Novel TEM approaches to imaging of microstructures in carbonates: Clues togrowth mechanisms in calcite and dolomite

Relating microstructures imaged by transmission electron microscopy (TEM) t o specific growth mechanisms requires imaging of both the surface microtopo graphy on crystal faces and its corresponding microstructure in the bulk cr ystal. Pt-C replicas of as-grown surfaces are ideally suited to this task, as they provide a high-resolution, nearly three-dimensional image of surfac e topography that can be correlated with microstructures visible in section s at high angle to the as-grown surface. Ultramicrotomy enables the prepara tion of ultrathin sections more quickly than conventional ion-thinning and can be used to investigate chemical heterogeneities by analytical electron microscopy (AEM). We evaluate the potential of both techniques for the stud y of microstructures in calcite and dolomite. (1) TEM images were obtained from Pt-C replicas of synthetic calcite. The as-grown (10 (1) over bar 4) f ace of a Mg/Mn-doped crystal growth showed growth hillocks aligned along pr eferred; orientations. In Pb/Mn/Sr-doped calcite, sections at high angle to as-grown faces showed a uniform microstructure in {10 (1) over bar 4} sect ors. The {01 (1) over bar 2} sectors contained concentric zones that altern ated from striated to uniform microstructures, suggesting periodic variatio ns in growth rate and possibly in the growth mechanism. An oscillatory-zone d Mn-doped calcite showed fine-scale banding (30-150 mm) and periodic rough ening of (10 (1) over bar 4) surfaces that suggest repetitive transitions b etween growth mechanisms. (2) In sedimentary dolomites, Pt-C replicas of su rfaces cleaved parallel to a (10 (1) over bar 4) face showed a hillocky top ography or smooth (10 (1) over bar 4) surfaces modified by non-equivalent f acets. Surfaces produced by cleavage at high angle to a (10 (1) over bar 4) face exposed sectors with fine-scale banding crosscut by sharp boundaries, suggestive of closely spaced growth hillocks intercalated with non-equival ent subsectors. Ultrathin sections prepared by microtomy showed some disrup tion of the crystal structure but, in some dolomite samples, domains with d istinctive microstructures similar in scale and shape to those seen in the Pt-C replica were identified. (3) The growth microstructures in {10 (1) ove r bar 4} sectors of dolomite present striking similarities to the larger sc ale compositional zoning patterns produced by growth spirals in calcite dop ed with trace elements. At this stage, Pt-C replicas and ultramicrotomy can distinguish among dolomites that are petrographically very similar.