Mammillary calcite

Figure 3. Map of Devils Hole. BR is Browns Room, and M is Millers Chamber. Mammillary calcite is the only speleothem morph below the water table. Rafts currently form on the surface on the main chamber and in Browns Room. Folia only form in Browns Room, and popcorn forms in both Millers Chamber and Browns Room.

In thin section, mammillary calcite resembles the palisade calcite of Folk and Assereto (1976), with very large (up to 23 mm long and 2 mm wide), columnar (Logan and Semeniuk, 1976) crystals oriented mostly normal to the surfaces it coats. Each columnar crystal is... 

Figure 4. Mammillary calcite coating the walls of Devils Hole about 23 m below water table. Note the gently rounded protuberances that characterize the coating. A core of mammillary calcite about 42 cm thick was recovered from approximately 30 meters below the water table. Photograph courtesy of Ray J. Hoffinan.

Figure 5. Photomicrograph of mammillary calcite in cross-polarized light. Note the closely packed comb structure of the crystals with the teeth (T) of the comb meeting the back (B) of the comb at an acute angle. White arrow indicates direction to the substrate.

Mammillary calcite on the hanging wall of Devils Hole is white, translucent, and unbanded. Footwall mammillary calcite, on the other hand, is strongly banded because silt and clay debris that settled on the up-facing surfaces were incorporated into the mammillary coating by continuing calcite precipitation. Well-developed bands on cross sections of up-facing mammillary calcite surfaces disappear abruptly wherever the depositional surface rolls over to vertical or overhanging. 

Growth rate of mammillary calcite ranges from about 0.3 to 1.3 mm/1,000 years (Ludwig et al., 1992 Plummer et al., 2000). 

Mammillary calcite is very dense, with porosities much less than 1%. Pore spaces are of two different types. The first type consists of irregularly shaped pore spaces with fluid inclusions that are commonly oriented parallel with the crystallite boundaries. In traditional speleothems, spindle-shaped inclusions mark the sites where columnar crystals have coalesced (Kendall and Broughton, 1978). In mammillary calcite, the irregular inclusions probably mark... 

Figure 7. Photomicrograph of mammillary calcite perpendiculm to die long crystal is. Note the very irregular crystal outlines. Pmtial coalescence of these irregular crystals may be responsible for the irregular nature of fluid inclusions in mammillary travertine. Plane polarized light.

The second pore and fluid inclusion type was formed when debris settling onto up-facing mammillary calcite surfaces trapped spaces in the plane of the crystal terminations (i.e., parallel to the growing surface) (Fig. 6). 

Figure 8. Accumulation of rafts (arrows) at a depth of about 28 meters below Browns Room in Devils Hole. Rafts have been cemented in place by continued precipitation of mammillary calcite. Thickness of bedrock block left of the arrow is about 15 cm.

Folia commonly exhibit internal banding. Most foliar bands, like the foot-wall-mammillary-calcite bands described earlier, are accumulations of sHt- and clay-sized detritus that settled on top of the folia during growth. Too-rapid sediment accumulation on the actively growing rhombohedral crystal surfaces disrupts crystal growth. Large numbers of very small seed crystals on top of the sediment accumulations (Fig. 12) mark the resumption of calcite precipitation. 

Flowstone precipitates from carbonate-saturated, externally derived water as it flows down the above-water-table walls of Browns Room. Its crystal structure is most similar to the speleothems described by Kendall and Broughton, (1978) and is petrographically distinct from folia, rafts, and mammillary calcite. By identifying flowstone deposits in Devils Hole, we have a second proxy record for the elevation of the water table... 

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