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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 507

Last Page: 508

Title: Cathodoluminescence in Carbonate Petrography: Some Aspects of Geochemical Interpretation: ABSTRACT

Author(s): Hans-G. Machel

Article Type: Meeting abstract


Commonly only Mn2+ and Fe2+ are considered to be responsible for the luminescence behavior of calcite and dolomite. However, a fairly large number of trace elements interact to produce certain luminescence characteristics in these minerals (as described by Gies in 1975 and 1976). The ions of these elements can be grouped into activators, sensitizors, and quenchers.

Activators are those ions that lead to active luminescence, undergoing excitation (entrapment of energy), temporary storage, and emission. Main activators in carbonates are Mn, Pb, and several rare earth elements. Sensitizors are those ions that undergo excitation and transmit some of this energy to the activators. Main sensitizors in carbonates are Pb and Ce. Quenchers

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are those ions that trap excitation energy, but whose outer electron transitions do not result in luminescence. Main quenchers in carbonates are Ni and Fe.

As opposed to visual determination, only a spectral analysis of the emitted radiation paired with a chemical analysis can detect which of all these elements participate in the luminescence of a particular carbonate crystal. A visual observation, such as "bright-orange luminescence," is merely a mixture of wavelengths, and this color can result from different spectral compositions due to correspondingly different trace element contents.

Activators, sensitizors, and quenchers have to be present in certain minimum and below certain maximum concentrations in order to be effective. The minimum concentration for the most important activator in non-hydrothermal carbonates, Mn2+, is well below 100 ppm, and it is even lower if the crystal contains any sensitizors in effective concentrations. Pb2+ and Ce2+ sensitize Mn-activated luminescence at concentrations as low as perhaps 30 ppm. The most effective quencher is Ni2+, which kills Mn-activated luminescence at concentrations as low as possibly 35 ppm. Fe2+ seems to effect initial quenching at about 30 to 60 ppm. Up to about 10,000 ppm, the luminescence behavior of calcite and dolomite depends on the Mn2+/Fe2+ ratio. No luminescence occurs above this level, whatever the Mn2+ concentration.

Mn2+ is the most important activator in carbonates because it leads to the most obvious luminescence; it is relatively abundant. Fe2+ is probably the most important, although not most effective, quencher due to its very high and variable abundance. If one attempts to interpret the luminescence behavior of carbonates in terms of the geochemical environment, however, the other activators, sensitizors, and quenchers have to be considered too. In particular, those elements associated with organic matter could be enriched in organic-rich (or even bituminous) carbonates, and elements primarily associated with clay minerals can be expected in impure, argillaceous limestones and dolostones, and their diagenetic carbonate phases.

The luminescence of carbonate cements, thought to result from Mn2+ and Fe2+ alone, has often been taken as an indicator of the redox-potential of diagenetic fluids. This is only permissible if it can be shown that the other elements are not involved to any significant degree.

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Copyright 1997 American Association of Petroleum Geologists