ORIGINAL PAPER E ´ milie Gaudry Æ Philippe Sainctavit Æ Farid Juillot Federica Bondioli Æ Philippe Ohresser Æ Isabelle Letard From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study Received: 27 January 2005 / Accepted: 16 October 2005 / Published online: 10 December 2005 Ó Springer-Verlag 2005 Abstract The best known cause for colors in insulating minerals is due to transition metal ions as impurities. As an example, Cr 3+ is responsible for the red color of ruby (a-Al 2 O 3 :Cr 3+ ) and the green color of eskolaite (a- Cr 2 O 3 ). Using X-ray absorption measurements, we connect the colors of the Cr x Al 2x O 3 series with the structural and electronic local environment around Cr. UV–VIS electronic parameters, such as the crystal field and the Racah parameter B, are related to those deduced from the analysis of the isotropic and XMCD spectra at the Cr L 2,3 -edges in Cr 0.07 Al 1.93 O 3 and eskolaite. The Cr–O bond lengths are extracted by EXAFS at the Cr K-edge in the whole Cr x Al 2x O 3 (0.07 £ x< 2) solid solution series. The variation of the mean Cr–O distance between Cr 0.07 Al 1.93 O 3 and a-Cr 2 O 3 is evaluated to be 0.01 5 A ˚ (1%). The variation of the crystal field in the Cr x Al 2x O 3 series is discussed in relation with the vari- ation of the averaged Cr–O distances. Keywords XAFS Æ Ruby Æ Color of minerals Introduction The color of minerals is due to the interaction of light with matter. When light goes through ruby (a- Al 2 O 3 :Cr 3+ ), the whole yellow-green and violet radia- tions are absorbed, while red and few blue radiations are transmitted. Therefore, ruby is red with a slight purple overtone. On the contrary, red light is absorbed in esk- olaite (a-Cr 2 O 3 ), so that this mineral looks green. Al- though it is now well known that color in ruby and eskolaite is due to the chromium ions (Nassau 1983; Burns 1993), a question remains unexplained. Why does the same chromium chromophore ion, with the same valence state and the same kind of distorted octahedral site, yield a red color in ruby and a green one in esk- olaite? In such oxide minerals, color is generally interpreted within the ligand field theory. This model is based on an electrostatic interaction between the central cation and the ligands of its coordination sphere. It is based mainly on the geometry and the symmetry around the central cation. This theory has been applied successfully to predict the number of 3d–3d transitions in optical absorption spectra and to identify the corresponding absorption bands (Lever 1984). Moreover, this theory enables an empirical calculation of the transition ener- gies (Liehr 1963). The UV–VIS absorption spectra of Cr 3+ -containing materials present two main absorption bands, which are already well explained by the ligand field theory applied to the system Al 2 O 3 :Cr 3+ (Poole 1964; Poole and Itzel 1963; McClure 1962, 1963; Reinen 1969; Sugano and Peter 1961; Liehr 1963; MacFarlane 1963; Graham 1960). The spectroscopic term of the fundamental state for the free Cr 3+ coloring ion in spherical symmetry is 4 F. In the octahedral symmetry, 4 F is split into three states of increasing energies that are 4 A 2g , 4 T 2g and 4 T 1g . The symmetry of the ground state is 4 A 2g . The absorption band at lower energy, labeled e 1 , arises from 4 A 2g toward 4 T 2g transitions. The broad band at higher energy, labeled e 2 , arises from 4 A 2g to- ward 4 T 1g transitions. The crystal field parameter D and the Racah parameter B are extracted from the values of e 1 and e 2 within the framework of the crystal field model. The D parameter is given by the energy e 1 . The B parameter is given by B ¼ ½ð 2   1 Þ=3½ð2 1   2 Þ=ð9 1  5 2 Þ (Marfunin 1979; Reinen 1969). The optical data given by Reinen (1969) lead to D=2.24 eV E ´ . Gaudry Æ P. Sainctavit (&) Æ F. Juillot Æ I. Letard Laboratoire de Mine´nalogie-Cristallographie, Universite´ Pierre et Marie Curie, UMR 7590, 4 place Jussieu, 75252 Paris Cedex 05, France E-mail: philippe.sainctavit@lmcp.jussieu.fr P. Sainctavit Æ P. Ohresser LURE, Baˆt 209D Centre Universitaire, B.P. 34, 91898 Orsay Cedex, France F. Bondioli Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita´ di Modena e Reggio Emilia, Via Vignolese 905, 41100 Modena, Italy Phys Chem Minerals (2006) 32: 710–720 DOI 10.1007/s00269-005-0046-x