Mejdahl (1983, 1985) suggested the use of separated potassium feldspars for dating, and showed the feasibility of this. We address here the question of what is the correct potassium content to use to evaluate the β dose rate from the potassium within the grains. Pure K-feldspar, KAlSi3O8, has 14.0 wt. % K. The grains in a sediment generally consist of a highly inhomogeneous collection and one can expect grains to be present with a range of K contents between 0 and 14%. The laboratory procedures designed to separate Kfeldspars are not perfect and usually result in the presence of other minerals such as quartz and plagioclase feldspars. The situation is further complicated by the fact that different mineral grains have different luminescence sensitivities and it is not unusual to find that most of the light that is being measured arises from a small fraction of the grains of an aliquot. The one observation that appears to make the problem tractable is that it is generally observed that there is a positive correlation between luminescence sensitivity and potassium content. Our approach to the problem has been to note that 14% is the maximum possible value of the K content, and that a minimum value can be determined from the average K content of the separated grains. The grains are separated using standard procedures. These are an HCl treatment to dissolve carbonates, sieving to select a grain size, a 6 minute HF etch to clean, but not dissolve the grains, an HCl treatment to dissolve precipitated fluorides, sieving to remove small fragments, selection of the < 2.58 g cm fraction using an aqueous solution of sodium polytungstate, and magnetic separation. After optical dating measurements the grains are recovered and about 0.3 g sent for commercial atomic-absorption analysis; the K contents so obtained for a variety of samples are given in the table.

The K content of the K-feldspars being measured in optical dating or in thermoluminescence dating

Luminescence dating : Laboratory procedures and protocols

Spectral information from minerals relevant for luminescence dating

https://www.sfu.ca/physics/research/workarea/huntley/108paper.pdf

Some observations on tunnelling of trapped electrons in feldspars and their implications for optical dating

Internal dose rate to K-feldspar grains from radioactive elements other than potassium

The paper describes a systematic study of the thermoluminescence emission spectra of thirty feldspars covering the whole composition range from high potassium (orthoclase) through high sodium (albite) to high calcium (anorthite). The study was stimulated by the need to understand the properties of feldspars in connection with the application of thermoluminescence to the dating of sediments. The data were obtained with a high-sensitivity Fourier transform spectrometer, which allows measurements at the low light levels found in natural samples. Three-dimensional displays in which intensity is plotted as a function of photon energy and temperature assist identification of a wide range of spectral features. A number of common features are found: an emission at 3.1 eV is strong in alkali feldspars with more than 80 mole.% orthoclase and occurs with lower intensity in most other alkali feldspars and some plagioclases; a broad band with a flat maximum near 2.6 eV is found in all alkali feldspars at temperatures near 200 degrees C when artificially irradiated, and at higher temperatures in some natural samples. Emission at 2.2 and 4.4 eV is characteristic of plagioclases with more than 75 mole.% of albite. Plagioclases with more than 50 mole.% anorthite, and alkali feldspars with less than 50 mole.% orthoclase, emit mainly in the range 1.5-2 eV. These features can be identified with luminescence centres suggested by previous workers. The application to practical thermoluminescence dating is discussed.

http://iopscience.iop.org/article/10.1088/0022-3727/26/12/024/pdf

J R Prescott

Papers

Three-dimensional thermoluminescence spectra of feldspars