Activity diagrams of borates: implications on common deposits

Most of the world's borate minerals are found in Neogene deposits and Quaternary lake deposits. Only a few of the borates are common geologically and commercially. A series of equilibrium activity diagrams were calculated for the common as well as some rare borate minerals in the systems of (1) Na2O-B2O3-H2O, (2) CaO-B2O3-H2O +/- CO2, (3) MgO-B2O3-H2O +/- CO2, (4) CaO-Na2O-B2O3-H2O, and (5) CaO-MgO-B2O3-H2O. Stability diagrams constructed with respect to variables of log[a(Mbn+)/(a(H+))(n)] and log[a(Mbn+)/a(Mc(n-1))+(a(H+))] versus both log[aH(2)O] and log[a(B)(OH)(3)] showed that some rare borates are thermodynamically not stable (tertschite, inderborite) at all in these systems. Still some common phases are thermodynamically occurred as metastable phases (tincalconite, meyerhofferite) in some deposits. On the contrary, some thermodynamically stable phases can form kinetically slower than the others and not found as common phases (inyoite). Some common and uncommon minerals such as ulexite, aksaite, and gowerite have small stability fields indicating that they can form at very limited thermodynamic conditions. Some phases such as pandermite, ginorite, ascharite, and suanite being structurally complex phases, form after less complex precursor minerals at the end of diagenesis due to burial and/or increasing temperature. Concentrations of cations and boron, pH, evaporation rate are other controlling variables of diagenetic processes. Through these diagrams, observed paragenetic relations and geochemical conditions can be depicted and expectant paragenetic phases can be predicted in any deposits