The genesis of gemstones revealed by photoluminescence:

synthetic and natural Mg-Al spinels

Leonardo Maini (1), David Ajò (1), Sylvana Ehrman (2)

(1) Istituto di Chimica Inorganica e delle Superfici, Consiglio Nazionale delle Ricerche, 35134 Padova, Italy;

(2) Scientific Methodologies Applied to Cultural Heritage, Inc., Silver Spring MD 20910 , U.S.A.; sjehrman@msn.com

It is well known that spectroscopic properties of transition-metal ions, even at trace levels, allow the optical spectra to be used as a non-destructive test for discerning between natural and synthetic gemstones, both loose or mounted. In particular, photoluminescence (PL) features of Cr^{3+ in Mg-Al spinels evidence cationic disorder. The most important peculiarities of the spinel structure are the cation inversion and - for some synthetic (defect) crystals - the presence of vacancies arising from an Al:Mg ratio higher than 2. According to the Crystal Field Theory, PL spectral differences between Cr-doped Mg-Al spinels have been interpreted in terms of a superimposition of partial spectra, associated to several classes of Cr3+ local environment, with different cation ligand field (LF). These classes have been defined considering only mean-structure’s parameters, such as inversion degree and vacancies’ content.}

^{In general, the PL spectra are characterized by a lines’ luminescence - due to strong-LF sites -between 14300 and 14700 cm-1, and broad bands - below 14300 cm-1 - related to weak and intermediate LF. Defect spinels show luminescent centres associated with weak, intermediate and strong LF, while stoichiometric natural samples are often characterized only by the strong-LF emissions. Focusing on the region between 14300 and 14700 cm-1, natural crystals exhibit narrow lines, associated to discrete Cr3+ classes, while the PL pattern markedly broadens in synthetic materials, even when diffraction data points to equally good crystallinity.}

^{In order to single out the "genesis factor" affecting the PL behaviour, stoichiometric Cr-doped spinels have been synthesised by means of two different methods: the Verneuil and the flux one; detailed spectra of these crystals - together with those of some hi-purity natural analogues - were collected both at 6K and 298K. Moreover, crystals with variable vacancies contents have been grown through the Verneuil method only, and their PL and absorption spectra were collected at 6K.}

^{The most fruitful result of the discussion about the above experimental data is the strict dependence of the spectroscopic behaviour on the crystal genesis. Natural, Verneuil and flux-grown spinels, even when exhibiting identical Mg:Al ratio and cation inversion, can therefore be easily distinguished; their PL behaviour, even at room temperature, is driven by factors apparently related to the thermal history (in particular the crystallization time), and cannot be described only by mean-structure parameters, as cation inversion and vacancies’ content.}

^{Sylvana Ehrman}

^SMATCH

^{612 Woodside Parkway}

^{Silver Spring MD 20910, U.S.A.
Phone: 001 301 565-9484}

^{E-mail: sjehrman@msn.com}