Over From The Panda's Thumb a stunning photo of a stalactite
Photograph by Quentin Cobb
This is the Big Stalactite from Doolin Cave, Ireland. At 7.3 meters it is the largest stalactite in Ireland and it contains about 33 tonnes of calcite.
The same geological processes work on a much smaller scale too. From the late Ordovician limestones of northern Georgia
These also are stalactites but they are a fraction of a millimeter in length and contain probably a few micrograms of calcite. In carbonate petrology jargon they are known as vadose cements because they precipitated in the vadose zone of a soil profile, i.e. the zone above the permanent groundwater table. Fresh water dissolves limestones and forms tiny cavities - micro mini caves - and calcite crystals precipitate in these cavities from calcite saturated water. Since the cavities are above the water table they are mostly filled with air. But tiny droplets of water are present adhering to the upper surfaces of cavity walls , held there by surface tension forces. Which is why vadose cements are often pendent in habit, that is they hang from the roofs of cavities and caves.
The different colors....? .. its the same crystal but on the left it has been stained using potassium ferricyanide, a dye commonly used in carbonate petrology studies to distinguish calcite with high amounts of iron (dyes blue) from non ferroan calcite. On the right is the same crystal viewed under cathodoluminesence. The crystal is bombarded with cathode rays in a vacuum chamber and the presence of activator ions like manganese makes the crystal glow in orange. Black bands are manganese poor (no activators) bands. Manganese gets incorporated in growing calcite crystals under oxygen poor reducing conditions. The alternating black and orange bands track the conditions as they fluctuated between oxidizing and reducing.
Not all vadose cements can be called micro-stalactites. In this case though the there has been dissolution of the limestone and formation of solution cavities. These vadose cements have formed on the roofs of these cavities.
Just like that giant stalactite from Ireland.
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