Absorption of light by exchange coupled pairs of tetrahedrally coordinated divalent manganese in the helvite-genthelvite solid solution

Abstract

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 105-111 - DOI: 10.2451/2011PM0008
Special Issue in memory of Sergio Lucchesi

Absorption of light by exchange coupled pairs of tetrahedrally coordinated divalent manganese in the helvite-genthelvite solid solution

Ulf Hålenius

Department of Mineralogy, Swedish Museum of Natural History, Stockholm, Sweden

ulf.halenius@nrm.se

Abstract

Intensified colours in the helvite-genthelvite solid solution series are demonstrated by means of single crystal optical absorption and electron microprobe techniques to be due to absorption of light by exchange coupled ^IVMn²⁺-^IVMn²⁺ pairs. The absorption bands caused by the pair transitions occur at energies, 21100, 22600, 23800, 26800 and 28400 cm^-1, that are comparable to those recorded for spin-forbidden single ion transitions in ^IVMn²⁺, but the absorption coefficients of the bands are proportional to the square of the Mn-contents of the samples instead of simply showing a Lambert-Beer relationship to the Mn-content. The helvite structure, comprising four corner sharing MnO₃S tetrahedra with a common sulphur anion apex, represents a favourable situation for formation of exchange coupled 3d-cation pairs. However, several common Mn-rich minerals, as, e.g., rhodonite, have comparably favourable structures that contain corner, edge or face sharing 3d-cation centred polyhedra. Consequently, may Mn²⁺-Mn²⁺ pair absorption be a frequent cause for colour intensification in minerals.


Key words: helvite; genthelvite; electronic spectroscopy; electron microprobe analyses.