Railsback's Some Fundamentals of Mineralogy and Geochemistry Stability and solubility of carbonate minerals of divalent cations The plot below shows the thermodynamic stability, mineral groups. For example, calcite is more solu- These facts help account for many of the and thus the solubility, of some carbonate minerals of ble than otavite, siderite, rhodochrosite, and interesting issues of carbonate petrology: #3 for why divalent cations. Blue is used to highlight the smithsonite, and aragonite is more soluble than waters may precipitate one CaCO3 mineral under rhombohedral carbonate minerals like calcite, where strontianite and cerussite. certain conditions but another CaCO3 mineral under 2+ 2+ the inter-radical cation (e.g., Ca in calcite) is nested 5) The two most abundant 2+ cations, Ca and other conditions; #4 for why both of the CaCO3 among six O2- anions. Red is used to highlight the Mg2+, fall outside the rhomobohdral and ortho- minerals are so soluble, giving us both caves and orthorhombic carbonates like aragonite, where the rhombic windows and therefore form relatively speleothems; and #5 and #6 for why dolomitization is inter-radical cation (e.g., Ca2+ in aragonite) is nested soluble carbonate minerals. so common and the dolomite problem so important. in a larger cation site among nine oxygens. 6) A mineral of two cations at the ends of a group is It would be a much less interesting world if our Things to note include more stable than the minerals of single cations. carbonate deposits consisted of carbonates of Cd2+ 1) Smaller cations make rhombohedral carbonate For example, dolomite is more stable than mag- or Pb2+ and thus did not offer so much mineralogical minerals, and larger cations form orthorhombic nesite and calcite, and alstonite is more stable than variablilty and such susceptibility to dissolution, carbonate minerals, as one would expect from the aragonite and witherite. reprecipitation, and diagenetic alteration. difference in coordination. 2) Within each group, cations -80 -14 with radii near the middle of Orthorhombic Carbonate their respective group form Window LEGEND Cations less soluble, and thus more -75 Rhombohedral Carbonate -13 One Two 2+ stable, minerals. Fe in Window Rhombohedral 2+ Cerussite Carbonates siderite and Cd in otavite (six-fold coord'n) Stable -70 Otavite2 luble are examples among the -12 Orthorhombic rhombohedral carbonate Ankerite Carbonates minerals. On the other hand, 1 (nine-fold coord'n) 2 Otavite More cations at the end of each -65 Rhodochrosite Hexagonal range are more soluble. Mg2+ -11 So More Carbonates in magnesite and Ca2+ in Siderite Hydrous G) of formation G) of -60 calcite are examples among Carbonates ∆ Smith- 2 the rhombohedral carbonate sonite Dolomite -10 minerals. These observa- -55 Sphaero- tions define the two "windows" cobaltite Stron- Alstonite Ksp is the solubility of greater stabilitiy in the tianite Dolomite 1 -9 product, the equilibrium diagram. -50 Calcite 25°C at constant for dissolution 2+ sp 3) Ca falls at the boundary Mag- from ions at 25°C (kJ/mol) 25°C at from ions or precipitation. For between the rhombohedral nesite Aragonite -8 and orthorhombic carbonate Free ( Energy -45 Vaterite example, for CaCO3 minerals, accounting for the Monohydrocalcite Log K 2+ 2- Mg-Calcites Witherite Ksp = aCa x aCO3 . existence of two common (CaCO3•H20) -40 -7 The results shown were calculated from data in polymorphs of CaCO3, calcite Ikaite (CaCO3•6H20) and aragonite. Gaspéite Robie et al. (1979), except for dolomite2, ankerite, and alstonite from Garrels et al. (1960); gaspéite, 4) The two most common Mg calcites and hydrous carbonates from Table 2.1 carbonate minerals, calcite -30 Nesquehonite (MgCO3•3H20) of Morse and MacKenzie (1990); rhodochrosite2 and aragonite, are both more -5 from Wartel et al. (1990); and otavite2 from Stipp et al. (1993). Log Ksp values for carbonates of two soluble than minerals of other cations (e.g., dolomite) were halved to convert to a 2+ 0.70.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 cations because Ca falls per-mole-CO3 basis. Data from Tareen et al. (1991) and Fazeli et al. (1991) would suggest an upward outside the windows of the two Cation Radius (Å) LBR CarbMinsvRadii07 Originally published in Car- bonates and Evaporites in 1999. Revised 8/2010. shift of points on the plot but little relative change..