Railsback's Some Fundamentals of Mineralogy and Geochemistry Cation sites in the common carbonate minerals Rhombohedral (Calcite-Type) Carbonates Orthorhombic (Aragonite-Type) Carbonates O O C O 1/ C O O 3 O O O O C O C 11/3 O O O O 1/3 O C C O M O O O M O C O C 2 O /9 O O O C O 1 O O 1 /3 O O 2/9 O C 2 O C /9 C O O O O
Six-fold coordination: Nine-fold coordination: From one plane of CO s (upper one here), From one 0001 plane of CO3s, 3 2- C Carbon bonds to two O2-s from each of two CO groups a bond to one O from each of three CO3 groups; 3 and a bond to one O2- from a third CO group; from other 0001 plane of CO3s, O Oxygen 3 2- from other plane of CO s (lower one here), a bond to one O from each of three CO3 groups. 3 2- Each O2- is bonded to two 2+ cations. M Cation bonds to two O s from one CO3 group and bonds to one O2- from each of two other CO groups. Bond 3 1 2- /3 Strength Each O is bonded to three 2+ cations. Radius Rhombohedral Orthorhombic Cation (Å) Mineral Mineral The common carbonate minerals (the 1) Ca2+ is at the threshold of cation sizes for the Ni2+ 0.69 Gaspeite anhydrous carbonates of divalent two groups, so that there are two polymorphs 2+ Mg 0.72 Magnesite cations) can be divided into two groups of CaCO3, calcite and aragonite. It's purely a Zn2+ 0.74 Smithsonite on the basis of their cation sites. The coincidence that the divalent cation that makes Co2+ 0.75 Sphaerocobaltite six-fold cation site in rhombohedral the most carbonate material (Ca2+) is at the Fe2+ 0.78 Siderite carbonates (a site in which the cation is threshold of cation sizes between the two Mn2+ 0.83 Rhodochrosite nested among six O2-s) can house carbonate mineral groups (see table at left). Cd2+ 0.95 Otavite relatively small cations. The nine-fold 2) The relatively small cation site in calcite means Ca2+ 1.00 Calcite Aragonite cation site in orthorhombic carbonates that the trace elements common in calcite are Sr2+ 1.13 Strontianite (a site in which the cation is nested the smaller cations (Mg2+, Zn2+, Fe2+, Mn2+, & Eu2+ 1.17 (synthetic) among nine O2-s) can house relatively Sr2+), whereas the relatively large cation site in Pb2+ 1.19 Cerussite large cations. aragonite means that the trace elements Ba2+ 1.35 Witherite This is significant to CaCO3 common in aragonite are the larger cations Ra2+ 1.40 (synthetic) because . . . (Sr2+, Pb2+, and Ba2+). LBR 10/1995 rev. 2/2008 SFMGCarbMinStrxrs04