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Correlation between Colour and Fluorescence of Lapis lazuli

A. Banerjee

Institut für Geowissenschaften der Universität Mainz, D-55099 Mainz

Z. Naturforsch. 48a, 1267-1268 (1993);

received September 29, 1993

Colour and fluorescence of lapis lazuli depend on its min- eral components. UV-VIS- and fluorescence-spectra of speci- men rich in lazurite differ from those rich in haüyne.

Lapis lazuli is defined as a rock of varying compo- sition. The following minerals have been identified in this rock by X-ray diffraction: lazurite, haüyne, so- dalite, nosean, mica, diopside, wollastonite, calcite and pyrite. Among them the first two minerals, lazu- rite and haüyne, are responsible for the unique blue colour of massive lapis lazuli [1]. As it has been proved [2], the blue colour of lapis lazuli varies in hue and intensity according to the predominance of either lazurite or haüyne. The present note deals with the correlation between colour and fluorescence of differ- ent types of lapis lazuli.

Reprint requests to Dr. A. Banerjee, Institut für Geowis- senschaften - Edelsteinforschung - , Universität Mainz, Saarstraße, D-55099 Mainz.

jn 1267

Thirty homogeneously coloured samples from Afghanistan and Chile were investigated by the follow- ing methods: UV-VIS-, Infrared- and Fluorescence- spectroscopy. The UV-VIS-spectra of the samples were measured by a spectrophotometer L a m b d a 3 (Perkin Elmer) from 300 to 700 nm. The samples can be divided into two groups according to their remis- sion spectra. Spectra of the samples belonging to group A are typical for samples from Afghanistan, and those of group B are typical for samples from Chile.

The samples of group A show two characteristic re- mission maxima at 375 and 460 nm, in contrast to the samples of group B, which show only the remission maximum at 460 nm (Fig. 1). Due to the similarity of the IR-spectra of the samples of group A to that of haüyne, it is concluded that their colour is due to haüyne. Similarly, it is concluded that lazurite causes the colour of the samples of group B.

The fluorescence spectra of the samples were mea- sured by a double beam fluorescence spectrophotome- ter (Spex, Model F 212). As it is shown (Fig. 2), the spectra of group A differ considerably f r o m those of group B. The emission peak at A = 460 nm for the excitation wavelength X = 380 nm is c o m m o n to both groups. But in contrast to the spectra of the samples of group A those of group B show five more emission peaks at 580, 601, 643, 660 and 669 nm for the same excitation wavelength X = 380 nm.

380.0

T r 400.0 480.0 WAVELENGTH

880.0 700.0

Fig. 1. UV-VIS spectra of lapis lazuli. (Below): lapis lazuli from Afghanistan (group A), (above): lapis lazuli from Chile (group B).

0932-0784 / 93 / 1200-1267$ 01.30/0. - Please order a reprint rather than making your own copy.

This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution-NoDerivs 3.0 Germany License.

On 01.01.2015 it is planned to change the License Conditions (the removal of the Creative Commons License condition “no derivative works”). This is to allow reuse in the area of future scientific usage.

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht:

Creative Commons Namensnennung-Keine Bearbeitung 3.0 Deutschland Lizenz.

Zum 01.01.2015 ist eine Anpassung der Lizenzbedingungen (Entfall der Creative Commons Lizenzbedingung „Keine Bearbeitung“) beabsichtigt, um eine Nachnutzung auch im Rahmen zukünftiger wissenschaftlicher Nutzungsformen zu ermöglichen.

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1268 Notizen

WAVELENGTH ( na )

Fig. 2. Emission spectra of lapis lazuli for excitation wavelength X = 380 nm. (Below): lapis lazuli from Afghanistan (group A), (above): lapis lazuli from Chile (group B).

As has been pointed out in [3], the colour of lapis lazuli can be explained as an anion-anion charge transfer. According to [4], the SJ ion is responsible for the colour of lapis lazuli. Sulphur has the configura- tion 3 s2 3 p4, so that the S J ion has a total number of 19 outer electrons in molecular orbitals. It is a transi- tion among these orbitals that produces a strong ab- sorption band at 660 nm in the yellow, leading to the blue colour with purple overtones. The role of sulphur

as cause of fluorescence of some minerals like sodalite was investigated in [5]. A comparison of the fluores- cence spectra of lazurite and haüyne with those of both groups shows that the lazurite-rich samples (group B) posses all the emission peaks of lazurite under the emission wavelength X = 380 nm, whereas the emission peak of the samples of group A coincide more or less with the emission peak of haüyne for the excitation wavelength X = 380 nm mentioned above.

[1] K. Scarratt, J. Gem. 20, 1 (1986).

[2] A. Banerjee and T. Häger, Ber. Dt. Min. Ges. 1, 12 (1990).

[3] K. Nassau, Physics and Chemistry of Colour 149, John Wiley, New York 1983.

[4] K. H. Schwarz and U. Hofman, Z. Anorg. Allg. Chem.

378, 152 (1970).

[5] D. Dirk, J. Electrochem. Soc. 101, 461 (1954).

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