An Atlas of Speleothem Microfabrics
L. Bruce Railsback, Department of Geology, University of Georgia.
COARSE SPELEAN CALCITE
Calcite is the most abundant mineral in speleothems, and spelean calcite can take a remarkable variety of habits and fabrics, as shown by Figures 2 and 3 of González et al. (1992). Many of these forms are elongate, and the degree to which they are elongate is described as grading from "bladed" to "fibrous" to "acicular". These three terms distinguish differing length-to-width ratios; precise definitions are given in the glossary.
Two terms that have been used to characterize aggregates of relatively large and elongate calcite crystals are "columnar" and "palisade" calcite.
Kendall and Broughton (1978) defined "columnar" as "crystals that are elongate but which are wider than 10 microns". They nonetheless used "columnar" interchangeably with "palisade" Folk and Assereto (1976) identified palisade calcite as "columnar crystals 2-10 mm long" that were "nearly parallel", at best suggesting that palisade crystals are a subset of columnar crystals. A sketch (Figure 2 of Folk and Assereto, 1976) indicated that palisade crystals have straight and almost perfectly parallel edges, but photographs (Figures 3 and 5 of Folk and Assereto, 1976) showed more irregular and non-parallel crystal edges.
In light of this confusion regarding "columnar" and "palisade", this atlas proposes that palisade calcite be defined as
"an assemblage of elongate calcite crystals with straight parallel sides and with their long direction parallel to the direction of growth" and that columnar calcite be defined as "an assemblage of elongate, roughly parallel, calcite crystals". These definitions follow the spirit (if not the reality) of the definition of "palisade" used by Folk and Assereto (1976), and they are in accord with usage of "columnar" in figures and figure captions in Kendall and Broughton (1978), Kendall (1993), González et al. (1992), and González et al. (1993). The accompanying sketch illustrates these definitions.
Causes of the growth of elongate or columnar calcite in speleothems have been the subject of vigorous debate by Kendall (1993) and González et al. (1993). González et al. (1992) argued that faster flow and greater supply of reactants (Ca2+ and CO32-) can favor growth of more elongate calcite fabrics like columnar calcite. However, they also noted that elongate calcite crystals can form under low-flow conditions. Figure 6 of González et al. (1992) presented a model for development of parallel elongate crystals via competitive growth, which may be a good model for development of most spelean columnar calcite.
More surprisingly, González et al. (1993) also argued that columnar calcite can form from the recrystallization of fine-grained equant calcite (as shown in Figure 1 of González et al. (1993)). Thus this atlas's treatment of columnar calcites as primary spelean material should be accompanied by the caveat that some columnar calcites have been interpreted as diagenetic.
This section is limited to coarse spelean calcite. Calcite in speleothems can be microcrystalline, and a separate section deals with such calcite. Calcite in speleothems can also be detrital rather than primary or spelean. A separate section covers detrital minerals in speleothems.