Railsback's Some Fundamentals of Mineralogy and Geochemistry Density of minerals VIII: The significance of crystal structure In the first page of this series, we saw that density of simple carbonate minerals is roughly proportional to atomic weight of the inter-radical 2+ Rhombohedral carbonates cation (for example, Pb in cerussite). On this 7.0 of divalent cations page, the emphasis is on the "roughly" in "roughly Orthorhombic carbonates Cerussite proportional". For example, one might look at the 6.5 of divalent cations upper plot here and ask "Why are magnesite and 6.0 aragonite denser than one would expect from the regression lines for the rhombohedral and 5.5 orthorhombic groups, respectively?" or "Why are ) calcite, otavite, and witherite less dense than 3 5.0 Otavite expected?". OrthorhombicRes mineralsidual Rhombohedral minerals (g/cm 4.5 plotted below The answer to these questions can be seen Sphaerocobaltite Smithsonite Siderite Witherite in the lower plot, where the anomaly or residual in 4.0 Rhodochrosite the upper diagram is plotted against cation radius. Gaspéite (Ni.6 Mg .4 ) Strontianite Among the rhombohedral carbonates, where the Density 3.5 inter-radical cation is in six-fold coordination, Magnesite smaller cations like Mg2+ let the structure collapse 3.0 Aragonite Calcite to give a proportionately large density. On the 2.5 other hand, larger cations like Cd2+ and Ca2+ stretch the rhombohedral structure to give a 0 20 40 60 80 100 120 140 160 180 200 220 proportionately small density. The same happens Atomic Weight of Cation among the orthorhombic carbonates, where the Magnesite inter-radical cation is in nine-fold coordination. In Orthorhombic 2+ Sphaerocobaltite Aragonite that case, Ca is a relatively small cation and 0.5 Gaspé- Smithsonite carbonates ite Siderite Strontianite allows collapse of the structure and an Rhodochrosite Cerussite anomalously great density, whereas a relatively 0.0 2+ Rhombohedral large cation like Ba stretches the structure to carbonates give a proportionately small density. Thus the Residuals -0.5 Otavite Calcite Witherite details or anomalies of the trends in density in the 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 upper plot are explained by the relationship of Cation Radius (Å) cation size to mineral structure shown in the lower plot..