Calcium-magnesium carbonate solid solutions from Holocene conglomerate cements and travertines in the Coast Range of California

Two calcium-magnesium carbonate solid solutions form Holocene travertines and conglomerate cements in fresh water stream channels of the Coast Range of California. Calcite does not yield the {015} diffraction maximum. The {006} diffraction maximum is lacking over most of the range of composition of calcite. Calcite has compositions from CaCO₃ to Ca_0.5Mg_0.5CO₃. Dolomite yields both the {006} and {015} diffraction maxima over its entire composition range, Ca_0.6Mg_0.4CO₃ to Ca_0.5Mg_0.5CO₃. The Ca-Mg carbonates form in isotopic equilibrium and thermodynamic disequilibrium from dispersion of Ca²⁺-rich water into CO₃²⁻-rich water within the alluvium. The stable isotope data suggest that all the Mg-rich carbonates are primary precipitates and not a result of Mg-substitution in precursor CaCO₃. There is a correlation between δC¹³ and Mg content of the carbonates which predicts a 5%. fractionation of C¹³ between dolomite and calcite at sedimentary temperatures. C¹⁴ is incorporated in Ca-Mg carbonates forming from C¹³-poor meteoric waters and C¹³-rich waters from Cretaceous sediments. C¹⁴ ages of the Ca-Mg carbonates are apparent, and cannot be corrected to absolute values. Solution rates of calcite decrease with increasing MgCO₃ content; dolomite dissolves slower than any calcite.