This is a polished slice of a septarian nodule from the famous Muddy Creek location in Orderville, Kane Co., Utah. The interior of these nodules are often brightly fluorescent and phosphorescent under UV, as is the case with this example. The size of this specimen is 14 x 12 x 1.25 cm, and it weighs 346 grams.
Septarian nodule from Orderville, Utah, showing bright fluorescent response under short wave UV (254nm).
Although the process of formation of these interesting geological curiosities is still debated, the predominant theory is that they originated as undersea concretions formed from marine organic matter and mud during the Cretaceous period. When the ocean receded the concretions were exposed to the air, causing them to dry and crack internally. Calcium carbonate originating from decomposing shells of marine organisms percolated into the cracks, and precipitated as calcite and aragonite. The end result is a thin layer of aragonite lining the cracks, separating the mudstone exterior from the calcite interior. The septarian nodules from Utah can also contain barite and siderite.
View of same specimen taken under visible light. Note the crystalline honey-yellow colored calcite filling the major voids in the nodule. The brown minerals are likely aragonite and non-fluorescent siderite.
The Orderville, Utah, septarian nodules often have brightly fluorescent interiors due to the calcite and aragonite content. The fluorescent response is similar under all UV wavelengths, but is typically brightest and warmer under long wave.
The above images show the fluorescent response of the septarian nodule under various UV wavelengths. Interestingly, the mudstone which encases the calcite/aragonite infilling is also fluorescent. Mouse over scrolling images for UV wavelength information.
The aragonite layer is also phosphorescent following exposure to all wavelengths of UV. However, the color of the phosphorescent response varies somewhat depending on the wavelength of excitation. The following paired images show the fluorescent response (top) and corresponding phosphorescence (bottom) for each UV wavelength.
Fluorescent response (top row) and phosphorescence (bottom row) during/after short wave (left), mid wave (middle) and long wave (right) UV. Note the subtle color shift of the phosphorescent response; white after short wave, greenish/white after mid wave, and yellow after long wave. The mudstone also shows dim phosphorescence after UV.
The following photographs show close-up detail of the calcite/aragonite interior of the nodule under visible light and UV. Field of view is approximately 9 cm across.
Mouse over scrolling images for wavelength information.