Quantitative Analysis of Mn2+ in Sedimentary Calcite
Using Cathodoluminescence Spectroscopy

Dirk Habermann Institut für Geologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany


Rolf. D. Neuser Institut für Geologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Detlev K. Richter Institut für Geologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Frank Bruhn Institut für Geologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

The discovery of Mn2+-activated cathodoluminescence (CL) in natural calcite has initiated efforts in measuring Mn concentrations by spectroscopy. The investigations document a complex relationship between Mn2+ content and luminescence intensity and were mostly discussed controversally.

The main activator elements of carbonate luminescence are Mn2+ and some REE2+/3+. Important quencher elements are Fe2+ (eg. Machel, 1983) and the much less effective Ni2+ (Marfunin, 1979). The intensity of the Mn-activated CL is dominantly controlled by the Mn and Fe concentration and the ratio of these two elements (e.g. Bruhn, 1995). Other factors such as sensitizer elements, lattice defects and the thermal history affect the Mn2+-activated luminescence (Mason, 1994). For sedimentary calcite, the latter effects are of minor importance (e.g. Mason, 1994) or they may be compensated by other factors.

The occurence of emisson peaks of some REE in the spectral region of Mn-activation (600-700 nm) may cause a weak correlation between luminescence intensity and Mn content. Detection and evaluation of overlapping peaks must be done by digital fitting of the emission spectrum.

For quantification of the Mn2+ content by CL spectroscopy at least assessment of all limiting factors is inevitable.

The influence of quenching is difficult to be recognized by emission spectroscopy, as both activator induced luminescence and intrinsic luminescence are quenched by the Fe2+ ion. Fe concentrations below ª 1500 ppm have no significant influence on the Mn-activated and intrinsic emission. In calcite with Fe concentrations above ª 2000 ppm the analysis of Mn2+ is coupled with an exponential increase of the fit error.

In carbonate sediments sensitizer elements such as Pb2+ and Ce3+ appear to play a secondary role. Ce3+ is a less effective sensitizer than Pb2+ (Marfunin, 1979). Pb2+ has also been identified as an activator, with an emission maximum at ª 480 nm (Machel et al., 1991). Consequently, sensitizing by Pb can be calibrated. However, Pb and Ce concentrations in sufficent quantities are uncommon in sedimentary calcite.

Our results document that quantitative analyses of Mn2+ in diagenetic calcite with Fe concentrations below ª 1500 ppm is possible using CL spectroscopy. The prerequisite for digital analysis of the CL spectrum with an non-commercial software are a "hot cathode" CL microscope and a high sensitve spectrometer with attached nitrogen cooled CCD detector (HRS-CL: see Neuser, 1995)

A powerful microprobe such as the proton probe (PIXE) with detection limits in the low ppm range is needed for calibration.

The sensibility of the quantitative analysis of Mn2+ in diagenetic calcite using CL spectroscopy is in the range of PIXE and thus considerably lower than that of the electron microprobe.

Detection of some REE using CL spectroscopy is even possible below the detection limits of PIXE for REE (Habermann et al., 1995) ranging from ª 20 ppm to n*100 ppm, depending on Fe concentration.


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