing silica activity (a (sub SiO (sub 2 aq ) ) ). Because braunite is much less abundant in the high-grade Wessels-type ore than in the low-grade Mamatwan-type ore, the maximum a (sub SiO (sub 2 aq ) ) ) in the solutions could be estimated at 10 (super -4) to 10 (super -3) at 300 degrees to 400 degrees C from the stable assemblage bixbyite-hausmannite-braunite. Kutnohorite, and to some extent calcite, was a major source of Ca reacting with silica released from braunite to form braunite II and andradite.