The chemical compositions of carbonate cements in sandstones were analyzed with an energy dispersive analyzer (KEVEX) and a scanning electron microscope in order to provide a baseline data base for one of the most common authigenic phases in sandstones. A total of 205 spectra was analyzed with respect to mineral standards using ZAF corrections. These spectra were acquired from 35 Cambrian to Cretaceous carbonate-cemented sandstones from our sedimentary rock collections.

Only 19% of the analyses were pure calcite (i.e., no Mg, Mn, or Fe were detected). Impure calcites accounted for 54% of the analyses, and dolomites and ankerites accounted for 27%. When the calcites were treated as a single group, the distribution of the components was as follows: calcite 91.6-100%; magnesite, 0-8.4%; rhodochrosite, 0-2.3%; and siderite, 0-4%. The dolomites and ankerites showed a larger range: calcite, 47.8-60.6%; magnesite, 20-52.2%; rhodochrosite, 0-12%; and siderite, 0-28.6%. The values of the calcite component in the dolomites that were significantly higher than 50% probably resulted from the beam (spot mode) encountering dolomite plus some calcite. In most cases, the calcite component was nearly 50%.

Many of these compositions displayed a large variation within a sample, even at the micron-level scale. A series of closely spaced analyses--all within an area 200 × 100 µm--from a sample rich in dolomite and/or ankerite ranged between 20 and 43% magnesite and 6 and 29% siderite. The calcites normally only ranged a few percent for each component in analyses that were spaced at a similar scale.

End_of_Article - Last_Page 308------------