Identification of Organic and Inorganic Activators of Photoluminescence in Calcite by Laser Microprobe Spectroscopy

Vicki A. Pedone, Karen R. Cercone, Robert C. Burruss

High-resolution laser-excited emission spectra provide new evidence that organic activation of photoluminescence can be distinguished from trace-element activation in calcite. Ultraviolet (362 nm), blue-violet (454 nm), and green (514 nm) wavelengths of an argon ion laser were focused through a petrographic microscope to generate photoluminescence from 2 to 5-µm spots. Emission spectra from pristine, organic-rich red algal skeletons and synthetic calcite crystals spiked with manganese were measured by an ISA U-1000 spectrometer. Biogenic and synthetic calcites display distinctly different spectra. With shorter exciting wavelengths, emission spectra of biogenic calcite show a blue shift in peak maximum from 620 to 480 nm. Spectra of manganese-spiked synthetic calcite how no change in the 630-nm peak maximum with shorter exciting wavelengths. The spectral characteristics are consistent with the known photoluminescence emission of high molecular weight aromatic hydrocarbon compounds and manganese-bearing calcite, respectively.

The absolute intensity of photoluminescence also shows a difference between biogenic and synthetic calcites. In biogenic calcite, the intensity increases with shorter exciting wavelengths; the intensity under 362-nm excitation is five and one-half times stronger than that under 514-nm excitation. In the manganese-spiked calcite, the intensity decreases with shorter exciting wavelengths; the intensity under 514-nm excitation is five times stronger than that under 362-nm excitation. The emission patterns are consistent with the known absorbance spectra of aromatic hydrocarbons and manganese-bearing calcite. In conjunction, measurements of peak shift and intensity demonstrate that variable-wavelength laser excitation must be used to distinguish organic from inorganic photoluminescence ac ivators in calcite.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.