Abstract

Four magnesian carbonate Holocene fresh water cements from the Coast Range of California were investigated using X-ray and electron microbeam techniques. These travertines arid conglomerate cements include three dolomite cements, which have compositions from Ca0.59Mg0.41CO3 to Ca0.55Mg0.45CO3. The fourth cement is a magnesian calcite of composition Ca0.77Mg0.23CO3. Both dolomite and magnesian calcite cements exhibit weakened and broadened basal plane reflections in powder XRD patterns.

All cements exhibit a heterogeneous microstructure in bright field TEM micrographs. The cements are mosaics of submicron-sized crystals with partial topotaxial orientation. Aside from mosaic spreading, diffraction maxima in electron diffraction patterns show no diffuseness or streaking. Ordering diffractions are present in electron diffraction patterns of the dolomite cements but are not observed in equivalent patterns from the magnesian calcite cement.

Cation disorder in the dolomites appears to be by substitution of Ca into Mg planes, rather than by a mixed layering mechanism. The calcite-type cement shows greater ranges of composition between individual crystals than do the dolomite-type cements. Dolomite (“protodolomite”) compositions are consistent with the existence of a metastable, partially ordered reactive intermediate. STEM microanalytical data demonstrate that broadening and diffuseness in powder XRD maxima are due to microscale compositional heterogeneity rather than structural phenomena.

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